Use of Bacillus subtilis as biocontrol agent. IV. Salt-stress tolerance induction by Bacillus subtilis FZB24 seed treatment in tropical vegetable field crops, and its mode of action

The rhizobacterium Bacillus subtilis FZB24 registered as biocontrol agent w as field tested as a promoter for salt- tolerance to two cultivars of eggpl ant and pepper in saline soil in the Sinai region (Egypt) under the conditi on of irrigation with ground saline water. The use of Bacillus subtilis for root bacterization was realized by watering the seedlings with bacterial s pore-(preparation-)suspension, titer 10(8) spores/ml. Starring from 8 weeks after transplanting and for 4 weeks, fruit yield (kg/m(2)), number of frui ts/plant, average fruit fresh weight, dry weight percentage and fruit size were evaluated. Compared with the unsaline-irrigated control the yield was reduced in both vegetable crops in all used cultivars, due to irrigating th e plants with saline groundwater to more than 90 %. By using Bacillus subti lis FZB24 in the plots irrigated with saline groundwater, the yield increas ed up to 550 % in eggplants, and up to 430 % in the pepper cultivars, as co mpared with unbacterized ones. Also in the other plant growth parameters, s ignificant promotions could be found in the bacterized plants in the saline plots. So, the bacterization caused 50 and 25 % reduction in salinity effe ct on the yield of eggplants and pepper, respectively, and consequently res ulted in a remarkable salt-stress tolerance induction, which varied its deg ree according to the used plant species. To have insight on the mode of act ions of the salt stress tolerance-inducing effect of Bacillus subtilis FZB 24, model experiments have been conducted with auxin precursors and IAA. in tomato seedlings under controlled, axenic conditions and under salt-stress conditions similar to the field experiment. Up to now, it has been hypothe sized for the mode of action of Bacillus subtilis FZB24, which acts as plan t growth and health promoter, and stress tolerance inducer, that the given bacterial production of auxin and auxin precursors during root colonization induces a push in the plane auxin synthesis with changing regulation of th e appropriate mechanisms. In the model experiments, the pretreatment of see dlings with millimolare amounts of auxin precursors, tryptophan, indole-3-p yruvic acid or indole-3-acetic aldehyde, 75 % growth reduction in untreated seedlings under salt stress could be compensated completely after 1 week. That was not observed to the same degree after preapplication of auxin (IAA ). So, the presented model experiment could support the hypothesis of salt- stress tolerance induction in Bacillus subtilis FZB24-treated plants. The r esults are discussed from the aspect of a general anti-stress effect of Bac illus subtilis FZB24.