3. 1 Drought and plant biotic stress interactions
The interaction between drought and plant disease stress has been
previously reviewed for a wide variety of crops and disease systems
(Kissoudis et al., 2014; Pandey et al., 2015a,b; Zarratini et al.,
2021). From these reviews it is apparent that the outcomes from plant
biotic and drought stress interactions are going to depend on
environmental conditions that may favor the establishment and spread of
the disease, the type of microorganism’s mode of infection and
virulence, and impacts of drought on plant defense mechanisms (Kissoudis
et al., 2014; Pandey et al., 2015a; Zarratini et al., 2021). Here we
will review how drought can affect plant-pathogen interactions by
studying the effect of drought on pathogen fitness on plant physiology
which can affect the plant-pathogen interaction.
Many fungal and bacterial pathogens need high soil or leaf moisture
levels to be able to survive in the plant surface and infect it. Many
root fungal pathogens such as root rot (Phytium sp.;Aphanomyces sp.) and downy mildew (Plasmopara sp.) are
reduced under drought conditions due to inadequate soil moisture (Pandey
et al., 2015a). For example, the occurrence of root rot and downy mildew
in sunflowers (Helianthus annuus ) was less severe under drought
conditions due to detrimental environmental conditions for the survival
of the pathogens (Pandey et al., 2015a). Similarly, foliar bacterial and
fungal diseases can be impeded by drought, as they favor high water
content in the leaf apoplast, which is usually associated with high
humidity (Freeman & Beattie, 2009). Additionally, it has been observed
that drought can reduce the spread of fungal pathogens as rain is needed
for the dispersal of fungal spores (Pandey et al., 2015a), as well as
the incidence of many bacterial leaf spot diseases, as they reproduce by
water-soaked lesions (Rudolph, 1984).
Drought, however, can also enhance the severity of some root diseases,
such as the incidence of smut on cereals (Urocystis agropyri ;
Colhoun, 1973), charcoal stalk rot in sorghum (Macrophomina
phaseoli ; Pandey et al., 2015a) and root rot in safflower
(Phytophthora sp.; Duniway, 1977). The increased infection and
severity observed in these root fungal pathogens can be possibly related
with a higher diffusibility of volatile fungal attractors emitted by
roots in dry soils (Kerr, 1964; Pandey et al., 2015a). Drought stress
can also increase herbivore performance, as seen in faba bean
(Vicia faba minor L.), where yield was decreased when plants
experienced both water stress and herbivory pressure from black bean
aphids (Aphis fabae ) (Raderschall et al., 2021). This could
increase the infection of viruses that are transmitted by aphid vectors.
Drought can also impair disease tolerance traits in plants, thereby
limiting their defense mechanisms. Low levels of ROS have been related
with the production of ABA in leaves and the regulation (closing) of
stomatal opening and pathogen attack (Qi et al., 2018). ABA is a plant
hormone that is synthesized during abiotic stress conditions, including
water stress, and helps the plant to maintain its turgor and water
potential by closing stomata, accumulating osmotolerant solutes, and
reducing leaf expansion at the expense of reduced growth (Kissoudis et
al., 2014). ABA production under drought is of special interest as it
downregulates the salicylic acid (SA) and jasmonic acid (JA) defense
mechanisms against plant pathogens (Kissoudis et al., 2014; Pandey et
al., 2015b). For example, drought induces the ABA signaling pathway,
which downregulates Calmodulin-binding protein 60g (CBP60g) and Systemic
Acquired Resistance Deficient 1 (SARD1). These two transcription factors
are important nodes in the crosstalk since they are needed for SA
production, required for suppression of pathogens (Choudhary &
Senthil-Kumar, 2021), as well as for other defense related proteins.
CBP60g plays a role early during defense response and SARD1 later during
the infection (Wang et al., 2011). Additionally, drought stress has been
shown to enhance the susceptibility of Arabidopsis to Pseudomonas
syringae by increasing ABA signaling, which suppress SA-mediated
defense responses (Mohr & Cahill, 2003, 2007; Choudhary &
Senthil-Kumar, 2022).
Oxidative damage produced by ROS accumulation under severe drought can
also produce membrane and cellular damage which results in plant solute
leakage through the membrane, making plants vulnerable to more severe
pathogen infection. For example, charcoal rot (Macrophomina
phaseolina ) can use osmotolerant amino acids, such as proline and
asparagine, produced by common bean (Phaseolus vulgaris ) to
tolerate drought, and therefore exacerbate the severity of the infection
and disease (Mayek-Perez et al., 2002; Ijaz et al., 2013).
As drought stress decreases the water potential of the whole
soil-plant-atmosphere continuum, this can result in damage in the
structure of the xylem, such as the pit membranes, which reduce xylem
conductivity and transpiration (Ladjal et al., 2005; Hillabrand et al.,
2016). This in turn can increase susceptibility of plants to disease,
such as Xylella fastidiosa which causes Pierce’s disease, as the
bacteria has better access to the xylem when the pit membranes are
damaged (Newman et al., 2003; Thorne et al., 2006).
Drought can also lead to improved disease tolerance traits in plants.
Under drought, ABA-induced stomatal closure can lead to decreased
pathogen infection, as seen with P. syringae (Melotto et al.,
2006). ABA also stimulates pre-invasion defense mechanisms such as
callose deposition in the phloem which reduces the spread of vascular
pathogens such as Phytium irregulare (Adie et al., 2007). In
addition, increased ABA levels during early drought have been shown to
increase the resistance of Nicotiana benthamiana to white mold
(Sclerotinia sclerotiorum ) and tomato (Solanum
lycopersicum ) to gray mold (Botrytis cinerea ) (Achuo et al.,
2006; Ramegowda et al., 2013). Increased cuticle thickening caused by
drought can also limit pathogen infection (Tang et al., 2007; Bi et al.,
2017). Finally, pathogen stress can lead to improved plant responses to
drought. Previous work has shown drought tolerance has been improved in
plants exposed to RNA viruses (Xu et al., 2008) and Arabidopsis plants
exposed to Verticillium spp. demonstrated enhanced tolerance to
drought due to increased water flow from de novo xylem formation
upon pathogen infection (Reusche et al., 2012).