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.