Fig. 1. Different sensitivity of two Chinese cabbage seedlings (cvs. Ryeokgwang and Buram-3-ho) to high temperature and/or drought stress under greenhouse conditions. (A) Plant growth of two cultivars grown for five weeks after planting at four different planting dates (4 Jul, 4 Aug, 5 Sep and 5 Oct) under a plastic greenhouse condition. Changes in average maximum (Tmax), average mean (Tmean) and average minimal (Tmin) temperatures (℃) for five weeks under the greenhouse conditions were demonstrated. (B) Plant growth of two Chinese cabbage cultivars by stopping irrigations. Three-week-old seedlings were undergone different irrigation regimes for additional two-weeks. Error bars represent the standard errors of the means of the four independent experimental replications. Means followed by the same letter are not significantly different at 5% level by least significant difference test.
Fig. 2. Bacterial tolerance to dehydration at different growing temperatures during in vitro liquid cultures. Two Bacillus species (B. aryabhattai H26-2 and B. siamensis H30-3) and two phytopathogenic bacteria Pectobacterium carotovorum subsp. carotovorum (Pcc) strain PCC21 and Xanthomonas campestris pv. campestris (Xcc) strain 8004 were cultured with increasing concentration (0, 4, 12 and 20%) of polyethylene glycol (PEG) 6000 at different temperatures (30, 40 and 50℃) for 48 h. Bacterial numbers were initially inoculated with 105 cfu/ml and indirectly measured using a spectrophotometer with optical density at 600 nm. Error bars represent the standard errors of the means of the four independent experimental replications. Means followed by the same letter are not significantly different at 5% level by least significant difference test.
Fig. 3. Effect of pretreatment with rhizobacteria on growth of Chinese cabbage seedling (cvs. Ryeokgwang and Buram-3-ho) under drought stress at two different temperatures. Two-weekold seedlings were treated with Bacillus aryabhattai H26-2 and B. siamensis H30-3 for one week and then undergone different concentrations (0, 2 and 4%) of polyethylene glycol (PEG) 6000-mediated drought stresses under two growth temperature regimes. The seedlings were sub-irrigated with water as mocks. Fresh weight (g) of the seedlings was measured after 12 days and 5 days for normal and high temperature conditions, respectively. Error bars represent the standard errors of the means of the four independent experimental replications. Means followed by the same letter are not significantly different at 5% level by least significant difference test.
Fig. 4. Protective effects of Bacillus siamensis H30-3 against fungal pathogens on Chinese cabbage plants. (A) Dual culture assay for in vitro inhibition of mycelial growth of Alternaria brassicicola and Colletotrichum higginsianum by B. siamensis H30-3. The fungal pathogens were co-cultured with the bacterial strain H30-3 for 15 and 12 days at 25℃ for A. brassicicola and C. higginsianum, respectively. (B) Inhibitory mycelial growth measured by half of the fungal colony diameter after co-culture. Error bars represent the standard errors of the means of the six independent experimental replications. Asterisks indicate significant differences as determined by Student’s t-test (P < 0.05). (C) Reduced black spot and anthracnose disease severities on Chinese cabbage plants by the antagonistic B. siamensis H30-3. Bacterial suspension (109 cfu/ml) of B. siamensis H30-3 was foliar sprayed at 1 day prior to challenge inoculations of the fungal pathogens. Disease severities were evaluated at 4 days after fungal inoculation based on 0-5 scales. Error bars represent the standard errors of the means of the four independent experimental replications. Means followed by the same letter are not significantly different at 5% level by least significant difference test.
Fig. 5. Antifungal metabolites production and induced defence response of Chinese cabbage plants by Bacillus siamensis H30-3. (A) PCR-based detection of bacterial genes encoding antimicrobial lipopeptides from B. aryabhattai H26-2 and B. siamensis H30-3 genomes. M, DNA size marker. bacD, bacilysin; bmyA, bacillomycin; fenD, fengycin; ituA, iturin A; srfA, surfactin; zwiA, zwittermicin A. (B) Characteristics of hydrolytic enzyme secretion, siderophore production and phosphate (P)-solubilisation originated from B. siamensis H30-3. In vitro productions of chitinase, protease and cellulase by B. siamensis H30-3 were examined on different agar media described in Materials and methods. (C) Expression of defence-related genes in Chinese cabbage leaves treated with B. siamensis H30-3. Expression of basic glucanase 2 gene BrBGL2, chitinase 1 gene BrChi1, glutathione-S-transferase 1 gene BrGST1 and ascorbate peroxidase 1 gene BrAPX1 was analyzed by semi-quantitative RT-PCR technique. BrActin7 was used as an internal control. The number of PCR cycles of each result is indicated within the right parenthesis.
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