MPK6 controls H2O2-induced root elongation by mediating Ca2+ influx across the plasma membrane of root cells in Arabidopsis seedlings

Mitogen-activated protein kinases (MPKs) play critical roles in signalling and growth, and Ca2+ and H2O2 control plant growth processes associated with abscisic acid (ABA). However, it remains unclear how MPKs are involved in H2O2- and Ca2+-mediated root elongation. Root elongation in seedlings of the loss-of-function mutant Atmpk6 (Arabidopsis thaliana MPK6) was less sensitive to moderate H2O2 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 H2O2 accumulation or the cytosolic Ca2+ increase were defective. Molecular and biochemical evidence showed that increased expression of the cell wall peroxidase PRX34 in Atmpk6 root cells enhanced apoplastic H2O2 generation; this promoted a cytosolic Ca2+ increase and Ca2+ influx across the plasma membrane. The plasma membrane damage caused by high levels of H2O2 was ameliorated in a Ca2+-dependent manner. These results suggested that there was intensified PRX34-mediated H2O2 generation in the apoplast and increased Ca2+ flux into the cytosol of Atmpk6 root cells; that is, the spatial separation of apoplastic H2O2 from cytosolic Ca2+ in root cells prevented H2O2-induced inhibition of root elongation in Atmpk6 seedlings.