Abstract
Application of plant growth promoting bacteria may induce plant salt stress tolerance, however the underpinning microbial and plant mechanisms remain poorly understood. In the present study, the specific role of phenazine production by rhizosphere-colonizing Pseudomonas in mediating the inhibitory effects of salinity on wheat seed germination and seedling growth in four different varieties was investigated using Pseudomonas chlororaphis 30-84 (wild type) and isogenic derivatives deficient or enhanced in phenazine production. The results showed that varieties differed in how they responded to the salt stress treatment and the benefits derived from colonization by P. chlororaphis 30-84. In all varieties, the salt stress treatment significantly reduced seed germination, and in seedlings, reduced relative water content, increased reactive oxygen species (ROS) levels in leaves, and in three of four varieties, reduced shoot and root production compared to the no salt stress treatment. Inoculation of seeds with Pseudomonas chlororaphis 30-84 wild type or derivatives promoted salt-stress tolerance in seedlings of the four commercial winter wheat varieties tested, but the salt-stress tolerance phenotype was not entirely due to phenazine production. For example, all P. chlororaphis derivatives (including the phenazine-producing mutant) significantly improved relative water content in two varieties, Iba and CV 1, for which the salt stress treatment had a large impact. Importantly, all P. chlororaphis derivatives enabled the salt inhibited wheat varieties studied to maintain above ground productivity in saline conditions. However, only phenazine-producing derivatives enhanced the shoot or root growth of seedlings of all varieties under nonsaline conditions. Notably, ROS accumulation was reduced, and antioxidant enzyme (catalase) activity enhanced in the leaves of seedlings grown in saline conditions that were seed-treated with phenazine-producing P. chlororaphis derivatives as compared to noninoculated seedlings. The results demonstrate the capacity of P. chlororaphis to improve salt tolerance in wheat seedlings by promoting plant growth and reducing osmotic stress and a role for bacterial phenazine production in reducing redox stress.
Highlights
Increasing soil salinity is a worldwide problem that is detrimental to plant growth, crop production, food security, and the livelihoods of farmers
In the absence of any varietal information on salt stress tolerance, we prescreened 20 winter wheat varieties for potential salt sensitivity based on root border cell production, given the potential for mucilage secreted by border cells to contribute to salt tolerance
The effects of bacterial seed-inoculation on growth was measured by comparing seedlings of Iba, CV 1, TAM 112 and TAM 113 receiving no inoculum or inoculum comprised of 30-84 ZN, 3084 WT, or 30-84 Enh grown in salt or no salt treatments
Summary
Increasing soil salinity is a worldwide problem that is detrimental to plant growth, crop production, food security, and the livelihoods of farmers. Saline soils are defined as having an electrical conductivity of the saturation extract (ECe) greater than 4 dS/ m (approximately 40 mM NaCl) at 25°C (Richards, 1954), and at this ECe, yields of most crops are reduced, many crops exhibit yield reductions at lower values (Munns, 2005). Under severe salinity (ECe > 16dS/m, ~160 mM NaCl), average yields of most agronomically important crops such as rice, wheat, maize, sorghum, sugarcane, potato, sugar beet soybean and sweet potato, are reduced by 50% (Panta et al, 2014). In addition to ion type and salt concentration, soil type, plant species (or varieties), plant developmental stage, stress distribution, cultivation system and weather conditions are strong determinants of the impacts of salt stress on plants (Morton et al, 2019)
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