It has been hypothesized that plants can get beneficial trade-offs from viral infections when grown under drought conditions. However, experimental support for a positive correlation between virus-induced drought tolerance and increased host fitness is scarce. We investigated whether increased virulence exhibited by the synergistic interaction involving Potato virus X (PVX) and Plum pox virus (PPV) improves tolerance to drought and host fitness in Nicotiana benthamiana and Arabidopsis thaliana. Infection by the pair PPV/PVX and by PPV expressing the virulence protein P25 of PVX conferred an enhanced drought-tolerant phenotype compared with single infections with either PPV or PVX. Decreased transpiration rates in virus-infected plants were correlated with drought tolerance in N. benthamiana but not in Arabidopsis. Metabolite and hormonal profiles of Arabidopsis plants infected with the different viruses showed a range of changes that positively correlated with a greater impact on drought tolerance. Virus infection enhanced drought tolerance in both species by increasing salicylic acid accumulation in an abscisic acid-independent manner. Viable offspring derived from Arabidopsis plants infected with PPV increased relative to non-infected plants, when exposed to drought. By contrast, the detrimental effect caused by the more virulent viruses overcame potential benefits associated with increased drought tolerance on host fitness.

In this study, we have used affinity chromatography to purify the lectins from a chlorolichen, Evernia prunastri (L.) Ach., and a cyanolichen, Peltigera canina (Ach.) Schard. These species secrete lectins that display arginase activity in addition to their role as recognition proteins. We found that fluorescently labeled lectins display efficient binding to their ligands on the cell wall. Binding was stronger when the lectin reacted with the producing (homologous) photobiont (alga or cyanobacteria) than with the nonproducing (heterologous) species used throughout the study. To address the specificity of lectin binding, we performed desorption experiments of cell-bound lectins with different hexoses. We found that Evernia lectin is only desorbed by galactose, consistent with its specific binding to a single polygalactosylated ligand. Conversely, Peltigera lectin is desorbed not only by galactose, but also by mannose. This indicates that Peltigera lectin recognizes not only a-D-galactose-containing ligands, but also ligands containing a-D-mannose moieties.