N,N-dimethyl-hexadecylamine modulates Arabidopsis root growth through modifying the balance between stem cell niche and jasmonic acid-dependent gene expression.

N,N-dimethyl-hexadecylamine (DMHDA) is released as part of volatile blends emitted by plant probiotic bacteria and affects root architecture, defense and nutrition of plants. Here, we investigated the changes in gene expression of transcription factors responsible of maintenance of the root stem cell niche and jasmonic acid signaling in Arabidopsis seedlings in response to this volatile. Concentrations of DMHDA that repress primary root growth were found to alter cell size and division augmenting cell tissue layers in the meristem and causing root widening. DMHDA triggered the division of quiescent center cells, which correlated with repression of SHORT ROOT (SHR), SCARECROW (SCR), and PLETHORA 1 (PLT1) proteins and induction of WUSCHEL-RELATED HOMEOBOX 5 (WOX5) transcription factor. Interestingly, an activation of the expression of the jasmonic acid-related reporter genes JAZ1/TIFY10A-GFP and JAZ10pro::JAZ10-GFP suggests that the halted growth of the primary root inversely correlated with expression patterns underlying the defense reaction, which may be of adaptive importance to protect roots against biotic stress. Our data help to unravel the gene expression signatures upon sensing of a highly active bacterial volatile in Arabidopsis seedlings.

The bacterial volatile N,N-dimethyl-hexadecylamine promotes Arabidopsis primary root elongation through cytokinin signaling and the AHK2 receptor.

N,N-dimethyl-hexadecylamine (DMHDA) is a volatile organic compound (VOC) produced by some plant growth-promoting rhizobacteria (PGPR), which inhibits the growth of pathogenic fungi and induces iron uptake by roots. In this report, through the application of a wide range of concentrations, we found that DMHDA affects Arabidopsis primary root growth and lateral root formation in a dose-dependent manner where 1 and 2 µM promoted root growth and higher (4-32 µM) concentrations repressed growth. Cytokinin-inducible TCS::GFP and ARR5::uidA gene constructs showed an increased expression in columella cells and root meristem, respectively, at 2 µM DMHDA, but their expression domains strongly diminished at growth repressing treatments. To test if either primary root growth promotion or repression could involve members of the cytokinin receptor family, the growth of WT and double mutant combinations cre1-12 ahk2-2, cre1-12 ahk3-3, and ahk2-2 ahk3-3 was tested in control conditions or supplemented with 2 µM or 16 µM DMHDA. Noteworthy, the root growth promotion disappeared in cre1-12 ahk2-2 and ahk2-2 ahk3-3 combinations, whereas all double mutants had higher repression than the WT at high doses. We further show that DMHDA fails to mimic the effects of ethylene in Arabidopsis seedlings grown in darkness that include an exaggerated apical hook, stem and root shortening, and root hair elongation. Our data help unravel how Arabidopsis senses a growth-modulating bacterial volatile through changes in cytokinin responsiveness.

The bacterial volatile dimethyl-hexa-decylamine reveals an antagonistic interaction between jasmonic acid and cytokinin in controlling primary root growth of Arabidopsis seedlings.

Chemical communication underlies major adaptive traits in plants and shapes the root microbiome. An increasing number of diffusible and/or volatile organic compounds released by bacteria have been identified, which play phytostimulant or protective functions, including dimethyl-hexa-decylamine (DMHDA), a volatile biosynthesized by Arthrobacter agilis UMCV2 that induces jasmonic acid (JA) signaling in Arabidopsis. Here, he found that the growth repressing effects of both DMHDA and JA are antagonized by kinetin and correlated with an inhibition of cytokinin-related ARR5::GUS and TCS::GFP expression in Arabidopsis primary roots. Moreover, we demonstrate that shoot supplementation of JA triggers JAZ1 expression both locally and systemically and represses cytokinin-dependent promoter activity in roots. A similar effect was observed after cotyledon wounding, in which an increase of JA-inducible LOX2:GUS expression represses root growth, which correlates with the loss of TCS::GFP detection at the very root tip. Our data demonstrate that the bacterial volatile DMHDA crosstalks with cytokinin signaling and reveals the downstream antagonistic interaction between JA and cytokinin in controlling root growth.