The drought-tolerant rhizobacterium, Pseudomonas putida AKMP7, suppresses polyamine accumulation under well-watered conditions and diverts putrescine into GABA under water-stress, in Oryza sativa

Drought-tolerant plant growth promoting rhizobacteria (PGPR) have been reported to alleviate water-stress in plants that they associate with. Here, we report the impact of a free-living, drought-tolerant PGPR strain, Pseudomonas putida AKMP7 on (a) morpho-physiological responses and (b) metabolism of the three major polyamines-namely, putrescine (Put), spermidine (Spd) and spermine (Spm) in well-watered and water-stressed Oryza sativa (BPT5204). The bacterium significantly improved the plant growth and biomass, decreased membrane damage, electrolyte leakage and plant proline levels under water-stress, indicating drought alleviation. Dehydration did not affect the root colonization efficiency of the bacterium. We observed that the concentrations of all three polyamines were downregulated in rice leaves under well-watered conditions by AKMP7 inoculation. Under water-stress, however, AKMP7 caused a significant decrease only in the levels of foliar Spm, while Put and Spd levels did not majorly fluctuate. Transcript levels of most of the polyamine biosynthetic genes studied (AIH involved in Put biosynthesis; SPD/SPM2, SPD/SPM3 and SAMDC2, involved in Spd and Spm biosynthesis) positively correlated with the low polyamine levels in the inoculated plants, vs. the non-inoculated ones under well-watered conditions. While similar trends were observed in the water-stressed, inoculated plants vs. water-stressed, non-inoculated plants, the reduction in only Spm (and not Put and Spd) was a potential consequence of the interplay of both, its biosynthesis and back-conversion. In water-stressed plants, AKMP7 caused a small downregulation in AIH, while stronger downregulations in the expressions of SPD/SPM2 as well as SAMDC2 were seen. Furthermore, the Spd to Put back-conversion gene, PAO3 exhibited a downregulation on AKMP7 treatment under water-stressed conditions. There was also an increase in the expression of BADH2 gene, leading to a concomitant increase in foliar GABA levels in inoculated, water-stressed plants vs. their non-inoculated counterparts. The overall conclusion from this study is- P. putida AKMP7 tightly regulates polyamine homeostasis in rice plants, both under well-watered and water-stressed conditions by lowering polyamine levels, through (a) transcriptional control of polyamine biosynthetic genes under well-watered conditions and (b) a combination of biosynthesis, back-conversion and catabolism of polyamines under water-stress.