1. INTRODUCTION
In agricultural environments, plants are often exposed to several stresses simultaneously. Changing climate suggests that combined biotic and abiotic stresses in agricultural settings may become more common, with opportunistic pathogens undergoing adaptations that will allow them to thrive under newly evolving conditions, while plants fight to maintain pathogen resistance capabilities in new ambient environments.
Reported thermo-tolerance mechanisms in plants can be diverse, with real-environment heat stress and adaptation being infinitely more varied than laboratory applied heat stresses in plant research. Four distinct thermo-tolerance mechanisms were proposed in Arabidopsis (Yeh et al., 2012), suggesting that plant responses to changing environmental temperatures can be highly complex.
Several reports have investigated different aspects of combinatorial plant stress, with varied conclusions (Saijo and Loo, 2020; Cappetta et al., 2020). While we are still at the beginning of understanding combinatorial stresses faced by plants in changing environments, several studies have concluded that transcriptional changes at the level of individual genes are highly variable and stress-specific (Zhang and Sonnewald, 2017). However, plants have a limited ”tool-box” with which they must generate adequate stress responses, and indeed, many reports have also demonstrated that central metabolic and signaling responses to different individual and combined stresses can share commonalities (Zhang and Sonnewald, 2017). In this context, of note are the Heat-Shock Protein (HSP) family, which were found in several cases to not only be important in heat stress, but to also be involved in plant immunity (Kumar et al., 2009; Park and Seo, 2015; di Donato and Geisler, 2019; Yu et al., 2016). HSPs are molecular chaperones responsible for protein folding, assembly, translocation, and degradation under both steady state and stress conditions. In addition to abiotic stresses, HSPs were reported to serve chaperone functions in quality control of Pattern Recognition Receptors (PRRs) and intracellular R-proteins important in plant defense against pathogens (Nekrasov et al., 2009; Liu et al., 2004; Lee et al., 2009).
Combined heat-biotic stress has been reported to have varying results in terms of plant resistance/ susceptibility to pathogens. In many cases, abiotic stress pre-exposure can weaken disease resistance, while pathogen infections often enhance abiotic stress responses (Atkinson and Urwin, 2012). Heat-related suppression of disease resistance has been reported for viruses and bacteria, usually as a results of the hypersensitive response / R-gene being compromised at high temperatures (Janda et al., 2019; Prasch and Sonnewald, 2013; Zhu et al., 2010).
In other cases, abiotic stress was shown to enhance disease resistance. In heat treated rice leaves, the heating resulted in accumulation of superoxide radicals and resistance to rice blast (Aver’yanov et al., 1993). Disease resistance following high temperature exposure was also reported in wheat against a rust pathogen (Qayoum and Line, 1985) and in tobacco protoplasts against a tomato virus (Jones et al., 1990). More recent reports have shown that plant heating can serve to combat subsequent pathogenic processes, leading to improved disease outcomes with several pathogens. In sweet basil, the incidence of gray mold (Botrytis cinerea ), white mold (Sclerotinia sclerotiorum ) and downy mildew (Peronospora belbahrii ) was found to be negatively correlated with high air and/or soil temperatures (Elad et al., 2017, 2016a). This disease amelioration effect was suggested to stem from host-induced mechanisms, rather than direct effects on the pathogens. In another report, soil polyethylene mulching was shown to reduce disease concomitantly with an increase in day-time soil temperatures (Shtienberg et al., 2010). Systemic disease resistance induced by heat was demonstrated in tomato and sweet basil, by exclusively heating the plant root zone, and observing disease resistance in the shoot/ canopy, which was measured to remain at ambient temperatures whilst the root zone was being heated (Elad et al., 2016a; Elad, 2018). Although the disease protectant effect achieved by root zone heating was demonstrated to be systemic, the molecular mechanisms driving this induced resistance were not examined.
In this work, we investigated the effectiveness of root-zone warming (RZW) as an inducer of disease resistance in tomato. We employ the term ”warming” throughout this work, to distinguish the treatment applied from ”heat-shock” protocols, in particular since the roots were only heated to 28ºC, a temperature which is not considered highly stressful in tomato breeding. We present evidence that RZW improves disease outcomes of the tomato fungal pathogens B. cinerea (Bc ) and Oidium neolycopersici (On ) and bacterial pathogenXanthomonas campestris pv. vesicatoria (Xcv ). RZW treatments were sufficient to activate the tomato immune system, inducing defense gene expression and an increase in ethylene and reactive oxygen species (ROS) production upon challenge. Our results suggest that mechanisms which govern acclimation to changing ambient temperatures may be exploited in agriculture to promote disease resistance.