Polyamines are long-chain, aliphatic amines involved in the regulation of plant growth and response to environmental stresses including oxidative stress, drought, salinity, metal toxicity, and chilling stresses (Gill and Tuteja, 2010). PAs are also involved in basic processes such as protein synthesis and RNA and DNA processing (Cohen, 1998; Jimenez-Bremont et al., 2014). The major PAs present in plants are putrescine [Put (diamine)], Spd (triamine), and Spm (tetramine) (Kusano et al., 2007). Specifically, for abiotic stress, PAs are known to be involved in stabilization of plant membranes and cellular structures by binding to membrane phospholipids. They also play a role in OA, modulating ion channels, act as reactive oxygen species scavengers, and can enhance antioxidant enzyme activity during stress (Pottosin and Shabala, 2014; Roychoudhury et al., 2011; Yamaguchi et al., 2007).
Exogenous application of Put, Spd, and Spm can be used to prime plants for stress tolerance. Gupta et al. (2012) reported that foliar spray of PAs in wheat (Triticum aestivum) significantly increased photosynthetic parameters, proline, total amino acids and soluble sugars, improved water status, grain yield, and reduced membrane damage under drought stress. Spd application to rice (Oryza sativa) significantly increased K+/Na+ ratio, Ca2+, and grain yield under salt stress (Saleethong et al., 2013). In a study with bermudagrass (Cynodon dactylon), Shi et al. (2013) reported that exogenous application of PAs (Spd, Spm, and Put) increased the activity of antioxidant enzymes and stress-related proteins under drought and salt stress. Because of their cationic nature, PA interacts with negatively charged macromolecules and stabilizes their structure during stress situations. The benefits of PAs application can be species specific and adequate rates of each PA for turfgrass species have yet to be determined; plants can exhibit phytotoxicity following PA applications. Therefore, determining whether PA applications may improve creeping bentgrass drought tolerance is warranted.
In addition to their role in stress tolerance, PAs are also known to regulate plant growth and development and are classified as plant hormones. PAs play a role in cell division and organ development (Davies, 1995). Recently, Shukla et al. (2015) showed that Spm and Spd applications promoted leaf growth and tillering rates in creeping bentgrass ‘Penn G2’ under optimal conditions, suggesting a role of PAs in regulating turfgrass shoots. Herein, we aim to determine whether creeping bentgrass shoot growth alterations by PAs may be associated with PA regulation of hormone accumulation in plant leaves.
Use of PAs as priming agents to better prepare plants for abiotic stress to improve turfgrass health and to promote growth has great potential practical application in the turf industry. Application of plant growth regulators is routinely employed in turf industry to regulate shoot growth, for seed head suppression and to improve turfgrass quality (Ervin and Koski, 2001). Therefore, the objectives of this study were to evaluate the effects of exogenous applications of Spd and Spm on growth physiology and accumulation patterns of phytohormones including GA isoforms (GA1, GA4, and GA20), salicylic acid (SA), jasmonic acid (JA), indole acetic acid (IAA), and ABA in creeping bentgrass under drought stress. We hypothesize that PAs may regulate GA isoforms to alter shoot growth and hormones involved in drought stress tolerance.
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