Combined effects of arsenic and Magnaporthe oryzae on rice and alleviation by silicon

Highlights: • Rice is exposed to multiple stressors including As and rice blast simultaneously. • Silicon can alleviate both stressors individually, but in combination it is unknown. • We surprisingly found that rice treated with As was less susceptible to rice blast. • As appears to disrupt the current thinking of how Si impacts blast-induced defenses. While the impacts of arsenic (As) and Magnaporthe oryzae on rice have been well-studied, a dearth of knowledge exists on how rice responds to their combined stress. Moreover, increasing exogenous silicon (Si) can alleviate M. oryzae infection and As uptake, but how increasing exogenous Si affects the combined stress of M. oryzae and As is unknown. We grew three cultivars of rice that varied in their susceptibility to As and M. oryzae under low (50 μM, Si_L) and high (1500 μM, Si_H) Si with and without As (4 μM, 80/20 As (III)/As(V)) and with or without M. oryzae infection and examined the impacts of treatments on plant As and Si concentrations, severity of disease by M. oryzae, and stress via targeted gene expression. Si_H treatments generally decreased shoot As concentrations by 20–70% compared to Si_L treatments depending on cultivar and M. oryzae exposure. There was no effect of Si or As treatments on percent of leaf diseased in the As-tolerant cultivar M206, but in the As-sensitive cultivar IR66, Si_H treatment decreased percent of leaf diseased in the absence of As and had no impact when As was present. In the M. oryzae-susceptible Sariceltik, plants receiving Si_H had significantly fewer lesions than those receiving Si_L and plants with the fewest lesions were in the Si_H + As treatments. Plants that were exposed to As + M. oryzae were the most stressed when grown under Si_L, but this stress response was lowered by Si_H treatments. A separate pathogenicity assay with Sariceltik showed that in contrast to our hypothesis, As exposure decreased lesion growth, particularly under Si_H treatments, and lessened the impact of M. oryzae on rice. These results suggest that rice grown under replete Si will be able to withstand combined stressors of M. oryzae and As, but will be highly stressed under Si deficient scenarios.