Treatment With Wheat Root Exudates and Soil Microorganisms From Wheat/Watermelon Companion Cropping Can Induce Watermelon Disease Resistance Against Fusarium oxysporum f. sp. niveum.

Companion cropping with wheat (Triticum aestivum L.) can enhance watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] wilt disease resistance against Fusarium oxysporum f. sp. niveum. However, the mechanism of resistance induction remains unknown. In this study, the effects of microbial community dynamics and the interactions between wheat and watermelon plants, particularly the effect of wheat root exudates on watermelon resistance against F. oxysporum f. sp. niveum, were examined using a plant-soil feedback trial and plant tissue culture approach. The plant-soil feedback trial showed that treating watermelon with soil from wheat/watermelon companion cropping decreased watermelon wilt disease incidence and severity, increased lignin biosynthesis- and defense-related gene expression, and increased ß-1,3-glucanase activity in watermelon roots. Furthermore, soil microbes can contribute to increasing disease resistance in watermelon plants. Tissue culture experiments showed that both exogenous addition of wheat root exudates and companion cropping with wheat increased host defense gene expression, lignin and total phenols, and increased ß-1,3-glucanase activity in watermelon roots. In conclusion, both root exudates from wheat and the related soil microorganisms in a wheat/watermelon companion cropping system played critical roles in enhancing resistance to watermelon wilt disease induced by F. oxysporum f. sp. niveum.

[Effects of Chinese onion' s root exudates on cucumber seedlings growth and rhizosphere soil microorganisms].

Taking the Chinese onion cultivars with different allelopathy potentials as the donor and cucumber as the accepter, this paper studied the effects of Chinese onion' s root exudates on the seedlings growth of cucumber and the culturable microbial number and bacterial community structure in the seedlings rhizosphere soil. The root exudates of the Chinese onion cultivars could promote the growth of cucumber seedlings, and the stimulatory effect increased with the increasing concentration of the root exudates. However, at the same concentrations of root exudates, the stimulatory effect had no significant differences between the Chinese onion cultivars with strong and weak allelopathy potential. The root exudates of the Chinese onion cultivars increased the individual numbers of bacteria and actinomyces but decreased those of fungi and Fusarium in rhizosphere soil, being more significant for the Chinese onion cultivar with high allelopathy potential (L-06). The root exudates of the Chinese onion cultivars also increased the bacterial community diversity in rhizosphere soil. The cloning and sequencing results indicated that the differential bacteria bands were affiliated with Actinobacteria, Proteobacteria, and Anaerolineaceae, and Anaerolineaceae only occurred in the rhizosphere soil in the treatment of high allelopathy potential Chinese onion (L-06). It was suggested that high concentration (10 mL per plant) of root exudates from high allelopathy potential Chinese onion (L-06) could benefit the increase of bacterial community diversity in cucumber seedlings rhizosphere soil.

[Effects of cinnamic acid on bacterial DNA polymorphism in rhizosphere soil of cucumber under NaCl stress].

By using PCR-DGGE technique, this paper studied the effects of different concentration (0, 25, 50, 100, and 200 mg x kg(-1) soil) cinnamic acid on the bacterial DNA polymorphism in rhizosphere soil of cucumber seedlings under the stress of 292.5 and 585 mg NaCl x kg(-1) soil. At all growth stages of cucumber seedlings, treatment 50 mg x kg(-1) of cinnamic acid had the similar band numbers and band gray scales in DGGE profiles to treatment 0 mg x kg(-1) of cinnamic acid, but the diversity index, richness index, and evenness index were the highest; while in treatments 100 and 200 mg x kg(-1) soil of cinnamic acid, the band numbers and band gray scales decreased, and the diversity index, richness index, and evenness index were lower. Our results demonstrated that low concentration cinnamic acid relieved the salt stress on soil microbes, while high concentration cinnamic acid aggravated the stress. The cloning and sequencing results showed that the main bacterial groups affected by salt stress were uncultured bacterial species, alpha-Proteobacteria, beta-proteobacteria, and gamma-proteobacteria, and a few were Firmicutes, Acidobacteria, and Actinobacteria.