The cobra mutation leads to stunted roots with an expanded epidermal layer resembling a cobra head. The corresponding protein, COBRA, is a glycosylphosphatidylinositol-anchored protein that extends its cellulose-binding domain into the apoplastic space and is required for the oriented deposition of cellulose microfibrils. In roots, the orientation of these microfibrils normally prevents radial expansion but allows longitudinal expansion. When COBRA localized in root cells is defective, aberrant orientation of cellulose microfibrils allows radial expansion of cells, leading to the cobra phenotype. Arabidopsis has twelve COBL paralogs that may play similar roles in other tissues. For stomatal guard cells in particular, the differential orientation of cellulose microfibrils is inherently important for stomatal function, allowing outward bending and thus stomatal opening when guard cells are turgid.
The researchers performed a mutant screen for aberrant stomatal morphology and identified a mutant with asymmetrical or lacking stomatal pores. This mutant was found to be defective for COBL7, expression of which was confirmed to be confined to guard cells. A close paralog, COBL8, was found to be not essential for stomatal pore formation, although cobl7 cobl8 double mutants show phenotypic additivity. COBL7 was found to be localized in the cell plate of dividing guard mother cells and later the ventral cell walls of guard cells. Surprisingly, COBL7 overexpressor lines show similar defects of stomatal pore formation and function as the mutants, suggesting that a tight balance of COBL7 is required for the correct positioning of cellulose microfibrils. In line with this hypothesis, both cobl7 mutants and overexpressors have reduced cellulose content, especially in ventral cell walls of guard cells, and defective cellulose microfibril anisotropy. Hence, COBL7 constitutes another relevant player in stomatal differentiation and an interesting target for further investigations.
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