| Some bacteria in the genus Bacillus, which are resistant to adverse environment, are one of the dominant microorganisms in the soil and plant microecological systems. Since Sanford reported that some antagonistic microorganisms in soil can restrain soil-borne pathogens in 1926, researches on Bacillus were focused on prevention and control of plant diseases. The plant growth-promotion effects of Bacillus were studied in recent years, and the mechanisms which have been reported mainly include plant growth regulating, biocontrol and induced plant systemic resistance. Bacillus spp. is one of the important sources of protease applied in industry, and the function of protease produced by Bacillus strains has so far constituted the primary research in biocontrol against fungal pathogens. However, little is known about the role of protease that secreted by Bacillus with evident biocontrol activity in plant growth-promotion. A large number of facts have been proved that amino acid fertilizer can significantly increase crop yield.Thus, the role and status of Bacillus extracellular protease in plant growth-promotion is worth of studying.In order to further reveal the importance of extracellular protease in pakchoi growth-promotion, Bacillus strains T2, which was potential biocontrol agent against a broad spectrum of plant pathogenic fungi with the ability to hydrolysis fish protein efficiently, were isolated. Subsequently, preliminary analyses of the growth-promotion ingredients in hydrolyzate are conducted. The main results and conclusions are as follows:1.Bacillus subtilis strain T2 with the highest ability to produce protease was selected out for further study from several broad-spectrum antagonistic bacteria obtained from the plant rhizosphere soil using abandoned coastal fish protein hydrolysis tests.2.It was revealed that the peptide and amino acid content in fish protein hydrolyzate become highest after 60h cultivation.3.Bacillus subtilis T2 was a potential biocontrol strain against a broad-spectrum plant pathogenic fungi. In order to determine the special role of extracellular protease in plant growth-promotion, a kind of deserted fish protein was selected as an only substrate to prepare degraded products of T2, acid-hydrolyzates and aqueous extracts, in which peptides and amino acids were included. Then, each of them was added to flowerpots respectively and their effects on pakchoi yield as well as the major nutritional quality were investigated through observations and biochemical analysis. The results of biochemical experiments revealed that the degraded products of T2 triggered the greatest increases in plant fresh and dry weight, Vc content, polysaccharides content, chlorophyll content and total-N content, enhanced the activity of nitrate reductase and catalase, and reduced nitrate content in pakchoi. It was revealed that the plant height, fresh weight, Vc content, polysaccharides content, chlorophyll content, total-N content, nitrate reductase activity and catalase activity were elevated by 78.24%, 34.35%, 15.24%, 32.79%, 28.16%, 9.18%, 27.87% and 73.68% respectively, and the nitrate content in pakchoi decreased by 16.36% compared to CK. However, acid-hydrolyzates and aqueous extracts did not stimulate pakchoi seedlings growth as much as the degraded products of T2.4.Roots played an important role in plant nutrition absorption and biosynthesis, and influenced the plant growth, nutritional status and production levels directly. In order to determine the special roles of extracellular protease in plant roots growth-promotion, degraded products of T2, acid-hydrolyzates and aqueous extracts of deserted fish protein were prepared, in which peptides and amino acids were included. Then, these three products were added to PG medium separately, and their effects on pakchoi root were investigated through observations under light microscopy and biochemical analysis. Results showed that within these three treatments, the hydrolysis protein products of T2 appeared to have the best function to increase root hair length, density, adventitious root formation and TTC reducing power.5.The obvious growth-promoting effect of T2 to packoi showed that there may be certain substances which can promote the plant growth engendered in the hydrolyzates. The result of qualitative detection revealed that the degraded products of T2 did not contain IAA. While content of L-tryptophan, which served as a precursor of indole-3-acetic acid (IAA), was much higher than other treatments. These results suggested that some specific peptides and amino acids from Bacillus subtilis T2 protease degraded products played an important role in plant growth-promotion, and L-tryptophan must be one of them. |
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