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).