3. DISCUSSION
With growing evidence that root zone warming induces systemic disease resistance in tomato, this work aimed to decipher the molecular mechanisms underlying this phenomenon. We have shown that RZW activates immunity in tomato, increasing the expression of defense genes and PRRs, along with the induction of ethylene and ROS. This activated immune system, in turn, results in systemic disease resistance to both biotrophic and necrotrophic pathogens, indicating that it is a mechanism common to several biotic signaling pathways.
Interestingly, we found that warming-induced immunity relies on the SA signaling pathway. Heating treatments were previously shown to affect SA signaling (Arofatullah et al., 2018; Sato et al., 2003; Widiastuti et al., 2013; Snyman and Cronjé, 2008).
Our work indicates that root zone warming treatments are most effective in promoting disease resistance to Bc after a short recovery period (Figure 3), though, interestingly, in the case of Xcvdisease resistance, different recovery periods had similar effects (Figure 4). Perhaps this is due to the different nature of the pathogens, with root-zone warming initially priming SA mediated pathways (Figure 9), which are also required for Xcv resistance (Xu et al., 2018), while the effect on attributes required for necrotrophic pathogen resistance require a longer period of ”acclimation”.
Evidences of commonalities between heat stress and biotic stress signaling pathways have been previously reported (Zhang and Sonnewald, 2017; Suzuki and Katano, 2018; Jacob et al., 2017). In particular, HSPs have been demonstrated to be involved in the response to both heat and biotic stressors (di Donato and Geisler, 2019; Yu et al., 2016). Over-expression of heat shock proteins in plants has been proposed as one of the potential strategies to combat heat stress. HSPs function as molecular chaperons, are involved in correct protein folding, assembly, translocation, degradation and they also provide stability to integral proteins and cell membranes under heat stress (Boston et al., 1996). HSPs reportedly serve chaperone functions in quality control of plant defense PRRs (Nekrasov et al., 2009; Lee et al., 2009). Interestingly, we found that several PRRs were induced by the same root zone warming treatment that was sufficient to promote pathogen resistance (Figure 6b), supporting the notion that PRR alterations in response to wounding or biotic cues can be sufficient to activate plant defense (Zhou et al., 2020; Saijo et al., 2018).
Low-level induction of HSPs concurrent with induced resistance toBc , On and Xcv suggest that the same mechanisms which exert abiotic stress tolerance also activate immune mechanisms in tomato, as was previously suggested (di Donato and Geisler, 2019; Yu et al., 2016; Zhang and Sonnewald, 2017). The activation of HSPs by pathogenic processes demonstrates the common signaling pathways which can underlie the plants’ response to several different types of stresses, which, ideally, would be those manipulated in order to generate resistance and agriculturally desirable cultivars in the face of combinatorial stress created by climate change. Of note is that, similar to what we found for root-zone warming induced immunity toBc , acquired thermo-tolerance also requires a short acclimation period, after which plants become more resistant to subsequent heat application (Baniwal et al., 2004; Charng et al., 2007) perhaps indicating that conserved machinery may be important in both types of stress.
Interestingly, transgenic tomato plants expressing the Arabidopsis NPR-I gene developed enhanced heat tolerance in addition to varying levels of resistance against several tomato pathogens, testifying to the connection between heat tolerance and biotic resistance (Lin et al., 2004). This also supports our results demonstrating that warming-induced resistance relies on an intact SA pathway (Figure 9). In addition to HSPs, ROS scavenging abilities were reported to be essential in both heat and biotic resistance (Piterková et al., 2013; Vallélian-Bindschedler et al., 1998; Suzuki and Katano, 2018), and thus, ROS homeostasis may be another underlying common mechanism that might explain why increased heat tolerance also affords induced immunity and pathogen resistance. Our results demonstrate that ROS inducing mechanisms are affected by root zone warming (Figure 6b).
The use of pesticides and chemicals in agriculture is hazardous to human health and lacks environmental sustainability. From a farmer’s perspective, any improvement that reduces the cost of chemical application is desirable. Heat induced systemic disease resistance represents an attractive strategy to aid in combatting pathogens, given its economical and environmentally friendly nature. Further, when combining different priming agents to potentiate plant immunity, our results indicate that warming treatments could best be combined with JA-pathway ISR inducers, to achieve combined effects by potentiating different immunity pathways simultaneously. Our results suggest that mechanisms which govern acclimation to changing ambient temperatures may be exploited in agriculture to promote disease resistance. Further research will elucidate whether a potential use of root zone warming as an eco-friendly disease control agent in agricultural systems is feasible.