FIGURE 1.

Schematic representation of effect of stress combination on plants. (A) Effect of combined stresses on plants is explained by representative examples of heat and drought (abiotic–abiotic stress) and drought and pathogen stress (abiotic–biotic stress) combination. (i) Depending on the nature of stresses, the two stresses can either not interact physically, but individually affect the plant leading to a net negative impact on plant growth or interact at plant interface and cause a net effect on the plant. Generally, abiotic stress combinations are examples of “only net effects and no stress interactions”. For example, simultaneous exposure to heat and salinity leads to enhanced retardation of physiological processes such as photosynthesis. (ii) Stress interactions are conspicuous in abiotic and biotic stress combinations wherein one stress factor affects the other stress factor per se. For example, exposure to combined drought and pathogen stress may result in a complex scenario encompassing an interaction of the two stresses along with the impact of the two stresses on the plant. Depending on the plant patho-system, the interaction may lead to enhanced or reduced susceptibility to a particular pathogen. Some pathogens also modulate drought tolerance of the plant. (B) Effect of multiple individual stresses (sequential stresses) on plants. Sequential stresses may either lead to priming or predisposition of plants to the subsequent stress as explained by examples of heat–pathogen and drought–pathogen stress combinations. (i) Priming: Exposure of plants to moderate heat stress (indicated by red arrow) may prime the plants to the subsequent pathogen infection. Mild stress can evoke stress memory in the form of epigenetic changes or transcriptomic changes in plants which may last short or long-term, leading to enhanced tolerance of stress to subsequent more severe stresses (same or different stress). (ii) Predisposition: A pre-occurring drought stress can pre-dispose plants to pathogen infection due to weakened plant defenses or any other metabolic changes occurring due to the drought stress. 1, Mittler, 2006; 2, Ahmed et al., 2013; 3, Gupta et al., 2016; 4, Sharma and Pande, 2013; 5, Xu et al., 2008; 6, Crisp et al., 2016; 7, Mayek-Perez et al., 2002.