Oxalate Mediates Fungal Infection By Affecting Stomatal Function
Posted on: Saturday, 11 December 2004, 03:00 CST
Several phytopathogenic fungi, including Sclerotinia sclerotiorum, produce millimolar concentrations of oxalate in infected tissues. Oxalate is an essential virulence factor of S. sclerotiorum because mutants that are deficient in oxalate biosynthesis are less pathogenic than wild-type fungus. Moreover, enzymes that catabolize oxalate protect plants from Sclerotinia infection when their genes are expressed in stably transformed plants. The detailed mechanisms by which oxalic acid affects host cells and tissues, however, are not understood. In this issue, Guimares and Stotz (pp. 3703-3711) examine the question of whether oxalate causes foliar dehydration following infection by disturbing guard cell function. The stomatal pores of Vicia faba leaves infected with S. sderotiorum open at night. This cellular response appears to be dependent on oxalic acid, because stomatal pores are partially closed when leaves are infected with an oxalate-deficient mutant of S. sclerotionim. The authors present evidence that the abnormal opening of stomatal pores may play a role in allowing S. sderotiorum to emerge through open stomata from the uninfected abaxial leaf surface for secondary colonization. Consistent with a role for oxalate in this process, the exogenous application of oxalic acid to the detached abaxial epidermis of V. faba leaves induces stomatal opening, apparently by causing potassium accumulation and starch breakdown in the guard cells. Oxalate also interferes with abscisic acid (ABA)-induced stomatal closure. A number of ABA-insensitive Arabidopsis mutants were found to be more susceptible to oxalate-deficient S. sclerotiorum than are wild-type plants, suggesting that ScIerotinia resistance is dependent on ABA. Thus, oxalate influences stomatal function by causing the accumulation of osmotically active molecules in guard cells and by inhibiting ABA-induced stomatal closure.
Figure 1. The fluorescent indicator yellow cameleon reveals dynamic changes in |Ca^sup 2+^]^sub cyt^ during pollen germination and elongation. These dynamic |Ca^sup 2+^]^sub cyt^ changes are affected by the presence of the papilla. Copyright American Society of Plant Physiologists Nov 2004
Source: Plant Physiology
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