Amino Acids From Root Exudates Induce <i>Bacillus</i> Spore Germination to Enhance Root Colonisation and Plant Growth Promotion

Strains of Bacillus species, plant growth-promoting rhizobacteria, have been commercialised as biofertilisers; they are ideal for this because these species form spores that can be stored stably for a long time. However, for these spores to exert their full beneficial effects, they must germinate. The specific germination signals in the rhizosphere, particularly those from plant root exudates, remain largely unknown. Here, we investigated the germination signals from different growth states of cucumber (Cucumis sativus) for spores of Bacillus velezensis SQR9 and Bacillus subtilis NCIB 3610. We identified the corresponding germination receptors and compared them biochemically between the Bacillus species. Larger plants better stimulated spore germination. Five amino acids-L-isoleucine, L-ornithine, L-valine, L-serine and β-alanine were-identified as spore germination signals. Combined application of a mixture of these amino acids with bacterial spores markedly enhanced the cucumber growth-promoting properties of B. velezensis SQR9. The germination receptor for these amino acids was GerA in both Bacillus species. Differences in spore germination efficiency between B. subtilis and B. velezensis may be attributable to variations in the GerA ligand-recognition sites. Expression of GerA from B. subtilis NCIB 3610 in B. velezensis SQR9 enhanced the spore germination rate of the latter. Our study highlights the pivotal role of amino acids in regulating spore germination of Bacillus and subsequent plant root colonisation, emphasising their potential to enhance the efficacy of Bacillus-based biofertilisers. Engineering of germination receptors is a promising approach to enhance the spore germination efficiency of biofertiliser strains.