Abstract
The root growth of most crop plants is inhibited by soil salinity. Roots respond by modulating metabolism, gene expression and protein activity, which results in changes in cell wall composition, transport processes, cell size and shape, and root architecture. Here, we focus on the effects of salt stress on cell wall modifying enzymes, cellulose microfibril orientation and non-cellulosic polysaccharide deposition in root elongation zones, as important determinants of inhibition of root elongation, and highlight cell wall changes linked to tolerance to salt stressed and water limited roots. Salt stress induces changes in the wall composition of specific root cell types, including the increased deposition of lignin and suberin in endodermal and exodermal cells. These changes can benefit the plant by preventing water loss and altering ion transport pathways. We suggest that binding of Na+ ions to cell wall components might influence the passage of Na+ and that Na+ can influence the binding of other ions and hinder the function of pectin during cell growth. Naturally occurring differences in cell wall structure may provide new resources for breeding crops that are more salt tolerant.
Highlights
Plant evolution has resulted in a large array of mechanisms to tolerate the stresses associated with increased soil salinity
Salinity causes transient alkalinisation of the apoplast, and this could limit growth in the context of the acid growth theory [4,5]: Auxin activates plasma membrane H+-ATPases and protons are extruded into the apoplast, apoplastic acidification induces cell wall loosening by activating expansins and other remodelling enzymes resulting in loosening of the cell wall
Growth could be limited by a decrease in free apoplastic protons causing a shift in the apoplastic pH away from the range that favors cell-wall loosening [4], in maize the inhibition of growth as a result of salinity was not associated with the capacity of the epidermal cells to acidify their walls [6]
Summary
Plant evolution has resulted in a large array of mechanisms to tolerate the stresses associated with increased soil salinity. RNA-seq analysis in bermudagrass genotypes differing in salt tolerance identified candidate genes encoding transcription factors involved in the regulation of lignin synthesis, reactive oxygen species (ROS) homeostasis controlled by peroxidases, and the regulation of phytohormone signalling that promotes cell wall loosening, and root growth under salinity [41]. These observations indicate that crop plants and their wild relatives are likely to have genetic variation in expansin, XTH and peroxidase activity. Genetic variation in root expansins, glucosylase, hydrolase and peroxidase activity could be used to develop segregating plant populations for testing the roles of these enzymes in ROS control, water transport and salt tolerance
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