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Do xylem vessels in roots have lignin?

Do xylem vessels in roots have lignin?



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I learned that water enters xylem vessels in the roots after absorption by osmosis, but also that xylem cells are impregnated with lignin that doesn't allow water to pass through them to prevent water loss. Does that mean xylem cells in roots of plants aren't impregnated with it?


We have to consider the main function of lignin in xylem vessels, which is providing mechanical support for the plant. For the parts of plant above the ground, the stem where the xylem is located has to counteract the gravitational force pulling the plant down. Here's where lignin comes in to strengthen the xylem to support the plant. Remember that this is for many land plants.

Compare this to creepers, vines and aquatic plants. Lignin is absent or only present in small amounts as for creepers and vines, they have another external support, while for aquatic plants, it's weight is distributed on the surrounding water apart from the stem.

Now we can liken the case for aquatic plants to the xylem vessels in the roots of land plants. The roots of land plants are firmly anchored in the soil, and the plant's weight is distributed along the roots and soil, not just on the central root (for tapped roots). Hence there is not a need for lignin in roots of land plants.

And yes there are perforations in lignin as mentioned by @Always Confused, hence water can still enter the xylem vessel even when lignin is waterproof.


Does xylem vessels have nucleus?

Xylem vessels are a long straight chain made of tough long dead cells known as vessel elements. The vessel have no cytoplasm. They are not living, but are made by living cells. They have a lignified cell wall and a central cavity.

Furthermore, do mature cells of xylem have nucleus? Unlike xylem, phloem is alive at maturity, but usually with a much reduced cell contents and no nucleus.

Moreover, do vessel elements have a nucleus?

Its elements are elongated, just like those of the xylem. In contrast to tracheids and wood vessels, mature phloem elements contain a protoplast and sometimes even a nucleus.

What are vessels in xylem?

The xylem vessels are long tubes which help in transportation of water and provide mechanical support. Each xylem vessel is formed by end to end union of a large number of short, wide, lignified dead cells. In these cells both the nucleus and cytoplasm are absent.


2.54 describe the role of xylem in transporting water and mineral ions from the roots to other parts of the plant

Xylem:
-transport water and mineral salts from roots up to shoots to leaves in transpiration stream.
*transpiration: evaporation of water from surface of plant
sucrose and amino acids) from where they're produced to areas around the plant.

COMMON EXAM QUESTIONS:
1. How does water enter the root from soil?
-Water molecules can only enter root hair cells by osmosis.

2. How do mineral ions enter the root from soil?
-Minerals enter root hair cells from soil via active transport.

3. What are the features of xylem vessels?
-no cytoplasm, no nucleus
-end walls of these cells break down to provide a continuous unbroken column of water all the way up the plant. The cell walls of xylem vessels made with protein lignin.

4. What causes water to move up the xylem?
-heat energy from the sun transpires water in the leaves and provides energy for this movement. (transpiration)
-water molecules are cohesive they have a hydrogen bond between each other. This allows them to pull on each other up the xylem vessel.


A Meristem

A meristem is a group of cells that continuously divide by mitosis for growth.

  • Apical meristems are found near root and shoot tips and they allow for growth in length.
  • Lateral meristems e.g. cambium allow for growth in width.

Meristimatic tissue divides to produce new cells. When these cells differentiate they give rise to :

  • DERMAL TISSUE:
  • forms the protective covering of plants
  • GROUND TISSUE:
  • Fills the interior of the plant
  • Involved in photosynthesis or food storage. Also gives strength and support to the plant
  • VASCULAR TISSUE:
  • Xylem transports water and minerals
  • Phloem transports food e.g glucose, amino acids.

These genes help keep plants green

PUBLISHED ON May 18, 2021

RIVERSIDE, Calif. — University of California scientists have discovered genetic data that will help food crops like tomatoes and rice survive longer, more intense periods of drought on our warming planet.

Over the course of the last decade, the research team sought to create a molecular atlas of crop roots, where plants first detect the effects of drought and other environmental threats. In so doing, they uncovered genes that scientists can use to protect the plants from these stresses.

Their work, published today in the journal Cell, achieved a high degree of understanding of the root functions because it combined genetic data from different cells of tomato roots grown both indoors and outside.

“Frequently, researchers do lab and greenhouse experiments, but farmers grow things in the field, and this data looks at field samples too,” said Neelima Sinha, a UC Davis professor of plant biology and the paper’s co-author.

The data yielded information about genes that tell the plant to make three key things.

Xylem are hollow, pipe-like vessels that transport water and nutrients from the roots all the way up to the shoots. Without transport in xylem, the plant cannot create its own food via photosynthesis.

“Xylem are very important to shore up plants against drought as well as salt and other stresses,” said lead study author Siobhan Brady, a professor of plant biology at UC Davis.

In turn, without plant mineral transport in xylem, humans and other animals would have fewer vitamins and nutrients essential for our survival. In addition to some typical players needed to form the xylem, new and surprising genes were found.

The second key set of genes are those that direct an outer layer of the root to produce lignin and suberin. Suberin is the key substance in cork and it surrounds plant cells in a thick layer, holding in water during drought.

Crops like tomatoes and rice have suberin in the roots. Apple fruits have suberin surrounding their outer cells. Anywhere it occurs, it prevents the plant from losing water. Lignin also waterproofs cells and provides mechanical support.

“Suberin and lignin are natural forms of drought protection, and now that the genes that encode for them in this very specific layer of cells have been identified, these compounds can be enhanced,” said study co-author Julia Bailey-Serres, a UC Riverside professor of genetics.

“I’m excited we’ve learned so much about the genes regulating this moisture barrier layer. It is so important for being able to improve drought tolerance for crops,” she said.

Genes that encode for a plant’s root meristem also turned out to be remarkably similar between tomato, rice, and Arabidopsis, a weed-like model plant. The meristem is the growing tip of each root, and it’s the source of all the cells that make up the root.

“It’s the region that’s going to make the rest of the root, and serves as its stem cell niche,” said Bailey-Serres. “It dictates the properties of the roots themselves, such as how big they get. Having knowledge of it can help us develop better root systems.”

Brady explained that when farmers are interested in a particular crop, they select plants that have features they can see, such as bigger, more attractive fruits. Much more difficult is for breeders to select plants with properties below ground they can’t see.

“The ‘hidden half’ of a plant, below ground, is critical for breeders to consider if they want to grow a plant successfully,” Brady said. “Being able to modify the meristem of a plant’s roots will help us engineer crops with more desirable properties.”

Though this study analyzed only three plants, the team believes the findings can be applied more broadly.

“Tomato and rice are separated by more than 125 million years of evolution, yet we still see similarities between the genes that control key characteristics,” said Bailey-Serres. “It’s likely these similarities hold true for other crops too.”


Transport in Animals

The Circulatory System

  • Large and complex animals have circulatory systems that consist of tubes, a transport fluid and a means of pumping the fluid.
  • Blood is the transport fluid which contains dissolved substances and cells.
  • The tubes are blood vessels through which dissolved substances are circulated around the body.
  • The heart is the pumping organ which keeps the blood in circulation.

The types of circulatory system exist in animals: open and closed.

In an open circulatory system

  • The heart pumps blood into vessels which open into body spaces known as haemocoel.
  • Blood comes into contact with tissues.

A closed circulatory system

  • Found in vertebrates and annelids where the blood is confined within blood vessels and does not come into direct contact with tissues.

Transport in Insects

  • In an insect, there is a tubular heart just above the alimentary canal.
  • This heart is suspended in a pericardial cavity by ligaments.
  • The heart has five chambers and extends along the thorax and abdomen.
  • Blood is pumped forwards into the aorta by waves of contractions in the heart.
  • It enters the haemocoel and flows towards the posterior.
  • The blood flows back into the heart through openings in each chamber called ostia.
  • The ostia have valves which prevent the backflow of blood.
  • Blood is not used as a medium for transport of oxygen in insects.
  • This is because oxygen is supplied directly to the tissues by the tracheal system.
  • The main functions of blood in an insect are to transport nutrients, excretory products and hormones.

What is a Xylem Cell? (with pictures)

A xylem cell is a cell which is responsible for providing support to a plant. These cells also make up the vascular system of plants, conducting water throughout the plant and providing circulation. These cells can be both alive and dead, and there are several different types of xylem cell which can be found within the parts of a plant known collectively as xylem.

Plant cells start out as undifferentiated parenchyma cells. These cells can store energy for the plant, and they can also differentiate and mature into various cell types, including xylem cells. The xylem of plants usually contains a number of parenchyma cells, leading some people to classify them as a type of xylem cell, although this is technically incorrect.

Support is created through trachieds and fibers, cells which contain a great deal of lignin in their cell walls. The lignin makes the cell walls rigid, making the xylem as a whole very stiff so that it will support the plant and keep it upright. Trachieds are also involved in conduction, as are cells known as vessel members. Vessel members are tubular xylem cells which are designed to force water up against the pull of gravity so that it can circulate into the upper reaches of the plant.

When xylem cells die, they are still useful to the parent plant, unlike dead animal cells, which are usually broken down and discarded because they no longer serve a function. Although a dead xylem cell is no longer able to perform complex biological functions, it can still act as part of a support network for the plant, because the lignin in the cell walls is intact. These cells can also continue to conduct water through the xylem after death, because their conductive properties are purely mechanical, created by the shape of the cell, rather than being biological in nature.

Without xylem cells, a plant would have no vascular system. Vascular plants are able to be much more complex than their non-vascular counterparts, and they could be considered an evolutionary step up from nonvascular plants. Vascular plants may also be referred to as “woody plants,” because their xylem gives them a woody texture and the ability to grow large, upright, and complex. Woody plants serve a number of important ecological functions, and they are also highly prized as ornamentals in gardens all over the world.

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a InfoBloom researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a InfoBloom researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.


RESULTS

MdMYB88 and MdMYB124 Positively Regulate Root Architecture under Long-Term Drought Stress

We previously found that MdMYB88 and its paralog MdMYB124 are dominantly expressed in roots of apple trees ( Xie et al., 2018). To further investigate their roles in root development, roots of 7-month-old nontransgenic and transgenic apple plants we generated before ( Xie et al., 2018) were examined. MdMYB88 and MdMYB124 were simultaneously silenced because the sequences of MdMYB88 and MdMYB124 are so similar that we cannot silence only one of them by RNAi approach. As shown in Figure 1, A and B, plants overexpressing MdMYB88 or MdMYB124 showed vigorous adventitious roots, as determined by adventitious root length. MdMYB88/124 RNAi plants had weak adventitious root systems, as compared with that of nontransgenic GL-3 plants, indicating potential roles for MdMYB88 and MdMYB124 in apple root development. Considering the important roles of roots in drought tolerance, we examined expression of both genes in apple roots in response to drought. Gene expression analysis revealed that MdMYB88 and MdMYB124 were induced slightly in the roots of Malus sieversii under simulated drought conditions, indicating their potential participation in drought tolerance ( Fig. 1C). We also tested expression of other MdMYBs, which displayed higher sequence similarity with MdMYB88 and MdMYB124, in MdMYB88/124 RNAi plants and found none of these genes were disrupted in their expression ( Supplemental Fig. S1A ). These results suggest that weak adventitious roots in MdMYB88/124 RNAi plants are due to disrupted expression of MdMYB88 and MdMYB124, but not other MdMYBs.

Root morphology of transgenic plants with altered MdMYB88 and MdMYB124 expression and MsMYB88 and MsMYB124 expression level changes in response to drought. A, Root morphology of nontransgenic plants (GL-3), MdMYB88 or MdMYB124 overexpression plants (OE), and MdMYB88/124 RNAi plants. B, Quantitation of adventitious root length of the plants shown in A. Data are means ± sd (n = 5). One-way ANOVA (Tukey test) was performed, and statistically significant differences are indicated by *P < 0.05. C, Relative expression level of MsMYB88 and MsMYB124 in M. sieversii roots under 20% PEG8000 treatment for 0 or 6 h. Data are means ± sd (n = 3).

Root morphology of transgenic plants with altered MdMYB88 and MdMYB124 expression and MsMYB88 and MsMYB124 expression level changes in response to drought. A, Root morphology of nontransgenic plants (GL-3), MdMYB88 or MdMYB124 overexpression plants (OE), and MdMYB88/124 RNAi plants. B, Quantitation of adventitious root length of the plants shown in A. Data are means ± sd (n = 5). One-way ANOVA (Tukey test) was performed, and statistically significant differences are indicated by *P < 0.05. C, Relative expression level of MsMYB88 and MsMYB124 in M. sieversii roots under 20% PEG8000 treatment for 0 or 6 h. Data are means ± sd (n = 3).

To further explore the roles of MdMYB88 and MdMYB124 in root development under drought, we applied long-term drought treatment on transgenic and nontransgenic plants ( Supplemental Fig. S1B ). As shown in Figure 2, drought treatment significantly affected plant height, stem diameter, dry weight of shoots, dry weight of roots, and root-to-shoot ratio. After 2 months of drought stress, MdMYB88/124 RNAi plants were much shorter, whereas MdMYB88 or MdMYB124 overexpression plants were taller, when compared to the height of GL-3 plants ( Fig. 2A). The stems of MdMYB88/124 RNAi plants were much thinner than those of GL-3 plants under drought. Overexpression of MdMYB88 or MdMYB124 increased stem diameter compared to that in the control after drought ( Fig. 2B). Dry weight of shoots and roots in MdMYB88/124 RNAi plants were clearly lower than that of GL-3 plants, resulting in a lower root-to-shoot ratio in MdMYB88/124 RNAi plants under drought stress ( Fig. 2, C–E). Consistently, MdMYB88 or MdMYB124 overexpression plants had a higher root-to-shoot ratio than that of GL-3 plants in response to long-term drought stress, proportional to the relatively higher dry weight of shoots and roots under drought ( Fig. 2, C–E). These data suggest that MdMYB88 and MdMYB124 positively regulate the drought tolerance of apple roots, at least in part, by mediating root architecture.

Quantitation of morphological traits of GL-3, MdMYB88, or MdMYB124 overexpression plants, and MdMYB88/124 RNAi plants under long-term drought conditions. A, Plant height. B, Stem diameter. C, Dry weight of roots. D, Dry weight of stem. E, Root-to-shoot ratio. Plants were subjected to long-term drought stress for 2 months in a greenhouse. Data are means ± sd (n = 9). One-way ANOVA (Tukey test) was performed, and statistically significant differences are indicated by *P < 0.05 or **P < 0.01.

Quantitation of morphological traits of GL-3, MdMYB88, or MdMYB124 overexpression plants, and MdMYB88/124 RNAi plants under long-term drought conditions. A, Plant height. B, Stem diameter. C, Dry weight of roots. D, Dry weight of stem. E, Root-to-shoot ratio. Plants were subjected to long-term drought stress for 2 months in a greenhouse. Data are means ± sd (n = 9). One-way ANOVA (Tukey test) was performed, and statistically significant differences are indicated by *P < 0.05 or **P < 0.01.

MdMYB88 and MdMYB124 Regulate Hydraulic Conductivity of Apple Roots under Long-Term Drought Conditions

Two fundamental capabilities of roots are supporting shoot components and transporting water and mineral elements to shoots ( Warren et al., 2015). Under drought stress, hydraulic conductivity, an indicator of the ability to transport water, decreases in both roots and shoots ( Moshelion et al., 2015). Changed root morphology of transgenic plants under drought stress prompted us to examine their root hydraulic conductivity in response to drought. After 2-month exposure to drought conditions, root hydraulic conductivity as measured by high pressure flow matter (HPFM) was reduced remarkably ( Fig. 3). Compared with GL-3 plants, roots of MdMYB88/124 RNAi plants had a much lower hydraulic conductivity, whereas MdMYB88 or MdMYB124 overexpression plants had a clearly higher root hydraulic conductivity ( Fig. 3 Supplemental Fig. S2 ). These data are suggestive of a stronger water transportation ability with MdMYB88 or MdMYB124 overexpression under long-term drought stress. We also measured the shoot hydraulic conductivity of the plants tested above and found that, similar to root hydraulic conductivity, shoot hydraulic conductivity of MdMYB88/124 RNAi plants was much lower than that of GL-3 plants under drought stress ( Supplemental Fig. S3 ). Consistently, MdMYB88 or MdMYB124 overexpression plants had a higher shoot hydraulic conductivity than that of GL-3 plants in response to drought stress ( Supplemental Fig. S3 ).

Root hydraulic conductivity of GL-3, MdMYB88, or MdMYB124 overexpression plants and MdMYB88/124 RNAi plants under long-term drought conditions. Plants were subjected to long-term drought stress for 2 months in a greenhouse. Data are means ± sd (n = 9). One-way ANOVA (Tukey test) was performed, and statistically significant differences are indicated by **P < 0.01.

Root hydraulic conductivity of GL-3, MdMYB88, or MdMYB124 overexpression plants and MdMYB88/124 RNAi plants under long-term drought conditions. Plants were subjected to long-term drought stress for 2 months in a greenhouse. Data are means ± sd (n = 9). One-way ANOVA (Tukey test) was performed, and statistically significant differences are indicated by **P < 0.01.

MdMYB88 and MdMYB124 Mediate Root Xylem Development under Long-Term Drought Conditions

Water is transported from roots to shoots by vessels vessel embolism and development therefore significantly affect hydraulic conductivity ( Olson et al., 2014). We next asked whether MdMYB88 and MdMYB124 were regulators of root xylem development in response to long-term drought treatment ( Fig. 3). We first stained roots of transgenic plants and GL-3 plants with Safranin O under control and drought treatments ( Fig. 4A Supplemental Fig. S4 ). Obviously, MdMYB88/124 RNAi plants had decreased vessel density in response to drought treatment. In comparison with that in GL-3 plants, MdMYB88 or MdMYB124 overexpression plants had higher vessel density under drought conditions ( Fig. 4A Supplemental Fig. S4 ). We quantified vessel density, vessel diameter (average length of major axis of vessels [mean Dmin], average length of minor axis of vessels [mean Dmax]), and lumen area ( Fig. 4B). As shown in Figure 4B, compared to that in GL-3 plants, vessel density and vessel diameter were lower in MdMYB88/124 RNAi plants under drought conditions, whereas those of MdMYB88 or MdMYB124 overexpression plants displayed greater vessel density and diameter ( Fig. 4B). Lumen area was quantified as the ratio of total vessel area compared to xylem area. In response to long-term drought stress, the lumen area was decreased in MdMYB88/124 RNAi plants but increased in MdMYB88 or MdMYB124 overexpression plants when compared to nontransgenic GL-3 plants ( Fig. 4B). We also noticed that root phloem thickness was significantly decreased in MdMYB88/124 RNAi plants under drought treatment compared with that in GL-3 plants, indicating that MdMYB88 and MdMYB124 might also regulate phloem development in response to drought ( Supplemental Fig. S4B ).

Xylem development in roots of GL-3, MdMYB88, or MdMYB124 overexpression plants and MdMYB88/124 RNAi plants under long-term drought conditions. A, Cross sections of roots from GL-3 and transgenic plants stained with Safranin O. Bars, 100 µm. B, Quantification of root xylem of plants shown in A. Mean Dmax, average length of major axis of vessels mean Dmin, average length of minor axis of vessels lumen area, total lumen area, relative to xylem area. n = 10.

Xylem development in roots of GL-3, MdMYB88, or MdMYB124 overexpression plants and MdMYB88/124 RNAi plants under long-term drought conditions. A, Cross sections of roots from GL-3 and transgenic plants stained with Safranin O. Bars, 100 µm. B, Quantification of root xylem of plants shown in A. Mean Dmax, average length of major axis of vessels mean Dmin, average length of minor axis of vessels lumen area, total lumen area, relative to xylem area. n = 10.

MdMYB88 and MdMYB124 Are Predominantly Expressed in Xylem Vessels and Cambium in Apple Roots

Previously, we found that MdMYB88 and MdMYB124 are predominantly expressed in the roots of apple plants ( Xie et al., 2018). To specifically investigate the localization of MdMYB88 and MdMYB124 transcripts in roots of apple, we performed an in situ hybridization ( Fig. 5). When using a sense probe, only background was detectable ( Fig. 5, A and B) however, strong signals were observed in the vessels and cambium of apple roots when using an antisense probe ( Fig. 5C). Enlarged images showed that transcripts of MdMYB88 and MdMYB124 were visualized in xylem vessels but not in xylem fiber cells ( Fig. 5D). In addition, weak signals were detected in the phloem of apple roots ( Fig. 5D).

Localization of MdMYB88 transcripts in roots of GL-3. A, In situ hybridization of MdMYB88 transcripts using sense probe. B, Enlarged image of A. C, In situ hybridization of MdMYB88 transcripts using antisense probe. D, Enlarged image of C. MdMYB88 transcript in roots is indicated by purple coloring. Bars, 100 µm.

Localization of MdMYB88 transcripts in roots of GL-3. A, In situ hybridization of MdMYB88 transcripts using sense probe. B, Enlarged image of A. C, In situ hybridization of MdMYB88 transcripts using antisense probe. D, Enlarged image of C. MdMYB88 transcript in roots is indicated by purple coloring. Bars, 100 µm.

MdMYB88 and MdMYB124 Mediate Expression of MdVND6 and MdMYB46 in Apple Roots under Simulated Drought Conditions

We next asked how MdMYB88 and MdMYB124 regulate xylem vessel development in apple roots. In Arabidopsis, a battery of NAC and MYB genes, including MYB46, VND6, VND7, and SND1, are known to mediate xylem vessel development ( Zhong et al., 2007b Ohashi-Ito et al., 2010 Kim et al., 2013). We then investigated expression of some of these genes in the roots of nontransgenic or transgenic plants under control or simulated drought conditions ( Fig. 6, A and B Supplemental Fig. S5 ). Reverse transcription quantitative PCR (RT-qPCR) analysis suggested a positive relationship between MdMYB88 and MdMYB124 presence and expression of both MdVND6 and MdMYB46 in the roots of apple under control or drought conditions ( Fig. 6, A and B). In contrast, no such relationship was found with MdVND7 and MdSND1 ( Supplemental Fig. S5 ). These data suggest that MdMYB88 and MdMYB124 may regulate root xylem vessel development by mediating expression of MdVND6 and MdMYB46.

MdMYB88 and MdMYB124 regulate MdMYB46 and MdVND6 expression by directly targeting their promoters. A and B, Expression level of MdVND6 and MdMYB46 in roots of GL-3, MdMYB88 or MdMYB124 overexpression plants, and MdMYB88/124 RNAi plants in response to drought stress. Plants were subjected to 20% PEG8000 for 0 or 6 h. Data are means ± sd (n = 3). C and D, Yeast one-hybrid analysis of interaction between MdMYB88 and MdVND6 (C) and MdMYB46 (D) promoters. AbA concentration is 500 ng/mL. E and F, ChIP-qPCR analysis of MdVND6 (E) and MdMYB46 (F) binding by MdMYB88 and MdMYB124. MDH is the negative control, also serves as the reference gene. Fragments MdVND6-a and MdMYB46-a serve as negative controls in E and F, respectively. Fragments MdVND6-b and MdMYB46-b both contain cis-element of AACCG. Data are means ± sd (n = 3). G and H, EMSA analysis of MdMYB88-His binding to the promoter region of MdVND6 (G) and MdMYB46 (H). Arrowheads indicate protein-DNA complex or free probe.

MdMYB88 and MdMYB124 regulate MdMYB46 and MdVND6 expression by directly targeting their promoters. A and B, Expression level of MdVND6 and MdMYB46 in roots of GL-3, MdMYB88 or MdMYB124 overexpression plants, and MdMYB88/124 RNAi plants in response to drought stress. Plants were subjected to 20% PEG8000 for 0 or 6 h. Data are means ± sd (n = 3). C and D, Yeast one-hybrid analysis of interaction between MdMYB88 and MdVND6 (C) and MdMYB46 (D) promoters. AbA concentration is 500 ng/mL. E and F, ChIP-qPCR analysis of MdVND6 (E) and MdMYB46 (F) binding by MdMYB88 and MdMYB124. MDH is the negative control, also serves as the reference gene. Fragments MdVND6-a and MdMYB46-a serve as negative controls in E and F, respectively. Fragments MdVND6-b and MdMYB46-b both contain cis-element of AACCG. Data are means ± sd (n = 3). G and H, EMSA analysis of MdMYB88-His binding to the promoter region of MdVND6 (G) and MdMYB46 (H). Arrowheads indicate protein-DNA complex or free probe.

MdMYB88 and MdMYB124 Directly Target MdVND6 and MdMYB46 Promoters

Previously, we identified one binding site of MdMYB88 and MdMYB124 using chromatin immunoprecipitation qPCR (ChIP-qPCR) and EMSA analyses: AACCG ( Xie et al., 2018). Regulation of MdVND6 and MdMYB46 expression by MdMYB88 and MdMYB124 under control and drought conditions prompted us to analyze MdVND6 and MdMYB46 promoter sequences. As expected, a cis-element of AACCG in the promoter region of MdVND6 and MdMYB46 was discovered ( Supplemental Fig. S6 ). By performing yeast one-hybrid (Y1H) analysis, direct binding of MdMYB88 to both promoters was detected ( Fig. 6, C and D). ChIP-qPCR analysis was then completed to further determine this direct binding in planta. Our results demonstrated MdMYB88 and MdMYB124 to be capable of binding to the AACCG site in promoters of MdVND6 and MdMYB46 ( Fig. 6, E and F). EMSA analysis further confirmed MdMYB88 to directly target MdVND6 and MdMYB46 promoters ( Fig. 6, G and H).

MdMYB88 and MdMYB124 Regulate Cellulose and Lignin Deposition in the Roots of Apple in Response to Long-Term Drought Conditions

In Arabidopsis, MYB46 is a master regulator for secondary wall-associated cellulose accumulation ( Kim et al., 2013). Furthermore, VND6 is a key regulator for xylem vessel differentiation, programmed cell death, and secondary wall formation ( Ohashi-Ito et al., 2010 Yamaguchi et al., 2010). Direct regulation of MdVND6 and MdMYB46 by MdMYB88 and MdMYB124 suggests that, in response to long-term drought stress, MdMYB88 and MdMYB124 may participate in the biosynthesis of secondary cell wall components. We then first examined contents of cellulose, lignin, and hemicellulose in roots of transgenic and nontransgenic plants under control or drought conditions. After 2-month drought treatment, MdMYB88/124 RNAi plants accumulated less cellulose and lignin compared with that in GL-3 plants. Under control conditions, MdMYB88 and MdMYB124 expression was positively associated with cellulose and lignin accumulation ( Fig. 7, A and B). Consistently, roots of plants overexpressing MdMYB88 or MdMYB124 contained more cellulose and lignin content under control or drought conditions than that of nontransgenic GL-3 plants ( Fig. 7, A and B). MdMYB88 and MdMYB124 did not regulate accumulation of hemicellulose in the roots under control or long-term drought conditions ( Fig. 7C).

Content of cellulose, lignin, hemicellulose, and expression level of genes associated with secondary cell wall biosynthesis in roots of GL-3, MdMYB88, or MdMYB124 overexpression plants and MdMYB88/124 RNAi plants under drought conditions. A to C, Contents of cellulose (A), lignin (B), and hemicellulose (C). Plants were subjected to long-term drought stress for 2 months in a greenhouse. Data are means ± sd (n = 9). One-way ANOVA (Tukey test) was performed, and statistically significant differences are indicated by *P < 0.05 or **P < 0.01. D to F, Relative expression levels of MdCesA4 (D), MdCesA8 (E), and MdC4H (F). Plants were subjected to 20% PEG8000 for 0 or 6 h. Data are means ± sd (n = 3).

Content of cellulose, lignin, hemicellulose, and expression level of genes associated with secondary cell wall biosynthesis in roots of GL-3, MdMYB88, or MdMYB124 overexpression plants and MdMYB88/124 RNAi plants under drought conditions. A to C, Contents of cellulose (A), lignin (B), and hemicellulose (C). Plants were subjected to long-term drought stress for 2 months in a greenhouse. Data are means ± sd (n = 9). One-way ANOVA (Tukey test) was performed, and statistically significant differences are indicated by *P < 0.05 or **P < 0.01. D to F, Relative expression levels of MdCesA4 (D), MdCesA8 (E), and MdC4H (F). Plants were subjected to 20% PEG8000 for 0 or 6 h. Data are means ± sd (n = 3).

In Arabidopsis, CELLULOSE SYNTHASE A4, A7, and A8 (CesA4, CesA7, CesA8), CINNAMATE 4-HYDROXYLASE (C4H), PHE AMMONIA LYASE 1 (PAL1), 4-COUMARATE:COA LIGASE 1 (4CL), ACAULIS 5 (ACL5), XYLEM CYSTEINE PEPTIDASE 1 (XCP1), and IRREGULAR XYLEM 9 (IRX9) ) are responsible for the biosynthesis of cellulose, lignin, and hemicellulose. We thus examined expression levels of these genes in roots of transgenic and nontransgenic plants under control or drought conditions. We found that expression levels of MdCesA4, MdCesA8, and MdC4H were decreased in MdMYB88/124 RNAi plants as compared with that in nontransgenic GL-3 plants under control and drought conditions ( Fig. 7, D–F). Consistently, the expression levels of these three genes were significantly elevated in plants overexpressing MdMYB88 or MdMYB124 under drought and control conditions. No variation in expression of MdIRX9, MdPAL1, Md4CL1, MdACL5, or MdXCP1 was detected under any conditions ( Supplemental Fig. S7 ).


Xylem to leaf Edit

As water evaporates from the leaf, a constantly occurring process, more water is taken in to replace it. The removal of water reduces the hydrostatic pressure (pressure exerted by a liquid). Since this pressure becomes lower at the top of the xylem vessel than at the bottom, this pressure difference causes water to move up the xylem vessels, just as in a straw.

This process is known as mass flow - as long with the fact that water molecules move together as a body of water - aided by water's property of being cohesive, and attracted to the lignin in the walls of the xylem vessels, known as adhesion.

Once water is in the leaf, it can be lost through the stomata, if there is a concentration gradient that it can go down, which are small pores in direct contact with the air outside. This process is known as transpiration.

Root pressure Edit

Plants can also increase the hydrostatic pressure at the bottom of the vessels, changing the pressure difference. They do this by cells surrounding the xylem vessels to use active transport to pump solutes across their membranes and into the xylem, lowering the water potential of the solution in the xylem, thus drawing in water from the surrounding root cells. The influx of water at the bottom of the xylem increases the pressure.


LIGNIFICATION AFFECTS AGRONOMIC TRAITS AND INDUSTRIAL APPLICATIONS

Lignin is important for enhancing rigidity to protect plants against pathogen attacks and mechanical stress (Zhang et al. 2006 ). It increases the strength of cell walls by cross-linking with cellulose and hemicellulose (Li et al. 2009 ). Culm mechanical strength is an important agronomic trait in crop breeding because lodging causes significant losses in yield (Li et al. 2009 Ookawa et al. 2014 ). Reduced lignin contents often affect lodging resistance (Ookawa et al. 2014 ).

In rice, most brittle culm mutants have decreased amounts of cellulose. The mutant in BC6 that encodes CesA, a cellulose synthesis enzyme, exhibits phenotypes with significantly less cellulose and thinner walls in sclerenchyma cells (Kotake et al. 2011 ). Mutants in BC11, which encodes CESA4, have reduced culm mechanical strength and irregular growth, e.g., dwarfism and abnormal leaf apices at the seedling stage plus drooping leaves, small panicles, and partial sterility at the mature stage (Zhang et al. 2009 ). In addition to lower cellulose contents, those culm mutants have a smaller amount of lignin (Oh et al. 2013 ).

When attempting to increase yields, reducing the occurrence of “shattering” is an important step in the domestication process for many crops. Seed dispersal involves abscission zone formation and lignification. In Arabidopsis, a seedpod has three tissues: Two laterally positioned valves that protect seeds a thin ridge of cells, or “replum”, where seeds are attached and two valve margins that connect the replum and valves (Lewis et al. 2006 ). The valve margin consists of a lignified layer and a separation layer. When that margin separates, the fruit opens for seed dispersal (Liljegren et al. 2004 Lewis et al. 2006 ). Functionally redundant SHP1 and SHP2 MADS box genes modulate differentiation of the dehiscence zone and promote lignification (Liljegren et al. 2000 ). The REPLUMPESS (RPL) and BP homeobox genes are key regulators for replum development and control the preferential expression of genes in the valve margin (Venglat et al. 2002 Roeder et al. 2003 ). Whereas the abscission zone in Arabidopsis is located at that margin, the zone in cereal crops is at the base of the pedicel (Tang et al. 2013 ). In Sorghum propinquum, a wild sorghum, shattering occurs when SpWRKY is expressed during the development of floral organs and seeds (Tang et al. 2013 ). SpWRKY is an ortholog of Medicago MtSTP and Arabidopsis AtWRKY12, both of which regulate cell wall biosynthesis and lignin deposition (Wang et al. 2010 Tang et al. 2013 ). In Glycine max, a NAC gene, SHATTERING1-5, activates the biosynthesis of secondary walls and promotes their thickening in fiber cap cells (Dong et al. 2014 ).

Lignin is a major concern in the pulp and paper industry because harsh chemical treatments are required in order to remove it from wood so that one can obtain pure cellulose (Peter et al. 2007 Bonawitz and Chapple 2013 ). The ability to alter lignin contents could also help in improving feed digestibility and lignocellulose saccharification for biofuel production (Ookawa et al. 2014 ). Genetic engineering techniques used to achieve such modifications usually exploit the regulation of lignin biosynthesis genes (Leple et al. 2007 Bonawitz and Chapple 2013 ). In transgenic poplar, downregulation of CCR reduces the lignin content and coloration of the outer xylem (Leple et al. 2007 ). Although their growth is negatively affected, those plants show improved pulping characteristics. Abnormal lignin deposition influences plant development primarily because of dwarfing, collapsed xylem tissue, and problems with the vascular system (Leple et al. 2007 Vanholme et al. 2008 Hirano et al. 2012 ). However, it remains unclear whether such irregularities result directly from those modifications to lignin deposition or are instead a consequence of the accumulation of certain intermediates or byproducts (Bonawitz and Chapple 2013 ).


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