Abstract: 【Objective】To screen a bacterial strain capable of efficiently degrading starch and to optimize its enzyme production conditions,providing novel methods and insights for starch degradation in tobacco leaves.【Methods】Starch-degrading bacterial strains were isolated from high-quality flue-cured tobacco varieties（Yunyan 97,Yunyan 99,and Qinyan 99） leaves in Shaanxi Province. A dominant strain was identified through preliminary screening based on the Hc value,secondary screening for amylase activity,morphological observation,physiological and biochemical tests,and molecular biological identification. Single-factor experiments and orthogonal experiments were conducted to optimize the enzyme production conditions of the dominant strain.【Results】A total of 250 starch-degrading bacterial strains were isolated,and Bacillus amyloliquefaciens ZJ-189 was identified as the dominant tarch-degrading strain with an amylase activity of 53.07 U/mL.Following ultraviolet（UV） mutagenesis,124 mutant strains were obtained from B. amyloliquefaciens ZJ-189. Through secondary screening for amylase activity,the highly efficient starch-degrading strain ZJ189-UV11 was selected,exhibiting an amylase activity of 74.42 U/mL,a 40.23% increase compared to the starter strain,with stable genetic characteristics. Orthogonal experiments revealed that the optimal culture conditions for strain ZJ189-UV11 after 24 h were a temperature of 42 ℃,pH of 7.5,agitation speed of 200 r/min,and an inoculation ratio of 1.2%.【Conclusion】Bacillus amyloliquefaciens ZJ-189 is identifid as a dominant strain for starch degradation. The culture conditions affecting the growth and amylase activity of the highly efficient mutant strain ZJ189-UV11 are optimized through single-factor experiments and orthogonal experiments.

Key words: Tobacco leaves, Bacillus amyloliquefaciens, UV mutagenesis, Enzyme production conditions optimization