RECEPTOR LIKE KINASE ACTIVITY MODULATES VIRAL INFECTION THROUGH PHOSPHORYLATION OF A CHLOROPLAST PROTEIN
An increasing number of chloroplast proteins have been found to interact with plant virus proteins. This is not surprising because these viruses cause various mosaic, mottles, and chlorosis symptoms on host leaves indicating damage to chloroplasts. A chloroplast protein, AtPsbP, was identified in a yeast two-hybrid screen as interacting with Alfalfa mosaic virus (AMV) coat protein (CP). AMV is a ssRNA virus with a wide host range including Arabidopsis. AtPsbP is an extrinsic subunit of photosystem II and with PsbQ is vital for water oxidation. We found that an RNAi knock-down of PsbP in Nicotiana tabacum, allowed increased replication of AMV and the development of quite severe disease symptoms in comparison to a wild-type N. tabacum. This suggested that PsbP plays an important role in plant resistance to AMV. PsbP, in addition to its role in photosynthesis, has been reported to interact with a wall-associated receptor kinase, WAK1, whereby it may affect plant defense signaling. We found that AtPsbP is a link between AtWAK1 and AMV CP at the plasma membrane. The formation of the AtWAK1-AtPsbP-AMV CP complex activated WAK1 kinase activity causing phosphorylation of PsbP and significant inhibition of AMV replication. We also found that the formation of the ternary complex induced the activation of the MAPK signal pathway. Analysis of the susceptibility of an Arabidopsis WAK1 knock-down indicated that WAK1, like PsbP, is critical for inhibiting AMV replication. Overall, we found a unique virus perception strategy, whereby a chloroplast protein (PsbP) interacts with a virus protein and then a Receptor-like kinase protein (WAK1) to transduce signals through the MAPK signaling pathway to activate defense responses.