Abstract
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, H2O2 and Ca2+, 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 H2O2 in roots by enhancing the transcription of RbohD and RbohF, and the elicited H2O2 further activated an influx of Ca2+ into root cells. Ca2+ signaling was transmitted via the stem from roots to leaves, where it elicited a fresh burst of H2O2, 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 H2O2 burst and subsequent crosstalk between H2O2 and Ca2+ signaling.
Highlights
Abiotic stresses such as salinity, drought, and extreme temperatures, have a significant impact on plant growth and crop yield worldwide
Salt and cold stresses significantly inhibited the growth of rape seedlings, which was manifested as a decrease in fresh weight (Fig. 1A)
MDA is the main product of membrane lipid peroxidation caused by excess reactive oxygen species (ROS) in stressed plants, the content of which can reflect the degree of membrane damage caused by abiotic stresses
Summary
Abiotic stresses such as salinity, drought, and extreme temperatures, have a significant impact on plant growth and crop yield worldwide. Whilst γ-PGA has been shown to alter the expression of certain genes, enhance the activity of antioxidant enzymes, and promote accumulation of osmoregulatory substances, the mechanism whereby the stress tolerance of plants is enhanced in response to γ-PGA treatment has yet to be fully elucidated[17,18]. It is not known whether γ-PGA is taken up by plant cells in the form of a polypeptide biomacromolecule. Investigation of both the role of H2O2 and any possible link between H2O2 and Ca2+ signaling is crucial in order to further elucidate the mechanism of the plant γ-PGA response
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