Tomato bHLH132 Transcription Factor Controls Growth and Defense and Is Activated by Effector XopD During Pathogenesis.

Effector-dependent manipulation of host transcription is a key virulence mechanism used by species causing bacterial spot disease in tomato and pepper. Transcription activator-like (TAL) effectors employ novel DNA-binding domains to directly activate host transcription, whereas the non-TAL effector XopD uses a small ubiquitin-like modifier (SUMO) protease activity to represses host transcription. The targets of TAL and non-TAL effectors provide insight to the genes governing susceptibility and resistance during infection. In this study, we investigated the extent to which the non-TAL effector strain Xe85-10 activates tomato transcription to gain new insight to the transcriptional circuits and virulence mechanisms associated with pathogenesis. Using transcriptional profiling, we identified a putative basic helix-loop-helix (bHLH) transcription factor, bHLH132, as a pathogen-responsive gene that is moderately induced by microbe-associated molecular patterns and defense hormones and is highly induced by XopD during infection. We also found that activation of transcription requires the XopD SUMO protease activity. Silencing mRNA expression results in stunted tomato plants with enhanced susceptibility to infection. Our work suggests that bHLH132 is required for normal vegetative growth and development as well as resistance to It also suggests new transcription-based models describing XopD virulence and recognition in tomato.