Malic acid (MA), one of the major organic acid exudates from roots, plays a significant role in the chemotaxis of beneficial bacteria to the plant's rhizosphere. In this study, the effect of a plant-growth-promoting rhizobacterium, Bacillus subtilis RR4, on the synthesis and exudation of MA from roots is demonstrated in rice. To test the chemotactic ability of strain RR4 towards MA, a capillary chemotaxis assay was performed, which revealed a positive response (relative chemotactic ratio of 6.15 with 10 μmol/L MA); with increasing concentrations of MA, an elevated chemotactic response was observed. Quantitative polymerase chain reaction, performed to analyze the influence of RR4 on the MA biosynthetic gene, malate synthase (OsMS), and the transporter gene, aluminium-activated malate transporter (OsALMT), demonstrated significant differential expression, with 1.8- and -0.58-fold changes, respectively, in RR4-treated roots. The gene expression pattern of OsMS corroborated the data obtained by high-performance liquid chromatography, which showed elevated MA levels in roots (1.52-fold), whereas the levels of MA in root exudates were not altered significantly although expression of OsALMT was reduced. Our results demonstrate that B. subtilis RR4 is chemotactic to MA and can induce biosynthesis of MA in rice roots.
Bacillus subtilis, a gram-positive soil bacterium, is widely used as a plant-growth-promoting agent. However, how Bacillus initially colonizes rice roots and evades the plant primary defense mechanisms, and how it influences root secretion of phytochemicals for further colonization remain obscure. To get an insight into how a plant perceives the bacterium upon initial root colonization, a microarray analysis was performed using rice roots treated with a rice rhizosphere isolate, B. subtilis RR4. About 891 transcripts (255 up-regulated and 636 down-regulated) were differentially expressed, indicating that the bacteria reprogram the plant to colonize it. In our experiments, RR4 mainly caused the suppression of transcripts encoding defense response enzymes such as chitinase, cell-wall-modifying enzymes such as pectinesterase, and genes associated with transport/exudation of phytochemicals, signifying that the bacteria modulate the gene expression of the plant to facilitate its colonization. Genes that regulate secondary metabolite production were up-regulated. Although the defense response genes in rice roots were suppressed initially, they were induced gradually at 4 and 10 days post-treatment. This was accompanied by an increased level of salicylic acid in the colonized rice roots. Thus, our results show that B. subtilis alters the transcriptome of rice roots for initial colonization by initially lowering the plants’ defenses, limiting root exudation and active cell growth, but boosting the plants’ defenses at a later stage.
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