Disease Interactions in a Shared Host Plant: Effects of Pre-Existing Viral Infection on Cucurbit Plant Defense Responses and Resistance to Bacterial Wilt Disease

Abstract

Both biotic and abiotic stressors can elicit broad-spectrum plant resistance against subsequent pathogen challenges. However, we currently have little understanding of how such effects influence broader aspects of disease ecology and epidemiology in natural environments where plants interact with multiple antagonists simultaneously. In previous work, we have shown that healthy wild gourd plants (Cucurbita pepo ssp. texana) contract a fatal bacterial wilt infection (caused by Erwinia tracheiphila) at significantly higher rates than plants infected with Zucchini yellow mosaic virus (ZYMV). We recently reported evidence that this pattern is explained, at least in part, by reduced visitation of ZYMV-infected plants by the cucumber beetle vectors of E. tracheiphila. Here we examine whether ZYMV-infection may also directly elicit plant resistance to subsequent E. tracheiphila infection. In laboratory studies, we assayed the induction of key phytohormones (SA and JA) in single and mixed infections of these pathogens, as well as in response to the feeding of A. vittatum cucumber beetles on healthy and infected plants. We also tracked the incidence and progression of wilt disease symptoms in plants with prior ZYMV infections. Our results indicate that ZYMV-infection slightly delays the progression of wilt symptoms, but does not significantly reduce E. tracheiphila infection success. This observation supports the hypothesis that reduced rates of wilt disease in ZYMV-infected plants reflect reduced visitation by beetle vectors. We also documented consistently strong SA responses to ZYMV infection, but limited responses to E. tracheiphila in the absence of ZYMV, suggesting that the latter pathogen may effectively evade or suppress plant defenses, although we observed no evidence of antagonistic cross-talk between SA and JA signaling pathways. We did, however, document effects of E. tracheiphila on induced responses to herbivory that may influence host-plant quality for (and hence pathogen acquisition by) cucumber beetles.

Plants, viruses and the environment: Ecology and mutualism

“…In wild gourds, the bacterial pathogen Erwinia trachiephila causes wilt disease, and induces the production of volatiles that attract the beetle vector to the plant and induce movement to healthy plants. When these plants are infected with ZYMV, the beetle-attracting volatiles are reduced protecting the plants from the bacterial pathogen (Shapiro et al, 2012(Shapiro et al, , 2013. Whether or not these benefits outweigh the costs varies, and depends on numerous environmental factors (Bao and Roossinck, 2013).…”

Move Over, Bacteria! Viruses Make Their Mark as Mutualistic Microbial Symbionts

“…Zucchini yellow mosaic virus infects wild gourds and reduces the production of volatile compounds that attract beetles to the plants. The beetles are vectors of a bacterial wilt pathogen, so the virus reduces transmission of the wilt bacteria (16). These relationships are never black and white; there are costs and benefits, and the players must find a balance.…”

An introduced crop plant is driving diversification of the virulent bacterial pathogenErwinia tracheiphila

“…E. tracheiphila contains an hrp T3SS locus, and E. tracheiphila suppresses salicylic acid production in a wild gourd host ( Cucurbita pepo ssp. texana ) (20, 44), suggesting that E. tracheiphila may use effectors for suppressing plant induced defenses during disease development.…”