Root Suberin Plays Important Roles in Reducing Water Loss and Sodium Uptake in Arabidopsis thaliana

Suberin is a cell-wall-associated hetero-polymer deposited in specific plant tissues. The precise role of its composition and lamellae structure in protecting plants against abiotic stresses is unclear. In Arabidopsis thaliana, we tested the biochemical and physiological responses to water deficiency and NaCl treatment in mutants that are differentially affected in suberin composition and lamellae structure. Chronic drought stress increased suberin and suberin-associated waxes in wild-type plants. Suberin-deficient mutants were not more susceptible than the wild-type to the chronic drought stress imposed in this study. Nonetheless, the cyp86a1-1 cyp86b1-1 mutant, which had a severely altered suberin composition and lamellae structure, exhibited increased water loss through the root periderm. Cyp86a1-1 cyp86b1-1 also recorded lower relative water content in leaves. The abcg2-1 abcg6-1 abcg20-1 mutant, which has altered suberin composition and lamellae, was very sensitive to NaCl treatment. Furthermore, cyp86a1-1 cyp86b1-1 recorded a significant drop in the leaf K/Na ratio, indicating salt sensitivity. The far1-2 far4-1 far5-1 mutant, which did not show structural defects in the suberin lamellae, had similar responses to drought and NaCl treatments as the wild-type. Our results provide evidence that the suberin amount and lamellae structure are key features in the barrier function of suberin in reducing water loss and reducing sodium uptake through roots for better performance under drought and salt stresses.

Bundle sheath suberisation is required for C4 photosynthesis in a Setaria viridis mutant

C4 photosynthesis provides an effective solution for overcoming the catalytic inefficiency of Rubisco. The pathway is characterised by a biochemical CO2 concentrating mechanism that operates across mesophyll and bundle sheath (BS) cells and relies on a gas tight BS compartment. A screen of a mutant population of Setaria viridis, an NADP-malic enzyme type C4 monocot, generated using N-nitroso-N-methylurea identified a mutant with an amino acid change in the gene coding region of the ABCG transporter, a step in the suberin synthesis pathway. Here, Nile red staining, TEM, and GC/MS confirmed the alteration in suberin deposition in the BS cell wall of the mutant. We show that this has disrupted the suberin lamellae of BS cell wall and increased BS conductance to CO2 diffusion more than two-fold in the mutant. Consequently, BS CO2 partial pressure is reduced and CO2 assimilation was impaired in the mutant. Our findings provide experimental evidence that a functional suberin lamellae is an essential anatomical feature for efficient C4 photosynthesis in NADP-ME plants like S. viridis and have implications for engineering strategies to ensure future food security.

Extracellular membrane tubules involved in suberin deposition in plant cell walls

Suberin is a fundamental plant biopolymer, found in protective tissues, such as seed coats, exodermis and endodermis of roots, the outer layers of stems and roots with secondary growth, as well as in wound-induced tissues. Its presence allows organs to resist various environmental stresses, such as pathogen attack, drought or excessive salt concentrations. Suberin is a mostly aliphatic polyester of long-chain fatty acids and alcohols, often co-occurring with lignin-like polymers in the same cells. Most suberizing cells appear to deposit suberin in the form of lamellae just outside of the plasma membrane, below the primary cell wall. The monomeric precursors of suberin are thought to be glycerated fatty acids, synthesized at the endoplasmic reticulum. However, it has remained obscure how these monomers are transported outside of the cell, where they will be polymerized to form suberin lamellae. Here, we demonstrate that extracellular vesicular-tubular structures accumulate specifically in suberizing cells. By employing various, independent mutational and hormonal challenges, known to affect suberization in distinct ways, we demonstrate that their presence correlates perfectly with root suberization. Surprisingly, no endosomal compartment marker showed any conspicuous changes upon induction of suberization, suggesting that this compartment might not derive from endosomal multi-vesicular bodies, but possibly form directly from endoplasmic reticulum subdomains. Consistent with this, we could block formation of both, suberin deposition and vesicle accumulation by a pharmacogenetic manipulation affecting early steps in the secretory pathway. Whereas many previous reports have described extracellular vesicle occurrence in the context of biotic interactions, our results suggest a developmental role for extracellular vesicles in suberin formation.One Sentence SummarySuberin lamellae formation is associated with extracellular membrane tubules.

Morphological structures and histochemistry of roots and shoots in Myricaria laxiflora (Tamaricaceae)

Myricaria laxiflora (Tamaricaceae) is an endangered plant that is narrowly distributed in the riparian zone of the Three Gorges, along the Yangtze River, China. Using bright-field and epifluorescence microscopy, we investigated the anatomical and histochemical features that allow this species to tolerate both submerged and terrestrial environments. The adventitious roots of Myr. laxiflora had an endodermis with Casparian bands and suberin lamellae; the cortex and hypodermal walls had lignified thickenings in the primary structure. In the mature roots, the secondary structure had cork. The apoplastic barriers in stems consisted of a lignified fiber ring and a cuticle at the young stage and cork at the mature stage. The leaves had two layers of palisade tissue, a hyaline epidermis, sunken stomata, and a thick, papillose cuticle. Aerenchyma presented in the roots and shoots. Several Myr. laxiflora structures, including aerenchyma, apoplastic barriers in the roots and shoots, were adapted to riparian habitats. In addition, shoots had typical xerophyte features, including small leaves, bilayer palisade tissues, sunken stomata, a thick, papillose cuticle, and a hyaline epidermis. Thus, our study identified several anatomical features that may permit Myr. laxiflora to thrive in the riparian zone of the Three Gorges, China.

Leaf cell wall properties and stomatal density influence oxygen isotope enrichment of leaf water

Oxygen isotopic composition (Δ18OLW) of leaf water can help improve our understanding of how anatomy interacts with physiology to influence leaf water transport. Leaf water isotope models of Δ18OLW such as the Péclet effect model have been developed to predict Δ18OLW, and it incorporates transpiration rate (E) and the mixing length between unenriched xylem water and enriched mesophyll water, which can occur in the mesophyll (Lm) or veins (Lv). Here we used two cell wall composition mutants grown under two light intensities and RH to evaluate the effect of cell wall composition on Δ18OLW. In maize (Zea mays), the compromised ultrastructure of the suberin lamellae in the bundle sheath of the ALIPHATIC SUBERIN FERULOYL TRANSFERASE mutant (Zmasft) reduced barriers to apoplastic water movement, resulting in higher E and Lv and, consequently, lower Δ18OLW. In cellulose synthase-like F6 (Cslf6) mutants and wildtype of rice (Oryza sativa), the difference in Δ18OLW in plants grown under high and low growth light intensity co-varied with their differences in stomatal density. These results show that cell wall composition and stomatal density influence Δ18OLW by altering the Péclet effect and that stable isotopes can facilitate the development of a physiologically and anatomically explicit water transport model.

The maize Aliphatic Suberin Feruloyl Transferase genes affect leaf water movement but are dispensable for bundle sheath CO2 concentration

ABSTRACTC4 grasses often outperform C3 species under hot, arid conditions due to superior water and nitrogen use efficiencies and lower rates of photorespiration. A method of concentrating CO2 around the site of carbon fixation in the bundle sheath (BS) is required to realize these gains. In NADP-malic enzyme (NADP-ME)-type C4 grasses such as maize, suberin deposition in the BS cell wall is hypothesized to act as a diffusion barrier to CO2 escape and O2 entry from surrounding mesophyll cells. Suberin is a heteropolyester comprised of acyl-lipid-derived aliphatic and phenylpropanoid-derived aromatic components. To disrupt BS suberization, we mutated two paralogously duplicated, unlinked maize orthologues of Arabidopsis thaliana ALIPHATIC SUBERIN FERULOYL TRANSFERASE, ZmAsft1 and ZmAsft2, using closely linked Dissociation transposons. Loss-of-function double mutants revealed a 97% reduction in suberin-specific omega-hydroxy fatty acids without a stoichiometric decrease in ferulic acid. However, BS suberin lamellae were deficient in electron opaque material, and cohesion between the suberin lamellae and polysaccharide cell walls was attenuated in double mutants. There were no other morphological phenotypes under ambient conditions. Furthermore, there was no significant effect on net CO2 assimilation at any intercellular CO2 concentration, and no effect on 13C isotope discrimination relative to wild type. Thus, ZmAsft expression is not required to establish a functional CO2 concentrating mechanism in in maize. Double mutant leaves exhibit elevated cell wall elasticity, transpirational, and stomatal conductance relative to WT. Thus, the ZmAsft genes are dispensable for gas exchange barrier function but may be involved in regulation of leaf water movement.One-sentence SummaryDouble mutants of two paralogously duplicated maize Aliphatic Suberin Feruloyl Transferase (ZmAsft) genes exhibit reduced aliphatic suberin content, cell wall cohesion defects, and elevated leaf transpiration, but no changes in CO2 assimilation relative to wild type.

GDSL-domain containing proteins mediate suberin biosynthesis and degradation, enabling developmental plasticity of the endodermis during lateral root emergence

ABSTRACTRoots anchor plants and deliver water and nutrients from the soil. The root endodermis provides the crucial extracellular diffusion barrier by setting up a supracellular network of lignified cell walls, called Casparian strips, supported by a subsequent formation of suberin lamellae. Whereas lignification is thought to be irreversible, formation of suberin lamellae was demonstrated to be dynamic, facilitating adaptation to different soil conditions. Plants shape their root system through the regulated formation of lateral roots emerging from within the endodermis, requiring local breaking and re-sealing of the endodermal diffusion barriers. Here, we show that differentiated endodermal cells have a distinct auxin-mediated transcriptional response that regulates cell wall remodelling. Based on this data set we identify a set of GDSL-lipases that are essential for suberin formation. Moreover, we find that another set of GDSL-lipases mediates suberin degradation, which enables the developmental plasticity of the endodermis required for normal lateral root emergence.

VviMYB41 orthologs contribute to the water deficit induced suberization of grapevine fine roots

ABSTRACTThe permeability of roots to water and nutrients is controlled through a variety of mechanisms and one of the most conspicuous is the presence of structures such as the Casparian strips and suberin lamellae. Roots actively regulate the creation of these structures developmentally, along the length of the root, and in response to the environment, including abiotic stresses such as drought. In the current study, we characterized the suberin composition along the length of grapevine fine roots during development and in response to water deficit. In parallel samples we quantified changes in expression of suberin biosynthesis- and deposition-related gene families (via RNAseq) allowing the identification of drought-responsive suberin-related genes. Grapevine suberin composition did not differ between primary and lateral roots, and was similar to that of other species. Under water deficit there was a global upregulation of suberin biosynthesis which resulted in an increase of suberin specific monomers, but without changes in their relative abundances, and this upregulation took place across all the developmental stages of fine roots. These changes corresponded to the upregulation of numerous suberin biosynthesis- and deposition-related genes which included orthologs of the previously characterized AtMYB41 transcriptional factor. Functional validation of two grapevine MYB41 orthologs, VviMYB41 and VviMYB41-like, confirmed their ability to globally upregulate suberin biosynthesis and deposition. This study provides a detailed characterization of the developmental and water deficit induced suberization of grapevine fine roots and identifies important orthologs responsible for suberin biosynthesis, deposition, and its regulation in grape.One sentence summaryOur study details the biochemical changes and molecular regulation of how grapevines decrease their root permeability during drought.

Suberized transport barriers in plant roots: the effect of silicon

Plant roots are the major organs that take up water and dissolved nutrients. It has been widely shown that apoplastic barriers such as Casparian bands and suberin lamellae in the endo- and exodermis of roots have an important effect on regulating radial water and nutrient transport. Furthermore, it has been described that silicon can promote plant growth and survival under different conditions. However, the potential effects of silicon on the formation and structure of apoplastic barriers are controversial. A delayed as well as an enhanced suberization of root apoplastic barriers with silicon has been described in the literature. Here we review the effects of silicon on the formation of suberized apoplastic barriers in roots, and present results of the effect of silicon treatment on the formation of endodermal suberized barriers on barley seminal roots under control conditions and when exposed to osmotic stress. Chemical analysis confirmed that osmotic stress enhanced barley root suberization. While a supplementation with silicon in both, control conditions and osmotic stress, did not enhanced barley root suberization. These results suggest that enhanced stress tolerance of plants after silicon treatment is due to other responses.