Plant growth regulating compounds from microbial resources: Structural diversity, biological activities, and applications in sustainable agriculture.

Plant hormones play important roles in plant growth and development and in response to abiotic and biotic stress. The endogenous phytohormones had been widely applied to improve the productivity and the quality of crop products, and enhance the resistance of plant to abiotic stress. Many microbes produce the endogenous phytohormones, like auxins, cytokinins, gibberellins, abscisic acids, jasmonic acids, ethylene, and their analogues, of which some had been commercialized through fermentation and applied in large scale in crop production and ornamental. Other than the endogenous plant growth regulators, microbes also produced structurally diverse secondary metabolites with plant growth regulating (PGR) activities, like amino acid derivatives, peptides, nucleosides, alcohols, ketones, aldehydes, and organic acids, esters, lactones and macrolides, amines, etc. With the limited amount of the compound isolated from the microbial cultures, they were only evaluated for the plant growth regulating activities on some model plants, like Arabidopsis thaliana, lettuce and rice, etc. To accelerate the discovery of the microbial secondary metabolites with PGR activities, it is necessary to set up sensitive, robust high-throughput screening system because of the low production titre of the secondary metabolites microbes produce. With the progress in gene editing and synthetic biology, it will help to increase the production tier of the discovered microbial secondary metabolites with PGR activities and accelerate their commercialization. So, this review focuses on endogenous/non-endogenous PGR compounds from microbial resources, especially for their diverse structure, bioactivities, modes of action and applications in sustainable agriculture, which will offer opportunities to discover novel PGRs or biostimulants for green and sustainable development of agriculture.