Development of root system architecture of Arabidopsis thaliana in response to colonization by Martelella endophytica YC6887 depends on auxin signaling

Many rhizobacteria promote plant growth by producing hormones that stimulate the development of plant root system and increase plant biomass. The aim of this study was to investigate the growth promotion activity of the bacterial strain Martelella endophytica YC6887 and elucidate the signaling pathways potentially involved in Arabidopsis interaction with M. endophytica YC6887. The growth regulation was evaluated by inoculation of strain YC6887 with wild-type Arabidopsis Col-0 seedlings and mutants defective in auxin aux1-7, axr4-2, eir1-1, ethylene ein2-1, etr1-3, jasmonic acid signaling jar1, and root hair deficient mutant rhd6. The auxin response was further determined by using transgenic line DR5::GUS and a polar auxin transport inhibitor, 1-N-naphthylphthalamic acid (NPA). M. endophytica YC6887 increased the number of lateral roots and plant biomass of Arabidopsis by producing phenylacetic acid. The growth promotion and improved lateral root development by the bacterium decreased in the auxin related mutants, whereas the ethylene and jasmonic acid mutants had a wild type response. The strain YC6887 increased root hair density in wild type Col-0 and recovered the root hair forming ability in root hair deficient mutant rhd6. Moreover, strain YC6887 treatment showed distinct response in DR5::GUS transgenic line compared to the control. Strain YC6887 lost its growth-promoting activity in the presence of NPA, an auxin transport inhibitor. This indicated that strain YC6887 activated the auxin signaling mechanism. Our results showed that M. endophytica YC6887 promoted plant growth in terms of plant biomass and root system development. Arabidopsis root system development upon M. endophytica YC6887 colonization was dependent on auxin signaling, but independent of ethylene and jasmonic acid signaling.