Abstract
Silicon (Si) has been widely employed in agriculture to enhance resistance against pathogens in many crop plants. However, the underlying molecular mechanisms of Si-mediated resistance remain elusive. In this study, the Arabidopsis-powdery mildew pathosystem was employed to investigate possible defense mechanisms of Si-mediated resistance. Because Arabidopsis lacks efficient Si transporters and thus is a low Si-accumulator, two heterologous Si influx transporters (from barley and muskmelon) were individually expressed in wild-type Arabidopsis Col-0 and a panel of mutants defective in different immune signaling pathways. Results from infection tests showed that while very low leaf Si content slightly induced salicylic acid (SA)-dependent resistance, high Si promoted PAD4-dependent but largely EDS1- and SA-independent resistance against the adapted powdery mildew isolate Golovinomyces cichoracearum UCSC1. Intriguingly, our results also showed that high Si could largely reboot non-host resistance in an immune-compromised eds1/pad4/sid2 triple mutant background against a non-adapted powdery mildew isolate G. cichoracearum UMSG1. Taken together, our results suggest that assimilated Si modulates distinct, multi-layered defense mechanisms to enhance plant resistance against adapted and no-adapted powdery mildew pathogens, possibly via synergistic interaction with defense-induced callose.
Highlights
Silicon (Si) is the second most abundant element on the earth
Plants with either very low or very high endogenous Si display enhanced resistance against powdery mildew In our pilot experiment, we surprisingly found that Arabidopsis thaliana wild-type Col-0 plants grown in regular soil matrix (Sun Gro Horticulture, Massachusetts) irrigated with deionized water without Si became slightly less susceptible to an adapted powdery mildew isolate Golovinomyces cichoracearum (Gc) UCSC1 compared to those grown in the same soil irrigated with 1.7 mM silicon or with tap water (Additional file 1: Figure S1)
Measurement of leaf Si content showed that plants irrigated with deionized water only had lower levels of Si than those irrigated with deionized water +Si or with tap water, and there was no significant difference between the latter two (Additional file 1: Figure S2b)
Summary
Silicon (Si) is the second most abundant element on the earth. it is not considered to be an essential element for plant growth, Si has long been recognized as a “beneficial” or “quasi-essential” substance to plants, mainly due to its important role in plant nutrition, under stressful conditions. Despite the extensive studies, the exact molecular mechanisms underlying the protective roles of Si in plants still remain elusive (Coskun et al 2019). This knowledge gap limits the exploitation of the full potential concerning practical application of Si in agriculture. It has been demonstrated that Si needs to be absorbed by plants through passive channel-type, selective Si transporters to realize its prophylactic effect (Ma 2010). Most studies on Si transport and its physiological roles in plants have been conducted with high accumulators, and to a lesser extent,
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