MPK 6 controls H 2 O 2 ‐induced root elongation by mediating C a 2+ influx across the plasma membrane of root cells in A rabidopsis seedlings

Summary Mitogen‐activated protein kinases ( MPK s) play critical roles in signalling and growth, and C a 2+ and H 2 O 2 control plant growth processes associated with abscisic acid ( ABA ). However, it remains unclear how MPK s are involved in H 2 O 2 ‐ and C a 2+ ‐mediated root elongation. Root elongation in seedlings of the loss‐of‐function mutant Atmpk6 ( A rabidopsis thaliana MPK 6) was less sensitive to moderate H 2 O 2 or ABA than that in wild‐type ( WT ) plants. The enhanced elongation was a result of root cell expansion. This effect disappeared when ABA ‐induced H 2 O 2 accumulation or the cytosolic C a 2+ increase were defective. Molecular and biochemical evidence showed that increased expression of the cell wall peroxidase PRX 34 in Atmpk6 root cells enhanced apoplastic H 2 O 2 generation; this promoted a cytosolic C a 2+ increase and C a 2+ influx across the plasma membrane. The plasma membrane damage caused by high levels of H 2 O 2 was ameliorated in a C a 2+ ‐dependent manner. These results suggested that there was intensified PRX 34‐mediated H 2 O 2 generation in the apoplast and increased C a 2+ flux into the cytosol of Atmpk6 root cells; that is, the spatial separation of apoplastic H 2 O 2 from cytosolic C a 2+ in root cells prevented H 2 O 2 ‐induced inhibition of root elongation in Atmpk6 seedlings.