Plant Responses to Combined Drought and Pathogen Infection: Current Understanding on the Role of Phytohormones

Abstract

Plants under natural conditions encounter a number of abiotic and biotic stresses often being inflicted simultaneously. Plant responses to a stress are governed by intricate network of the hormone signaling pathways. Abscisic acid (ABA) forms the major component of the plant response to drought and cold stress. Salicylic acid (SA), jasmonic acid (JA), and ethylene act as key regulators of plant response to pathogen infection. In fact, the extensive cross talk among the different hormone-mediated signaling pathways determines plant response to a particular stress. A large number of studies focus on hormone signaling under individual drought and pathogen stresses and the cross talk between the two stress responses. However, owing to the relatively few studies on combined drought and pathogen stresses, our understanding of phytohormonal signaling under combined stress is still obscure. Recent studies on combined drought and pathogen infection indicate that plants when simultaneously exposed to the two stresses often exhibit a transcriptional and metabolic response different from that exhibited under single stress conditions. This is also applicable to the phytohormonal signaling. The nature, time, and severity of the two stresses in combination modulate hormonal concentrations as well as the hormone signal transduction pathways involved. In this chapter, we provide a compendious description of the role of the three major hormones, namely, ABA, SA, and JA, in combined drought and pathogen infection. A brief description of the role of auxins, cytokinins, and gibberellins has also been provided. Taking leads from few studies, we have discussed the potential role of hormones in conferring combined drought and pathogen stress tolerance to plants. We also briefly discussed the effect of different “stress elicitors” on hormone signaling.