Abstract
Roles of the major polyamines (mPA), putrescine, spermidine, and spermine (Spm), in various developmental and physiological processes in plants have been well documented. Recently, there has been increasing focus on the link between mPA metabolism and defense response during plant-stress interactions. Empirical evidence is available for a unique role of Spm, distinct from the other mPA, in eliciting an effective defense response to (a)biotic stresses. Our understanding of the precise molecular mechanism(s) by which Spm modulates these defense mechanisms is limited. Further analysis of recent studies indicates that plant Spm functions differently during biotic and abiotic interactions in the regulation of oxidative homeostasis and phytohormone signaling. Here, we summarize and integrate current knowledge about Spm-mediated modulation of plant defense responses to (a)biotic stresses, highlighting the importance of Spm as a potent plant defense activator with broad-spectrum protective effects. A model is proposed to explain how Spm refines defense mechanisms to tailor an optimal resistance response.
Highlights
Polyamines are ubiquitous, small aliphatic polycations found in eukaryotic organisms
These findings suggest that Spm is a stress-associated signaling molecule (Yamakawa et al, 1998) due to its unique role in inducing several components of the plant defense response, including: (i) genes coding for pathogenesis related (PR) and resistance (R) proteins (Yamakawa et al, 1998; Gonzalez et al, 2011); (ii) mitogen-activated protein kinases (MAPK) (Takahashi et al, 2003; Gonzalez et al, 2011); (iii) several defense-associated transcription factors (Mitsuya et al, 2009; Gonzalez et al, 2011); (iv) phytoalexin biosynthesis (Marco et al, 2014; Mo et al, 2015); and, (v) the hypersensitive response (HR) (Takahashi et al, 2004; Sagor et al, 2009)
Put is successively converted to Spd by Spd synthase, and to Spm by Spm synthase. The latter reactions require the addition of aminopropyl groups, supplied from decarboxylated S-adenosylmethionine (SAM), which is a product of SAM decarboxylase (SAMDC)
Summary
Polyamines are ubiquitous, small aliphatic polycations found in eukaryotic organisms. An Arabidopsis mutant deficient in Spm biosynthesis exhibits hypersensitivity to salt and drought stresses, and the phenotype is mitigated by exogenous Spm, but not Put or Spd (Yamaguchi et al, 2006; Kusano et al, 2007) Together, these findings suggest that Spm is a stress-associated signaling molecule (Yamakawa et al, 1998) due to its unique role in inducing several components of the plant defense response, including: (i) genes coding for pathogenesis related (PR) and resistance (R) proteins (Yamakawa et al, 1998; Gonzalez et al, 2011); (ii) mitogen-activated protein kinases (MAPK) (Takahashi et al, 2003; Gonzalez et al, 2011); (iii) several defense-associated transcription factors (Mitsuya et al, 2009; Gonzalez et al, 2011); (iv) phytoalexin biosynthesis (Marco et al, 2014; Mo et al, 2015); and, (v) the hypersensitive response (HR) (Takahashi et al, 2004; Sagor et al, 2009). A model is proposed to explain how Spm regulates various oxidative and hormone signaling pathways, which tailor an optimal defense response to various external stresses
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