ABCG Transporters Are Required for Suberin and Pollen Wall Extracellular Barriers inArabidopsis
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Abstract:
Effective regulation of water balance in plants requires localized extracellular barriers that control water and solute movement. We describe a clade of five Arabidopsis thaliana ABCG half-transporters that are required for synthesis of an effective suberin barrier in roots and seed coats (ABCG2, ABCG6, and ABCG20) and for synthesis of an intact pollen wall (ABCG1 and ABCG16). Seed coats of abcg2 abcg6 abcg20 triple mutant plants had increased permeability to tetrazolium red and decreased suberin content. The root system of triple mutant plants was more permeable to water and salts in a zone complementary to that affected by the Casparian strip. Suberin of mutant roots and seed coats had distorted lamellar structure and reduced proportions of aliphatic components. Root wax from the mutant was deficient in alkylhydroxycinnamate esters. These mutant plants also had few lateral roots and precocious secondary growth in primary roots. abcg1 abcg16 double mutants defective in the other two members of the clade had pollen with defects in the nexine layer of the tapetum-derived exine pollen wall and in the pollen-derived intine layer. Mutant pollen collapsed at the time of anther desiccation. These mutants reveal transport requirements for barrier synthesis as well as physiological and developmental consequences of barrier deficiency.Keywords:
Suberin
Suberin
Cutin
Sporopollenin
Middle lamella
Phloroglucinol
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Suberin
Endodermis
Secondary cell wall
Middle lamella
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A municipal solid-waste bottom slag was used to grow maize plants under various abiotic stresses (high pH, high salt and high heavy metal content) and to analyse the structural and chemical adaptations of the cell walls of various root tissues. When compared with roots of control plants, more intensive wall thickenings were detected in the inner tangential wall of the endodermis. In addition, phi thickenings in the rhizodermis in the oldest part of the seminal root were induced when plants were grown in the slag. The role of the phi thickenings may not be a barrier for solutes as an apoplastic dye could freely diffuse through them. The chemical composition of cell walls from endodermis and hypodermis was analysed. Slag-grown plants had higher amounts of lignin in endodermal cell walls when compared to control plants and a higher proportion of H-type lignin in the cell walls of the hypodermis. Finally, the amount of aliphatic suberin in both endo- and hypodermal cell walls was not affected by growing the plants on slag. The role of these changes in relation to the increase in mechanical strengthening of the root is discussed.
Endodermis
Suberin
Fibrous root system
Middle lamella
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Suberin
Endodermis
Secondary cell wall
Callose
Cutin
Cell membrane
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Summary Wood is the most important repository of assimilated carbon in the biosphere, in the form of large polymers (cellulose, hemicelluloses including glucuronoxylan, and lignin) that interactively form a composite, together with soluble extractives including phenolic and aliphatic compounds. Molecular interactions among these compounds are not fully understood. We have targeted the expression of a fungal α‐glucuronidase to the wood cell wall of aspen ( Populus tremula L. × tremuloides Michx.) and Arabidopsis ( Arabidopsis thaliana (L.) Heynh), to decrease contents of the 4‐ O ‐methyl glucuronopyranose acid (mGlcA) substituent of xylan, to elucidate mGlcA's functions. The enzyme affected the content of aliphatic insoluble cell wall components having composition similar to suberin, which required mGlcA for binding to cell walls. Such suberin‐like compounds have been previously identified in decayed wood, but here, we show their presence in healthy wood of both hardwood and softwood species. By contrast, γ‐ester bonds between mGlcA and lignin were insensitive to cell wall‐localized α‐glucuronidase, supporting the intracellular formation of these bonds. These findings challenge the current view of the wood cell wall composition and reveal a novel function of mGlcA substituent of xylan in fastening of suberin‐like compounds to cell wall. They also suggest an intracellular initiation of lignin–carbohydrate complex assembly.
Suberin
Secondary cell wall
Softwood
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Based on the characterization of the chemical composition of endodermal and hypodermal cell walls isolated from seven monocotyledonous and three dicotyledonous plant species, a model of the composition of apoplastic barriers in roots is proposed. Depending on the species, endodermal and hypodermal cell walls of roots contained varying amounts of the biopolymers suberin, lignin, cell wall proteins, and carbohydrates. Although analysis of the chemical composition of these apoplastic barriers of roots is now possible, it is pointed out that conclusions from these data concerning the functional properties of these cell walls can not easily be drawn. However, in analogy to suberized periderms it is argued that the suberin should play a role in establishing an apoplastic transport barrier in roots, albeit not a perfect barrier. Furthermore, due to the combined occurrence of suberin, lignin and cell wall proteins it is argued that endodermal and hypodermal cell walls also have an important function as barriers towards pathogens. Finally, it is pointed out that additional experimental approaches combining the investigation of transport properties and of the chemical composition of apoplastic transport barriers in roots are necessary before the function of endodermal and hypodermal cell walls in roots can be fully understood.
Suberin
Endodermis
Protoplast
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Citations (344)
The simultaneous occurrence of heat stress and drought is becoming more regular as a consequence of climate change, causing extensive agricultural losses. The application of either heat or osmotic stress increase cell-wall suberization in different tissues, which may play a role in improving plant resilience. In this work, we studied how the suberization process is affected by the combination of drought and heat stress by following the expression of suberin biosynthesis genes, cell-wall suberization and the chemical composition in Arabidopsis roots. The Arabidopsis plants used in this study were at the onset of secondary root development. At this point, one can observe a developmental gradient in the main root, with primary development closer to the root tip and secondary development, confirmed by the suberized phellem, closer to the shoot. Remarkably, we found a differential response depending on the root zone. The combination of drought and heat stress increased cell wall suberization in main root segments undergoing secondary development and in lateral roots (LRs), while the main root zone, at primary development stage, was not particularly affected. We also found differences in the overall chemical composition of the cell walls in both root zones in response to combined stress. The data gathered showed that, under combined drought and heat stress, Arabidopsis roots undergo differential cell wall remodeling depending on developmental stage, with modifications in the biosynthesis and/or assembly of major cell wall components.
Suberin
Osmotic shock
Osmosis
Secondary cell wall
Endodermis
Lateral root
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Plant cell walls are imperative to the normal growth and development of plants as they serve many functions, including protecting the protoplast and providing rigidity to the stem. In this study two poplar and Arabidopsis endoglucanases, which have been hypothesized to play a role in secondary cell wall development, were examined. The Class B endoglucanases, PtGH9B5 and AtGH9B5, are secreted enzymes that have a predicted GPI anchor, while the Class C
endoglucanases, PtGH9C2 and AtGH9C2, are also predicted to be secreted but contain a
carbohydrate-binding module (CBM). The poplar endoglucanases were up-regulated in
Arabidopsis using a 35S promoter as well as the Arabidopsis CesA8 promoter, respectively.
Additionally, Arabidopsis t-DNA insertion lines of each Arabidopsis gene were analyzed, and an
RNAi construct was created to down-regulate AtGH9C2 in Arabidopsis. All of the transgenic
plant lines were examined for changes in cell morphology and patterning, growth and
development, cell wall crystallinity, microfibril angle, and proportion of cell wall carbohydrates.
Mis-regulation of PtGH9B5/AtGH9B5 resulted in changes in glucose and xylose content,
suggesting that this endoglucanase may be involved in regulating the amount of cellulose
and/or xylans in the developing secondary cell wall. Furthermore, mis-regulation of
PtGH9C2/AtGH9C2 resulted in a change in crystallinity, which was inversely correlated with a change in plant height and rosette diameter. This suggests that this endoglucanase may be
involved in modifying cell wall crystallinity at the time of primary growth cessation and/or early
secondary cell wall development. Together, these results support the role of these endoglucanases in secondary cell wall development, though their exact enzymatic function remains to be discovered.
Secondary cell wall
Expansin
Microfibril
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Acyl lipids in Arabidopsis and all other plants have a myriad of diverse functions. These include providing the core diffusion barrier of the membranes that separates cells and subcellular organelles. This function alone involves more than 10 membrane lipid classes, including the phospholipids, galactolipids, and sphingolipids, and within each class the variations in acyl chain composition expand the number of structures to several hundred possible molecular species. Acyl lipids in the form of triacylglycerol account for 35% of the weight of Arabidopsis seeds and represent their major form of carbon and energy storage. A layer of cutin and cuticular waxes that restricts the loss of water and provides protection from invasions by pathogens and other stresses covers the entire aerial surface of Arabidopsis. Similar functions are provided by suberin and its associated waxes that are localized in roots, seed coats, and abscission zones and are produced in response to wounding. This chapter focuses on the metabolic pathways that are associated with the biosynthesis and degradation of the acyl lipids mentioned above. These pathways, enzymes, and genes are also presented in detail in an associated website (ARALIP: http://aralip.plantbiology.msu.edu/). Protocols and methods used for analysis of Arabidopsis lipids are provided. Finally, a detailed summary of the composition of Arabidopsis lipids is provided in three figures and 15 tables.
Cutin
Suberin
Galactolipids
Sphingolipid
Organelle
Membrane Lipids
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Citations (292)
Acyl lipids in Arabidopsis and all other plants have a myriad of diverse functions. These include providing the core diffusion barrier of the membranes that separates cells and subcellular organelles. This function alone involves more than 10 membrane lipid classes, including the phospholipids, galactolipids, and sphingolipids, and within each class the variations in acyl chain composition expand the number of structures to several hundred possible molecular species. Acyl lipids in the form of triacylglycerol account for 35% of the weight of Arabidopsis seeds and represent their major form of carbon and energy storage. A layer of cutin and cuticular waxes that restricts the loss of water and provides protection from invasions by pathogens and other stresses covers the entire aerial surface of Arabidopsis. Similar functions are provided by suberin and its associated waxes that are localized in roots, seed coats, and abscission zones and are produced in response to wounding. This chapter focuses on the metabolic pathways that are associated with the biosynthesis and degradation of the acyl lipids mentioned above. These pathways, enzymes, and genes are also presented in detail in an associated website (ARALIP: http://aralip.plantbiology.msu.edu/). Protocols and methods used for analysis of Arabidopsis lipids are provided. Finally, a detailed summary of the composition of Arabidopsis lipids is provided in three figures and 15 tables.
Cutin
Suberin
Galactolipids
Sphingolipid
Organelle
Membrane Lipids
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Citations (1,321)