Growth-Stimulatory Effect of Quorum Sensing Signal Molecule N-Acyl-Homoserine Lactone-Producing Multi-Trait Aeromonas spp. on Wheat Genotypes Under Salt Stress.

Muhammad Shoib Nawaz,Muhammad Ahmad,Sami Ullah,Kaneez Fatima,Asma Imran,Ayesha Arshad,Lubna Rajput

doi:10.3389/fmicb.2020.553621

Abstract

Salinity is one of the major threats to agricultural productivity worldwide. Soil and plant management practices, along with inoculation with plant-beneficial bacteria, play a key role in the plant’s tolerance toward salinity stress. The present study demonstrates the potential of acyl homoserine lactone (AHL)-producing plant growth promoting rhizobacteria (PGPR) strains of Aeromonas sp., namely, SAL-17 (accession no. HG763857) and SAL-21 (accession no. HG763858), for growth promotion of two wheat genotypes inherently different for salt tolerance potential. AHLs are the bacterial signal molecules that regulate the expression of various genes in bacteria and plants. Both Aeromonas spp., along with innate plant-growth-promoting (PGP) and salt tolerance traits, showed AHL production which was identified on tandem mass spectrometry as C6-HSL, 3-OH-C5-HSL, 3-OH-C6-HSL, 3-oxo-C7-HSL C10-HSL, 3-oxo-C10-HSL, 3-OH-C10-HSL, 3-oxo-C12-HSL and C6-HSL, and 3-oxo-C10-HSL. The exogenous application of purified AHLs (mix) significantly improved various root parameters at 200 mM NaCl in both salt-sensitive (SSG) and salt-tolerant (STG) genotypes, where the highest increase (≈80%) was observed where a mixture of both strains of AHLs was used. Confocal microscopic observations and root overlay assay revealed a strong root colonization potential of the two strains under salt stress. The inoculation response of both STG and SSG genotypes was evaluated with two AHL-producing strains (SAL-17 and SAL-21) and compared to non-AHL-producing Aeromonas sp. SAL-12 (accession no. HG763856) in saline (EC = 7.63 ms/cm2) and non-saline soil. The data reveal that plants inoculated with the bacterial consortium (SAL-21 + SAL-17) showed a maximum increase in leaf proline content, nitrate reductase activity, chlorophyll a/b, stomatal conductance, transpiration rate, root length, shoot length, and grain weight over non-inoculated plants grown in saline soil. Both STG and SSG showed relative effectiveness toward inoculation (percent increase for STG: 165–16%; SSG: 283–14%) and showed a positive correlation of grain yield with proline and nitrate reductase activity. Furthermore, principal component analysis (PCA) and categorical PCA analysis clearly showed an inoculation response in both genotypes, revealing the effectiveness of AHL-producing Aeromonas spp. than the non-AHL-producing strain. The present study documents that the consortium of salt-tolerant AHL-producing Aeromonas spp. is equally effective for sustaining the growth of STG as well as SSG wheat genotypes in saline soil, but biosafety should be fully ensured before field release.

Highlights

Salinity is edaphic stress that has affected 45 million hectares out of 230 million hectares of irrigated land, causing annual losses of about US$ 12 billion worldwide (FAO, 2020), and is a major threat to global agricultural productivity
The present study has demonstrated the production of different Acyl homoserine lactones (AHLs), varying in acyl chain length (C5–C12), from halotolerant, plant-beneficial Aeromonas spp. strains isolated from wheat rhizosphere and their subsequent growth-promoting effect on two wheat genotypes under salt stress
The 16S rRNA gene sequences of Aeromonas spp. strains SAL-17, SAL-21, and SAL-12 were aligned to highly similar sequences using multiple sequence alignment, and phylogeny was determined by maximum likelihood method (Jukes and Cantor, 1969) using a MEGA6 software package (Kumar et al, 2016)

Summary

Introduction

Salinity is edaphic stress that has affected 45 million hectares out of 230 million hectares of irrigated land, causing annual losses of about US$ 12 billion worldwide (FAO, 2020), and is a major threat to global agricultural productivity. Salinity decreases the agricultural production of all major crops and deteriorates the structure and the ecological functioning of the soil. The first phase of plant response to salinity is characterized by the release of phytohormones, mainly abscisic acid (Ismail et al, 2014), the expression of reactive oxygen species (ROS)-scavenging enzymes (Bharti et al, 2013), and the accumulation of osmoprotectants such as proline (Sneha et al, 2013; Iqbal et al, 2014). The second phase of plant response is characterized by Na+ exclusion from xylem parenchyma cells via plasma membrane porter HKT1 (Lv et al, 2012; Munns et al, 2012), SOS 1 Na+/H+ antiporter (Ariga et al, 2013), Na+ storage into vacuoles via vacuolar Na+/H+ antiporter (Kronzucker and Britto, 2011), or Na+ compartmentalization (Garcia de la Garma et al, 2015)

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Discussion

Conclusion