Pastinvestigationsof stomatal development found no change in stomatal conductance in systemic virus-free leaves of resistant N. glutinosaand C. quinoa hostswhen inoculated with TMV (Murray, et al., 2016). This agrees with observations from the present study regarding N.
glutinosaunder watered conditions, but is not reflected in responses observed under drought. It appears that effects of N. glutinosa’s response to virus infection are dependent on water availability. Defence responses of N. glutinosaagainst virus infection likely influence water use, thus, potentially exacerbate damaging effects of drought on plants.
Further investigation is required to determine whether this hypothesis can be extended to other resistant host species.Previous studies indicate aspects of the HR affect host-plant water use. The TMV-resistance HR in N.
glutinosais conferred by the N-gene (Whitham, et al., 1994). One aspect ofthe HR involves transient restriction of xylem transport within localised infected lesions before necrosis, resulting in lowered water potential for accelerated cell desiccation; partly due to guard cell closure and reduced transpiration (Wright, et al., 2000). This may support results of the present study: increased water loss from leaves of TMV-inoculated resistant host-plants (indicated by a lower LWC (Fig. 6b)) might be a result of the HR.
In this case, N. glutinosacould be expected to perform worse under drought conditions. Studies demonstrate TMV-infection causes stomatal closure by inducing abscisic acid (ABA) synthesis in infected lesions during the HR (Chaerle, et al., 1999).
Whilst stomatal closure is expected to benefit plants by limiting water loss via thetranspiration stream (Yamaguchi-Shinozaki, & Shinozaki, 2006; Hetherington, 1998), TMV-inoculated N. glutinosadoes not perform better under drought in the present study. ABA synthesis during the HR potentially indicates drought stress caused by HR-induced necrosis (Whenham & Fraser, 1981). Thus, one explanation for reduced performance of TMV-inoculated N. glutinosa under drought may be that HR-induced necrosis exacerbates damage caused by environmental drought in resistant hosts. Alternatively, poor performance of TMV-inoculated N. glutinosa under drought could be explained by limited uptake of soil-water andwater-soluble minerals via plant roots by the reduced transpiration streamduring the HR. Combinedwith drought stresses experienced by plants in the present study, plants’ abilities to tolerate drought during the onset of the HR against TMV may be impaired.
Moreover, salicylic acid, a component of the HR, may reduce plasmodesmatal permeability in TMV-infected N. glutinosa, limiting the systemic spread of the virus (Krasavina, et al., 2002). Reduced plasmodesmatal permeability may also limit efficiency of symplastic water transport; detrimentally affecting growth under drought. The dynamicity of symbiotic interactions between plants and viruses in nature are important acknowledgements.
Implications of infection depend on multi-factorial interactions including host species, environment and ecological contexts; contrasting reports of effects of viral infection on drought symptoms in different host species demonstrate the complexity of these interactions (Xu, et al., 2008; Garrett, et al., 2006). Thus, responses to combined viral and drought stresses will likely vary between plant species; inter-species generalisations should not be made. Anunexpected conclusion of this study was that establishment and systemic spread of TMV through susceptible hosts potentially depends on the water supply to the plant. The lack of increasein TMV-infection level with weekspost-inoculation in N.
tabacum hostsunder droughtcompared with those in wateredconditions suggests water limitation is detrimental to viral establishment and success (Fig. 7). A possible limitation here is bias introduced by sampling from different leaves between assays one and three weeks post-inoculation; TMV-infection level may be higher in inoculated leaves taken one weekpost-inoculation. Nevertheless, drought appears to limit viral infectivity in susceptible hosts. Investigation into water requirements of viruses may provide potential to combat plant disease by cultivation under partially water-limited conditions, which fulfil plants’growth requirements but limit viral establishment.