Drought has become the most important limiting factor to crop productions. Research thus far has been devoted to identifying drought-responsive genes (DRGs) via breeding and engineering approaches. Still, these efforts have not resulted in a solution to combat drought's effects because the ectopic expression of most DRGs causes adverse effects that reduce plant growth and yields. Lately, we discovered that two DRGs, Response to Desiccation (RD)29A and RD29B, induced by Paenibacillus polymyxa CR1, a plant growth-promoting rhizobacterium capable of priming drought tolerance and concurrently stimulating plant growth, play pivotal roles in defense responses against drought. In this study, we employ the ChlP and qRT-PCR analyses and further clarify that P. polymyxa CR1 reformats the chromatin/transcriptional memory of RD29s, positioned as upstream controllers that fine-tune the temporal dynamic of stress-regulating transcription factors (TFs) in elaborating induced systemic drought tolerance without growth penalties. Two genes coordinate the upregulation of NAC TFs, while feedback inhibiting CBF TFs, which regulate downstream DRG expressions. This supports that RD29s are unique, feasible transgene candidates for improving plants' survival capacity in both optimal and drought conditions. However, the mode of action of RD29A and RD29B are partly independent, exerting distinct roles in disparate ecological states. When subjected to increasing NaCl concentrations, the KO mutant of RD29A (rd29a) displayed enhanced tolerance compared to WT and rd29b plants, proposing that RD29B, but not RD29A, a key player in conferring WT-like tolerance to salinity stress; further studies will be needed to optimize/maximize their applications in engineering for-profit drought and/or broad-spectrum stress tolerant crops.
Keywords: Drought; Histone modification; Induced systemic tolerance; Plant growth-promoting rhizobacteria (PGPR); Response to desiccation (RD)29.
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