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The Role of Plant Cell Wall Arabinose in Salt Stress Sensing and Adaptation
thesisposted on 10.06.2019 by Omar Mohamed Zayed
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Plant cell wall is critical for the regulation of cell shape, cell growth, and responses to abiotic stress and pathogen infection. The plant cell wall is composed of several monosaccharides including glucose, galactose, mannose, xylose, fucose, rhamnose, and arabinose. Arabinose-containing polymers account for ~20 % of the total cell wall saccharides in rice and Arabidopsis. Arabinose is a plant-specific monosaccharide that is required for the decoration of several cell wall polysaccharides, including rhamnogalacturonan I (RGI)-arabinan, arabinoxylan, and rhamnogalacturonan II (RGII). Arabinose is also involved in the modification of some cell wall glycoproteins, including arabinogalactan-proteins (AGPs), extensins, and leucine-rich repeat extensin (LRX) proteins. In addition, arabinose is conjugated to signaling peptides like CLAVATA3 and some cytoplasmic arabinosylated flavonols, such as quercetin 3-O-l-arabinoside and myricetin. The only known enzyme in the final step of the arabinose de novo biosynthesis pathway is the Golgi-localized UDP-D-xylose 4-epimerase (MUR4), which converts UDP-xylose to UDP-arabinose. There is a 50% reduction of cell wall arabinose in mur4 mutant, indicating that other enzymes may also be involved in the de novo biosynthesis pathway. Under salt stress, mur4 mutant plants exhibit reduced root elongation and abnormal cell-cell adhesion. The roles of three MUR4 paralogs, MURL, DUR, and MEE25, in arabinose biosynthesis and salt stress tolerance are described. Data are also shown regarding the importance of AGPs in salt tolerance. Analysis of higher order mutants of mur4 with its three paralogs reveals that the three proteins also contribute to the biosynthesis of UDP-Ara and are critical for root elongation. The salt-hypersensitivity of the mur4 mutant is rescued by exogenous arabinose or gum Arabic (a commercial AGP product). Taken together, my work reveals the importance of arabinose metabolism in salt stress tolerance and provides new insights into the enzymes involved in UDP-Ara biosynthesis in plants. Plants have evolved cell-wall integrity sensing and signaling pathways to maintain cell-wall homeostasis in response to stress conditions, but the cellular components involved in the perception and transduction of cell-wall signals are largely unknown. I found that the cell wall-localized leucine-rich repeat extensins (LRX) 3/4/5 interact with RAPID ALKALINIZATION FACTOR (RALF) peptides RALF22/23 to transduce cell wall signals. Mature RALF22/23 peptides convey signals to the plasma membrane-localized FERONIA (FER) to induce intracellular stress responses. The lrx345 and fer mutants and RALF22/23 overexpressing transgenic plants display similar phenotypes, including retarded growth and increased sensitivity to salt stress. These results suggest that LRX3/4/5, RALF22/23, and FER function as a module to regulate plant growth and salt stress tolerance. Further analyses show that the LRXs-RALF-FER module negatively regulates the accumulation of the phytohormones jasmonic acid (JA), salicylic acid (SA), and abscisic acid (ABA), and the simultaneous over-accumulation of these stress hormones can be detrimental to plants. Based on genetic and biochemical data, we propose that salt-induced perturbations of the cell wall may be sensed by the LRXs, triggering the release of RALF peptides in the extracellular space; these peptides are then perceived by FER, inducing its internalization and down-regulating its function as part of a homeostatic mechanism to halt growth and to acclimate to salt stress through the activation of ABA, JA and SA signaling. Taken together, my work offers valuable insights into how salt stress is sensed in the apoplast by the LRXs-RALFs-FER signaling module, which subsequently modulates hormone signaling to establish a homeostatic mechanism coordinating growth and stress responses. In brief, my study contributes to the understanding of the role of MUR4 family of enzymes in plant arabinose biosynthesis and the role of arabinose-containing macromolecules in salt stress sensing and adaptation.