Soybean abiotic stress tolerance is improved by beneficial rhizobacteria in biosolids-amended soil

Abstract

Soybean responds to heat and water deficit by producing ethylene, which in high concentrations reduces plant biomass. One way for soybean to tolerate stress is to associate with rhizobacteria that lower the ethylene concentration by hydrolyzing the molecular precursor with the extracellular enzyme 1-aminocyclopropane-1-carboxylate deaminase (ACC deaminase). Organic fertilizer could enhance soybean stress tolerance by providing essential nutrients to the plant and substrates to rhizobacteria with ACC deaminase activity. The objectives of this study were 1) to evaluate the effects of organic fertilizer and mineral N fertilizer on rhizosphere ACC deaminase activity and ethylene production of soybean exposed to heat or water deficit and subsequently 2) determine if fertilizer-induced responses improve soybean tolerance to heat and water deficit. Soybean was grown in soil amended with organic fertilizer (biosolids or anaerobic digestate) or with mineral N fertilizer (urea) for 9 wk in a greenhouse, then exposed to no stress, heat stress or water deficit for 7 d. In digestate-amended soils, we detected more ACC deaminase (acdS) gene copies in the rhizosphere, while biosolid-treated soil had 33% greater ACC deaminase activity in the rhizosphere than the soil receiving mineral N fertilizer. The ACC deaminase activity was negatively correlated with ethylene production from the soybean-soil system. At the same time, soybean biomass was less affected by heat or water deficit in biosolids and digestate-treated soils, as they were 23–36% taller and had 11–22% more biomass than soybean receiving mineral N fertilizer. The increase in acdS gene copies and ACC deaminase activity in the rhizosphere of organically-fertilized soils has potential for improving soybean tolerance to heat and water deficit.