4.2 Complex biotic and abiotic stress interactions and feedbacks
In addition to trade-offs between growth and defense, increasing tolerance to one stress may be at the expense of tolerance to another stress (Rizhsky et al., 2004; Mittler, 2006; Atkinson & Urwin, 2012). For example, heat stress causes stomata to open, but having open stomata could lead to more water loss, which would be detrimental under drought conditions. Such interaction of stress factors is said to occur when the presence of the initial stress leads to an acclimation response which alters the plant’s normal response when subjected to a second stress. Recent reviews looking at the interactive effects of two or more stresses have found outcomes of these combined stresses can be positive, negative, or neutral depending on timing, nature, and severity of each stress (Mittler & Blumwald, 2010; Suzuki et al., 2014; Rivero et al., 2022). Additionally, the presence of abiotic stress has been found to enhance host susceptibility towards some pathogens, or reduce susceptibility to some pathogens, thus, the effect of multiple stresses is not simply additive. Thus, challenging growth conditions associated with climate change necessitates breeding programs need to evaluate durability of resistant cultivars in presence of abiotic stresses.
Abiotic stress can also have a positive impact on the outcome of pathogen stress. In barley, increasing salt-induced osmotic stress directly correlates with resistance to powdery mildew (Blumeria graminis f. sp. hordei race A6) (Wiese et al. 2004), and drought stress can enhance resistance to B. cinere a in tomatoes (Achuo et al. 2006). Additionally, pathogens interfere with water relations during pathogenesis by inducing stomatal closure to reduce water loss from infected tissue, which can have a positive effect on plant tolerance against abiotic stress conditions (Goel et al. 2008; Beattie, 2011). Drought-stressed tomato leaves accumulate high levels of defense compounds that reduce the herbivore Spodoptera exigua ’s ability to feed (English-Loeb et al. 1997). Additionally, infection with plant viruses can provide protection against drought stress (Xu et al. 2008), as seen with tobacco (Nicotiana ), beet (Beta vulgaris ), and rice. This was shown to be due to virus-induced accumulation of osmoprotectants and antioxidants anthocyanins. Future work is needed to identify key targets for breeding which address the complex nature of the plant growth environment with responses to both abiotic and biotic stress.
TARGETS FOR FUTURE RESEARCH EFFORTS
Several studies have been done at the whole genome level to analyze gene expression under single and combined abiotic and biotic stresses (Vos et al., 2015; Coolen et al., 2016; Davila et al., 2017; also see Fujita et al., 2006; Atkinson & Urwin, 2012, and Suzuki et al., 2014; Rivero et al., 2022 for review). From this work and subsequent meta-analytic studies, key signaling pathways and genes sit at the intersection of biotic and abiotic stress responses. Below we outline those key targets, understanding that plant response to multiple stresses often produces gene expression and signaling patterns unique to those of a singular stress (Zarattini et al 2021).