Plant-Growth-Promoting Bacteria Effects on Stress-Tolerating Salt Capacity

Abstract

Environmental stressors, including climate-induced saltwater intrusion and sea level rise, are challenging coastal agricultural production. The increasing intensity of saltwater intrusion contributes to crop yield declines. Salt stress causes water to leave roots through epidermal cells, ultimately decreasing plant growth. Plant-microbe mutualisms, species interactions where both partners benefit, can be harnessed to enhance crop production. For example, plants benefit from metabolites and nutrients that microbial mutualists release, while microbes benefit from plant-derived organic carbon resources. However, how salt-tolerant soil bacteria with plant-growth-promoting potential influence plant growth under excess salt conditions is unknown. Therefore, this experiment evaluates how plant-growth-promoting bacteria (PGPB) can buffer or protect plants from salinity stress. I tested how a simplified salt marsh bacterial community influenced Scarlett rice (Oryza sativa) seedlings exposed to a saltwater gradient. I isolated soil bacteria Priestia aryabhattai, Streptomyces violarus, Bacillus hominis, and Bacillus paramycoides, from a coastal North Carolina salt marsh. These bacteria can potentially provide growth-promoting metabolites that could help protect the plant host against salt stress. I observed variation in morphological and growth rates across the four coastal marsh sediment bacterial isolates. To test whether these bacteria protect rice plants from salt stress, we set up a replicated (n=9) factorial experiment where I added a simplified PGPB community (seeds treated with bacteria or no addition) across four salt levels (freshwater 0 PSU, 2 PSU, 5 PSU, 10 PSU using Instant Ocean) to total 72 experimental units. I measured plant height each week, for 4 weeks. Plant height was significantly higher when the simplified PGPB community was added compared to no addition control samples at lower salinity levels (0 and 2 PSU), but this PGPB effect was not observed at higher salinity levels (5 and 10 PSU). This suggests, the simplified bacterial community experienced a reduction in growth-promoting functions at these salinity levels during the early growth of the rice plants. This work is important for understanding sustainable approaches to enhancing crop resilience against climate change-induced salinity stress. Future research should evaluate the compatibility of these bacterial strains in soil and test their ability to combat more salinity levels, as there might be a threshold where they become suppressed.