CPK1‐HSP90 phosphorylation and effector XopC2–HSP90 interaction underpin the antagonism during cassava defense‐pathogen infection

Summary Cassava is one of the most important tropical crops, but it is seriously affected by cassava bacteria blight (CBB) caused by the bacterial pathogen Xanthomonas phaseoli pv manihotis ( Xam ). So far, how pathogen Xam infects and how host cassava defends during pathogen–host interaction remains elusive, restricting the prevention and control of CBB. Here, the illustration of HEAT SHOCK PROTEIN 90 kDa (MeHSP90.9) interacting proteins in both cassava and bacterial pathogen revealed the dual roles of MeHSP90.9 in cassava– Xam interaction. On the one hand, calmodulin‐domain protein kinase 1 (MeCPK1) directly interacted with MeHSP90.9 to promote its protein phosphorylation at serine 175 residue. The protein phosphorylation of MeHSP90.9 improved the transcriptional activation of MeHSP90.9 clients (SHI‐RELATED SEQUENCE 1 (MeSRS1) and MeWRKY20) to the downstream target genes ( avrPphB Susceptible 3 ( MePBS3 ) and N‐aceylserotonin O‐methyltransferase 2 ( MeASMT2 )) and immune responses. On the other hand, Xanthomonas outer protein C2 (XopC2) physically associated with MeHSP90.9 to inhibit its interaction with MeCPK1 and the corresponding protein phosphorylation by MeCPK1, so as to repress host immune responses and promote bacterial pathogen infection. In summary, these results provide new insights into genetic improvement of cassava disease resistance and extend our understanding of cassava–bacterial pathogen interaction.