EXPLORING THE MOLECULAR MECHANISM OF ROOT-MEDIATED RESPONSES TO RALSTONIA

Bacterial Wilt, caused by Ralstonia solanacearum, is among the most devastating plant diseases in the world. This pathogen causes significant loss in crops such as tobacco, potato, and tomato. R. solanacearum root infection and xylem colonization determine disease outcome. To date, little is known about the defense mechanism mediated by roots to prevent R. solanacearum vascular colonization during the initial infection stages. Plant early responses are important since they may impact disease outcomes. Here, we report the formation of root hairs and primary root growth inhibition in tomato seedlings as Ralstonia-induced phenotypes that depend on tomato genotype and Ralstonia species. The Ralstonia-induced root phenotypes are independent of a functional type III secretion system and exopolysaccharide production (EPS). We also found that R. solanacearum K60 infection increased auxin levels throughout the root meristem in wilt-susceptible tomato roots. Our data suggest proper auxin signaling and transport are important for susceptibility to R. solanacearum K60. Blocking auxin transport pharmacologically or genetically led to fewer wilting symptoms, suggesting that auxin is important during early infection stages and disease outcomes in tomato. We previously found that a tomato mutant defective in auxin transport and signaling, known as diageotropica (dgt), has enhanced resistance to R. solanacearum K60. We characterized the resistant response in the dgt mutant, and we found that the resistant response in the dgt mutant may be due to increased lignin content preventing pathogen vasculature colonization. DGT encodes a cyclophilin protein that regulates auxin transport and signaling. Mutations in the cyclophilin DGT promote resistance to R. solanacearum K60. DGT has been reported to regulate auxin transport and signaling. However, the molecular mechanism regarding how DGT mediates these processes is still unknown. We used Yeast Two-Hybrid to identify candidate protein interactors, and we found that SlbZIP1/SlbZIP29, Sl14-3-3, and SlMYB110 may interact with DGT to regulate both development and defense responses. Understanding the role of DGT, auxin, and lignin in defense responses to R. solanacearum K60 in tomato is necessary for Solanaceae crop improvement.