Differential Responses to the Combined Stress of Heat and Phytophthora capsici Infection Between Resistant and Susceptible Germplasms of Pepper (Capsicum annuum L.)

Stresses caused by heat and Phytophthora capsici infection reduce pepper (Capsicum annuum) yields; however, little is known about their combined effects on pepper. In this study, the combination of heat and P. capsici infection resulted in worse symptoms, including increased heat stress injury, cell death, relative electrical conductivity, and decreased root activity, than either individual stress. The symptoms were more obvious in the susceptible pepper cultivar ‘Early Calwonder’ (EC) than in the resistant landrace line ‘Criollo de Morelos 334’ (CM334). Under heat stress alone, the expression levels of three tested C. annuum heat-shock protein (CaHSP) genes were enhanced in both ‘CM334’ and ‘EC’; however, under the combined stress, their expression levels increased in ‘CM334,’ but decreased in ‘EC.’ Among four defense-related genes, under stress treatments, only two were up-regulated in ‘CM334’ and ‘EC,’ respectively, and both showed higher levels under the combined stress than under the individual stresses. Under the combined stress, the expression levels of three tested Ca2+-signaling pathway genes were enhanced in ‘EC,’ but inhibited in ‘CM334’ compared with either heat or P. capsici infection. Compared with the effects caused by either heat or P. capsici infection, the CaWRKY expression levels were restored to nearly normal levels under the combined stress, and the number of up-regulated genes was higher in ‘CM334’ than in ‘EC.’ Thus, the molecular responses of pepper to the combined stress of heat and P. capsici infection are different from those caused by individual stresses, and the stresses interaction is different between resistant and susceptible pepper germplasms.