Non-negligible Effect of Native Rhizobacteria on Cooperation with Plant Growth Regulators Improve Tolerance to Cadmium: A Case Study Using Duckweed Spirodela polyrhiza as Indicating Plant

Plant growth regulators (PGRs) are well known for their ability to improve plants' tolerance to heavy metals. However, in these treatments, the contribution of native rhizobacteria on heightened tolerance to heavy metals is usually not considered, and thus PGR ability is overestimated. In this study, using duckweed Spirodela polyrhiza as indicating plant and according to the responses of it grown under nonsterile and sterile conditions, native rhizobacteria were able to cooperate with PGRs and thereby improved duckweed's tolerance to Cd stress. All the randomly selected bacterial isolates and salicylic acid exerted synergistic effects that improved Cd tolerance, suggesting that active rhizobacteria are not necessarily plant growth-promoting rhizobacteria (PGPRs). Comparative physiological and transcriptomic analyses showed that the cooperation between the native bacterium Pseudomonas and salicylic acid can greatly reduce Cd accumulation and decrease oxidative stress, thus lowering the requirement of antioxidant defense in duckweed; moreover, the synthesis of flavonoids that facilitates reactive oxygen species scavenging in duckweed was significantly induced. This study recommends that the contribution of native rhizobacteria on improved plant tolerance to heavy metals should be considered when applying PGRs.

Automated summary

Plant growth regulators (PGRs) are often overestimated in their ability to improve plants' tolerance to heavy metals due to the neglect of native rhizobacteria's contribution. In this study, it was found that native rhizobacteria can cooperate with PGRs to enhance the tolerance of duckweed Spirodela polyrhiza to Cd stress. The synergistic effects of bacterial isolates and salicylic acid improved Cd tolerance, indicating that active rhizobacteria are not necessarily plant growth-promoting rhizobacteria (PGPRs). Physiological and transcriptomic analyses revealed that the cooperation between the native bacterium Pseudomonas and salicylic acid can reduce Cd accumulation, decrease oxidative stress, and induce the synthesis of flavonoids in duckweed. Therefore, the contribution of native rhizobacteria should be considered when applying PGRs to improve plant tolerance to heavy metals.