Abstract
Mixed-linkage glucan (MLG) is a cell wall polysaccharide containing a backbone of unbranched (1,3)- and (1,4)-linked β-glucosyl residues. Based on its occurrence in plants and chemical characteristics, MLG has primarily been associated with the regulation of cell wall expansion due to its high and transient accumulation in young, expanding tissues. The Cellulose synthase-like F (CslF) subfamily of glycosyltransferases has previously been implicated in mediating the biosynthesis of this polymer. We confirmed that the rice (Oryza sativa) CslF6 gene mediates the biosynthesis of MLG by overexpressing it in Nicotiana benthamiana. Rice cslf6 knockout mutants show a slight decrease in height and stem diameter but otherwise grew normally during vegetative development. However, cslf6 mutants display a drastic decrease in MLG content (97% reduction in coleoptiles and virtually undetectable in other tissues). Immunodetection with an anti-MLG monoclonal antibody revealed that the coleoptiles and leaves retain trace amounts of MLG only in specific cell types such as sclerenchyma fibers. These results correlate with the absence of endogenous MLG synthase activity in mutant seedlings and 4-week-old sheaths. Mutant cell walls are weaker in mature stems but not seedlings, and more brittle in both stems and seedlings, compared to wild type. Mutants also display lesion mimic phenotypes in leaves, which correlates with enhanced defense-related gene expression and enhanced disease resistance. Taken together, our results underline a weaker role of MLG in cell expansion than previously thought, and highlight a structural role for MLG in nonexpanding, mature stem tissues in rice.
Highlights
Mixed-linkage glucan (MLG) is a cell wall polysaccharide containing a backbone of unbranched (1,3)- and (1,4)-linked b-glucosyl residues
As the strain is a measure of the brittleness of the material, these results show that MLG contributes to the flexibility properties of the cell wall in both seedlings and mature stems, and to cell wall strengthening in stems but not in seedlings
The same PR marker genes were upregulated in noninoculated, lesion mimic-free fully expanded leaves of younger plants prior to flowering in cslf6 mutant plants compared to wild type (Supplemental Fig. S13). These results indicate that loss of MLG activates defense responses and cell death, which correlates with enhanced disease resistance in cslf6 plants
Summary
Mixed-linkage glucan (MLG) is a cell wall polysaccharide containing a backbone of unbranched (1,3)- and (1,4)-linked b-glucosyl residues. MLG accumulates in cells that are not typically associated with growing tissues such as fibers and xylem vessels in the leaf (Trethewey and Harris, 2002; Doblin et al, 2009), which contain thick, lignified, secondary cell walls that provide structural and mechanical support Another important role of MLG is as a storage compound in barley (Hordeum vulgare), oats (Avena sativa), and Brachypodium distachyon grains where levels can represent as much as 70% to 80% of the endosperm cell wall (Fincher, 2009b; Guillon et al, 2011). These studies demonstrated the involvement of these gene families in MLG biosynthesis by overex-
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