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Blue light photoreceptors are required for the stability and function of a resistance protein mediating viral defense in Arabidopsis

This light-perciving ability of plants requires the activities of proteins termed photoreceptors. In addition to various growth and developmental processes, light also plays a role in plant defense against pathogens and is required for activation of several defense genes and regulation of the cell death response. However, the molecular or biochemical basis of light modulated regulation of defense signaling is largely unclear. We demonstrate a direct role for blue-light photoreceptors in resistance (R) protein-mediated plant defense against Turnip Crinkle Virus (TCV) in Arabidopsis. The blue-light photoreceptors, cryptochrome (CRY) 2 and phototropin (PHOT) 2, are specifically required for maintaining the stability of the R protein HRT, and thereby resistance to TCV. Exogenous application of the phytohormone salicylic acid elevates HRT levels in phot2 but not in cry2 background. These data indicate that CRY2 and PHOT2 function distinctly in maintaining post-transcriptional stability of HRT. HRT-mediated resistance is also dependent on CRY1 and PHOT1 proteins, but these do not contribute to the stability of HRT. HRT interacts with the CRY2/PHOT2-interacting protein COP1, a E3 ubiquitin ligase. Exogenous application of a proteasome inhibitor prevents blue-light-dependent degradation of HRT, suggesting that HRT is degraded via the 26S proteasome. These and the fact that PHOT2 interacts directly with the R protein RPS2 suggest that blue-light photoreceptors might be involved in regulation and/or signaling mediated by several R proteins.