Bacillus sp B55, a bacterium naturally associated with
Nicotiana attenuata roots, promotes growth and survival of wild-type and, particularly, ethylene (
ET)–insensitive
35S-ethylene response1 (
etr1)
N. attenuata plants, which heterologously express the mutant
Arabidopsis thaliana receptor ETR1-1. We found that the volatile organic compound (
VOC) blend emitted by B55 promotes seedling growth, which is dominated by the S-containing compound dimethyl disulfide (
DMDS).
DMDS was depleted from the headspace during cocultivation with seedlings in bipartite Petri dishes, and
35S was assimilated from the bacterial
VOC bouquet and incorporated into plant proteins. In wild-type and
35S-etr1 seedlings grown under different sulfate (SO
4−2) supply conditions, exposure to synthetic
DMDS led to genotype-dependent plant growth promotion effects. For the wild type, only S-starved seedlings benefited from
DMDS exposure. By contrast, growth of
35S-etr1 seedlings, which we demonstrate to have an unregulated S metabolism, increased at all SO
4−2 supply rates. Exposure to B55
VOCs and
DMDS rescued many of the growth phenotypes exhibited by
ET-insensitive plants, including the lack of root hairs, poor lateral root growth, and low chlorophyll content.
DMDS supplementation significantly reduced the expression of S assimilation genes, as well as Met biosynthesis and recycling. We conclude that
DMDS by B55 production is a plant growth promotion mechanism that likely enhances the availability of reduced S, which is particularly beneficial for wild-type plants growing in S-deficient soils and for
35S-etr1 plants due to their impaired S uptake/assimilation/metabolism.
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