Abstract
SummaryAGD2‐LIKE DEFENCE RESPONSE PROTEIN 1 (ALD1) triggers plant defence against bacterial and fungal pathogens by regulating the salicylic acid (SA) pathway and an unknown SA‐independent pathway. We now show that Nicotiana benthamiana ALD1 is involved in defence against a virus and that the ethylene pathway also participates in ALD1‐mediated resistance. NbALD1 was up‐regulated in plants infected with turnip mosaic virus (TuMV). Silencing of NbALD1 facilitated TuMV infection, while overexpression of NbALD1 or exogenous application of pipecolic acid (Pip), the downstream product of ALD1, enhanced resistance to TuMV. The SA content was lower in NbALD1‐silenced plants and higher where NbALD1 was overexpressed or following Pip treatments. SA mediated resistance to TuMV and was required for NbALD1‐mediated resistance. However, on NahG plants (in which SA cannot accumulate), Pip treatment still alleviated susceptibility to TuMV, further demonstrating the presence of an SA‐independent resistance pathway. The ethylene precursor, 1‐aminocyclopropanecarboxylic acid (ACC), accumulated in NbALD1‐silenced plants but was reduced in plants overexpressing NbALD1 or treated with Pip. Silencing of ACS1, a key gene in the ethylene pathway, alleviated the susceptibility of NbALD1‐silenced plants to TuMV, while exogenous application of ACC compromised the resistance of Pip‐treated or NbALD1 transgenic plants. The results indicate that NbALD1 mediates resistance to TuMV by positively regulating the resistant SA pathway and negatively regulating the susceptible ethylene pathway.
Highlights
Salicylic acid (SA) induces an important plant defence pathway against pathogens by producing pathogenesis-related proteins (Durrant and Dong, 2004; Fu and Dong, 2013; Gao et al, 2015; Tsuda et al, 2009; Vlot et al, 2009)
We report that the expression of Nicotiana benthamiana AGD2-LIKE DEFENCE RESPONSE PROTEIN 1 (ALD1) (NbALD1) is induced following infection by turnip mosaic virus (TuMV) and that NbALD1 participates in plant defence against TuMV by both SA-dependent and SA-independent pathways simultaneously
We show that the ethylene pathway, regulated negatively by NbALD1 and mediating the susceptibility of N. benthamiana to TuMV, functions in NbALD1 or pipecolic acid (Pip)-mediated resistance in an SA-independent manner
Summary
Salicylic acid (SA) induces an important plant defence pathway against pathogens by producing pathogenesis-related proteins (Durrant and Dong, 2004; Fu and Dong, 2013; Gao et al, 2015; Tsuda et al, 2009; Vlot et al, 2009). Research over the last decade has shown that AGD2-LIKE DEFENCE RESPONSE PROTEIN 1 (ALD1) triggers the basal defence response and systemic acquired resistance (SAR) against bacterial infection in plants by regulating the SA pathway (Cecchini et al, 2015; Song et al, 2004a). Compared with wild-type plants, ald mutants of Arabidopsis thaliana were more susceptible to Pseudomonas syringae and had reduced SA accumulation (Song et al, 2004a), while the overexpression of ALD1 conferred resistance to the pathogen by inducing the expression of PAD4 and ICS1, key components of the SA pathway that are both essential for the response (Cecchini et al, 2015; Song et al, 2004b). Natural oviposition by Pieris brassicae or treatment with egg extract induced SAR and inhibited growth of P. syringae in an ALD1- and FMO1-dependent manner, implicating the ALD1-mediated pathway in a response to insects (Hilfiker et al, 2014)
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