Creeping bentgrass (Agrostis stolonifera) is the most widely used putting green species on golf courses where on shorelines or areas with salt afflicted soils. Recycled and reclaimed water, which can have high salt content, is increasingly being implemented or required for turfgrass irrigation. These practices cause both salt and drought stresses that limit turfgrass health, functionality, and can increase inputs and costs of management. Polyamine (PA) including putrescine (Put), spermidine (Spd), and spermine (Spm) are involved in abiotic stress tolerance in plants. However, their roles in stress tolerance can be highly species specific. It is not well-known whether PA plays a major role in stress tolerance of turfgrass species or whether they can be harnessed and used in turf management industry. Therefore, our specific objectives were to determine PA induced drought and salt stress tolerance mechanisms in plants, to evaluate the effects of salt stress on endogenous PA content in salt stressed and non-stressed plants, and to identify differentially expressed genes by transcriptome analysis due to PA and drought stress treatment of creeping bentgrass. Under salt stress, leaf Na+ content increased while leaf K+ and Ca2+ content decreased when compared with non-stressed plants. Salt stress decreased turf quality and leaf osmotic potential but increased leaf electrolyte leakage and canopy temperature depression. Endogenous PA content was altered due to salt stress in creeping bentgrass leaf tissues. Put and Spm content was two to five times higher in salt stressed leaf tissue compared with controls in early salt stressed stages. Application of a relatively low concentration of Spd (500 μM•L-1) promoted tillering rates under optimal growth condition in hydroponics. Association of Spd (500 or 750 μM•L-1) or Spm (500 μM•L-1) treatment to increased membrane health was revealed as greater photochemical efficiency, higher quantum yield, increased leaf relative water content, less electrolyte leakage, and less lipid peroxidation (malondialdehyde content) in PA treated plants compared to control plants. Transcriptome analysis using RNA-sequencing evaluated differentially (DE) expressed genes due to drought and Spd application. De novo assembly and transcriptome alignment showed 22% and 19% of genes were either up- or down-regulated due to drought while 20% and 34% of genes were either up- or down- regulated in response to Spd application, respectively. Gene ontology and enrichment analysis indicated that these DE genes were primarily associated with energy metabolism, transport, antioxidants, photosynthesis, signaling, and stress defense processes. This research is the first to provide transcriptome data for creeping bentgrass under drought stress. DE genes identified here could be further investigated for use as molecular markers or for functional analysis in responses to drought and Spd. Since endogenous PAs are regulated by abiotic stress, exogenous application improved abiotic stress tolerance, and genes involved in stress tolerance were differentially expressed due to PA treatment, PA metabolic pathways may be useful in chemical technologies or in breeding strategies that aim to improve creeping bentgrass stress tolerance.
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