The microbe-secreted isopeptide poly-γ-glutamic acid induces stress tolerance in Brassica napus L. seedlings by activating crosstalk between H(2)O(2) and Ca(2+)

Poly-γ-glutamic acid (γ-PGA) is a microbe-secreted isopeptide that has been shown to promote growth and enhance stress tolerance in crops. However, its site of action and downstream signaling pathways are still unknown. In this study, we investigated γ-PGA-induced tolerance to salt and cold stresses in Brassica napus L. seedlings. Fluorescent labeling of γ-PGA was used to locate the site of its activity in root protoplasts. The relationship between γ-PGA-induced stress tolerance and two signal molecules, H(2)O(2) and Ca(2+), as well as the γ-PGA-elicited signaling pathway at the whole plant level, were explored. Fluorescent labeling showed that γ-PGA did not enter the cytoplasm but instead attached to the surface of root protoplasm. Here, it triggered a burst of H(2)O(2) in roots by enhancing the transcription of RbohD and RbohF, and the elicited H(2)O(2) further activated an influx of Ca(2+) into root cells. Ca(2+) signaling was transmitted via the stem from roots to leaves, where it elicited a fresh burst of H(2)O(2), thus promoting plant growth and enhancing stress tolerance. On the basis of these observation, we propose that γ-PGA mediates stress tolerance in Brassica napus seedlings by activating an H(2)O(2) burst and subsequent crosstalk between H(2)O(2) and Ca(2+) signaling.