Crosskingdom growth benefits of fungus-derived phytohormones in Choy Sum

Soil-borne beneficial microbes establish symbioses with plant hosts, and play key roles during growth and development therein. In this study, fungal strains FLP7 and B9 were isolated from the rhizosphere microbiome associated with Choy Sum (Brassica rapa var. parachinen-sis) and barley (Hordeum vulgare), respectively. Sequence analyses of the internal transcribed spacer and 18S ribosomal RNA genes combined with colony and conidial morphology identified FLP7 and B9 to be isolates of Penicillium citrinum. Plant-fungus interaction assays revealed that B9, but not FLP7, showed significant growth promotion effect in Choy Sum cultivated in normal soil, whereas FLP7 enhanced Choy Sum growth under phosphate-limiting condition. In comparison to the mock control, B9-inoculated plants showed a 34% increase in growth in aerial parts, and an 85% upsurge in the fresh weight of roots when cultivated in sterilized soil. The dry biomass of inoculated Choy Sum increased by 39% and 74% for the shoots and roots, respectively. Root colonization assays showed that P. citrinum associates directly with the root surface but does not enter/invade the roots of inoculated Choy Sum plants. Preliminary results also indicated that P. citrinum can promote growth in Choy Sum via volatile metabolites too. Interestingly, we detected relatively higher amounts of indole acetic acid and cytokinins in axenic P. citrinum culture filtrate through liquid-chromatography mass-spectrometry analyses. This could plausibly explain the overall growth promotion in Choy Sum. Furthermore, the phenotypic growth defects associated with the Arabidopsis ga1 mutant could be chemically complemented by the exogenous application of P. citrinum culture filtrate, which also showed accumulation of fungus-derived active gibberellins. Our study underscores the importance of trans-kingdom beneficial effects of such mycobiome-derived phytohormone-like metabolites in host plant growth.