Calcium-dependent protein kinases play an essential role in a plant defence response

Abstract

Calcium‐dependent protein kinases (CDPKs) comprise a large family of serine/threonine kinases in plants and protozoans. We isolated two related CDPK cDNAs (NtCDPK2 and NtCDPK3) from Nicotiana tabacum. These CDPK transcripts are elevated after race‐specific defence elicitation and hypo‐osmotic stress. Transiently expressed myc‐epitope‐tagged NtCDPK2 in Nicotiana benthamiana and N.tabacum leaves showed a rapid transient interconversion to an activated form after elicitation and hypo‐osmotic stress. The Avr9 race‐specific elicitor caused a more pronounced and sustained response. This transition is due to phosphorylation of the CDPK. Immuno complex kinase assays with epitope‐tagged NtCDPK2 showed that stress‐induced phosphorylation and interconversion of NtCDPK2 correlates with an increase in enzymatic activity. The function of NtCDPK2 in plant defence was investigated by employing virus‐induced gene silencing (VIGS) in N.benthamiana. CDPK‐silenced plants showed a reduced and delayed hypersensitive response after race‐specific elicitation in a gene‐for‐gene interaction, and lacked an accompanying wilting phenotype. Silencing correlated with loss of CDPK mRNA, whereas mRNA accumulation of mitogen‐activated protein kinase WIPK remained unaltered.