Determination And Application Of IAA Producing Ability Of Three Tomato Endophytic Strains

Auxin can affect many physiological activities,and plays a key role in regulating various physiological activities of the growth and development in plant.It has been widely accepted that many rhizobacteria and endophytes also promote plant growth,and affect plant development and yield by synthesizing auxin indoleacetic acid（IAA）.In this study,plant growth promoting effect and IAA production capacity of the endophytic strains were screened and evaluated.Furthermore,the relationship between the growth promoting effect and IAA production capacity for the endophytes was also investigated.This study will lay the materials and theoretical foundation for understanding IAA synthesis mechanism and pathway of plant endogenous bacteria,and exploring more efficient and targeted microbial inoculants with good growth promoting effects.Based on the re-identification,three tomato endophyte strains Enterobacter cloacae mr7,Bacillus aryabhattai mr31 and Bacillus subtilis B21,previously isolated and preserved in the laboratory,were selected for the determination and research of IAA production ability.The strains were cultured in a medium containing tryptophan and without tryptophan,respectively.The supernatant was collected by centrifuging from the culture solution,and the colorimetric solution was added to observe color change of the mixed solution.The results showed that the three tested tomato endophytes could produce IAA when cultured in tryptophan-containing medium,but no IAA was produced in the medium without tryptophan.The result indicated that these three plant endophytes produced IAA through the tryptophan-dependent IAA synthesis pathway.The growth promotion effects of the three endophytic strains were evaluated plants wtih wheat seeds as target by culturing seeds on the plate.The sterilization conditions of wheat seeds were first optimized to result in the sterilization conditions combining alcohol and sodium hypochlorite.Furthermore,we optimized the concentration of the bacterial solution to incubate wheat treatment with 1,2,4,8,16,32 times dilution of bacterial solution,and then with 1,10,100,1000,and 10,000-fold gradient dilutions of bacterial solution,in the growth experiment.The results showed that high concentration of bacterial solution inhibite the tested wheat seeds to germinate,and the germination rate of wheat seeds in most plates was 100%for the 10^-2-10^-4dilutions.Combined with the previous IAA screening,three plant endophytes showed obvious plant growth promoting effects,and their growth promoting effects have a significant correlation with IAA.However,high concentrations of IAA may have an inhibitory effect on the germination of wheat seeds.Finally,the growth promoting effect of the plant endophyte strains were evaluated in pot experiments.In the pot experiment,seven different conditions such as seed soaking,root application and spraying were designayed to optimize the application methods of probiotic bacteria by seeding wheat.The results showed that the three tested strains showed obvious growth promoting effects on the plants.All strains had a more obvious promotion effect on wheat germination with the germination rate between 62.5%-100%.Of which mr7 is slightly lower.Compared with the control,the leaf length,root length and plant height of wheat treated by the three strains increased significantly.Among them,the leaf length increased by 2.4%-29.8%,the root length increased by 4.4%-21.0%,and the plant height increased 2.3%-26.4%.In general,mr31 has a slightly low promoting effect.In terms of biomass,the plant weight treated by the three strains increased by 0-54.6%,5.7%-64.1%,and 32.4%-88.5%,respectively,compared with the control.Among the 7 different treatment methods,seed soaked with the bacterial solution and then sprinkled into the soil showed better promoting effects in the germinated pot for treatments.In this study,we demonstrated that the IAA production capacity of three plant endophytes and their growth promoting conditions.The results indicated that the endophytes produced IAA in in vitro fermentation experiments through tryptophan-dependent IAA synthesis pathway,and showed obvious promotion effects on seed germination rate,leaf length,root length,plant height and plant weight in the plate and pot experiments.The research on the IAA production capacity and growth promotion effect of three tomato endophytes will be lay a foundation for understanding plant-microbe interactions and the development and application of more efficient plant probiotic microbial agents.