Abstract
Our previous work showed that a consortium of three plant growth-promoting rhizobacterium (PGPR) strains (Bacillus cereus AR156, Bacillus subtilis SM21, and Serratia sp. XY21), termed as BBS for short, was a promising biocontrol agent. The present study investigated its effect on drought tolerance in cucumber plants. After withholding watering for 13 days, BBS-treated cucumber plants had much darker green leaves and substantially lighter wilt symptoms than control plants. Compared to the control, the BBS treatment decreased the leaf monodehydroascorbate (MDA) content and relative electrical conductivity by 40% and 15%, respectively; increased the leaf proline content and the root recovery intension by 3.45-fold and 50%, respectively; and also maintained the leaf chlorophyll content in cucumber plants under drought stress. Besides, in relation to the control, the BBS treatment significantly enhanced the superoxide dismutase (SOD) activity and mitigated the drought-triggered down-regulation of the expression of the genes cAPX, rbcL, and rbcS encoding cytosolic ascorbate peroxidase, and ribulose-1,5-bisphosphate carboxy/oxygenase (Rubisco) large and small subunits, respectively, in cucumber leaves. However, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity was undetected in none of the culture solutions of three BBS constituent strains. These results indicated that BBS conferred induced systemic tolerance to drought stress in cucumber plants, by protecting plant cells, maintaining photosynthetic efficiency and root vigor and increasing some of antioxidase activities, without involving the action of ACC deaminase to lower plant ethylene levels.
Highlights
Due to their sessile nature, plants are constantly faced with abiotic and biotic stresses during their whole life time
Aiming at assessing its potential for inducing cucumber drought tolerance, here, we examined the effects of BBS on a range of physiological indicators of drought tolerance, the activities of antioxidant enzymes, and expression profiles of the genes encoding cytosolic ascorbate peroxidase and ribulose1,5-bisphosphate carboxy/oxygenase (Rubisco) large and small subunits in cucumber plants experiencing drought stress
Treated cucumber plants displayed much darker green and lighter wilt symptoms than those of control plants, which were unable to rehydrate at night or in an early morning (Figure 1)
Summary
Due to their sessile nature, plants are constantly faced with abiotic and biotic stresses during their whole life time. A range of abiotic and biotic elicitors can confer tolerance to drought stress in plants The former includes alginatederived oligosaccharides [5], ketoconazole [6], CO2 laser [7], 2aminoethanol [8], abscisic acid (ABA) [9], and brassinosteroids [10,11], while the latter comprises rhizobia [12,13], mycorrhizal fungi [14,15,16,17,18], endophytic fungus [19,20] as well as additional types of beneficial microorganisms [21,22,23,24]. Plant growthpromoting rhizobacterium (PGPR) such as Bacillus cereus AR156 can enhance disease resistance [25]
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