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

Calcium-dependent protein kinases (CDPKs) comprise a large family of serine /threonine kinases in plants and protozoans. We isolated two related CDPK c DNAs (NtCDPK2 and NtCDPK3) from Nicotiana tabacum. These CDPK transcripts a re 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 activ ated 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. Immunocomplex kinase assays with epi tope-tagged NtCDPK2 showed that stress-induced phosphorylation and intercon version of NtCDPK2 correlates with an increase in enzymatic activity. The f unction of NtCDPK2 in plant defence was investigated by employing virus-ind uced gene silencing (VIGS) in N. benthamiana. CDPK-silenced plants showed a reduced and delayed hypersensitive response after race-specific elicitatio n in a gene-for-gene interaction, and lacked an accompanying wilting phenot ype. Silencing correlated with loss of CDPK mRNA, whereas mRNA accumulation of mitogen-activated protein kinase WIPK remained unaltered.