Monodehydroascorbate reductase 2 and dehydroascorbate reductase 5 are crucial for a mutualistic interaction between Piriformospora indica and Arabidopsis

Ascorbate is a major antioxidant and radical scavenger in plants. Monodehydroascorbate reductase (MDAR) and dehydroascorbate reductase (DHAR) are two enzymes of the ascorbate–glutathione cycle that maintain ascorbate in its reduced state. MDAR2 (At3g09940) and DHAR5 (At1g19570) expression was upregulated in the roots and shoots of Arabidopsis seedlings co-cultivated with the root-colonizing endophytic fungus Piriformospora indica, or that were exposed to a cell wall extract or a culture filtrate from the fungus. Growth and seed production were not promoted by Piriformospora indica in mdar2 (SALK_0776335C) and dhar5 (SALK_029966C) T-DNA insertion lines, while colonized wild-type plants were larger and produced more seeds compared to the uncolonized controls. After 3 weeks of drought stress, growth and seed production were reduced in Piriformospora indica-colonized plants compared to the uncolonized control, and the roots of the drought-stressed insertion lines were colonized more heavily by the fungus than were wild-type plants. Upregulation of the message for the antimicrobial PDF1.2 protein in drought-stressed insertion lines indicated that MDAR2 and DHAR5 are crucial for producing sufficient ascorbate to maintain the interaction between Piriformospora indica and Arabidopsis in a mutualistic state.