Plant Growth-Promoting Activity of Beta-Propeller Protein YxaL Secreted from Bacillus velezensis Strain GH1-13

Abstract

YxaL is conserved within Bacillus subtilis species complex associated with plant and soil. The mature protein YxaL contains a repeated beta-propeller domain, but the subcellular location and function of YxaL has not been determined. The gene encoding the mature YxaL protein was PCR amplified from genomic DNA of B. velezensis strain GH1-13 and used for recombinant protein production. A rabbit polyclonal antibody against the purified YxaL was generated and used for western blotting to determine the constitutive expression and secretion of YxaL, which exhibited a half-life of 1.6 h in the culture medium of strain GH1-13. In this study, we show that seed treatments of Arabidopsis thaliana and rice (Oryza sativa L) with less than 1 mg L-1 of purified YxaL in a soaking solution were effective at improving the root growth of plants. The seedlings of the treated Arabidopsis seeds markedly increased transcription of a 1-aminocyclopropane-1-carboxylate synthetase marker gene (ACS 11) but reduced expression of auxin-and abscisic acid-responsive marker genes (IAA1, GH3.3, and ABF4), especially when provided exogenous auxin. The horticulture experiments showed that pepper (Capsicum annum) seeds treated with 1 mg L-1 YxaL in soaking solution increased shoot growth and improved tolerance to drought stress. We hypothesize that YxaL secreted from plant growth-promoting Bacillus cells has a significant impact on plant roots, with the potential of improving plant growth and tolerance against stress.