Pseudomonas syringae pv. tomato DC3000 induces defense responses in diverse maize inbred lines

Many phytopathogens translocate virulence (effector) proteins into plant cells to circumvent host immune responses during infection. One such pathogen is Pseudomonas syringae pv. tomato DC3000, which secretes at least twenty-nine effectors into host cells, of which a subset elicits host defense responses in crop plant species. However, it is unknown whether P. syringae pv. tomato DC3000 activates immune responses in diverse maize inbreds. Here, we screened a diverse maize germplasm collection for effector-dependent recognition of this bacterial pathogen. As a control, we infiltrated Pseudomonas syringae DC3000(D36E), a derivative of P. syringae pv. tomato DC3000 that lacks all endogenous effectors. In our evaluations, we observed a variety of responses to P. syringae pv. tomato DC3000 in maize and scored the phenotypes as either no observable response (N) or as one of three responses: weak chlorosis (WC), chlorosis (C) with minimal cell death, and hypersensitive reaction (HR)-like cell death. Of the twenty-six maize inbreds screened, 13 were scored as N, 2 as WC, 2 as C, and 9 as HR-like cell death. Importantly, no maize line responded to P. syringae DC3000(D36E), demonstrating the responses observed are likely dependent upon recognition of one or more Pseudomonas effectors. Importantly, maize inbreds that recognize P. syringae pv. tomato DC3000 accumulated detectable hydrogen peroxide as well as an increase in transcript expression of a subset of maize defense genes. Collectively, our results will likely stimulate new research aimed at identifying the cognate maize disease resistance proteins that recognize the activities of one or more bacterial effectors.