Spermidine Exodus and Oxidation in the Apoplast Induced by Abiotic Stress Is Responsible for H2O2 Signatures That Direct Tolerance Responses in Tobacco

Abstract

Polyamines (PAs) exert a protective effect against stress challenges, but their molecular role in this remains speculative. In order to detect the signaling role of apoplastic PA-derived hydrogen peroxide (H 2 O 2 ) under abiotic stress, we developed a series of tobacco (Nicotiana tabacum cv Xanthi) transgenic plants overexpressing or downregulating apoplastic polyamine oxidase (PAO; S-pao and A-pao plants, respectively) or downregulating S-adenosyl-L-methionine decarboxylase (samdc plants). Upon salt stress, plants secreted spermidine (Spd) into the apoplast, where it was oxidized by the apoplastic PAO, generating H 2 O 2 . A-pao plants accumulated less H 2 O 2 and exhibited less programmed cell death (PCD) than did wild-type plants, in contrast with S-pao and samdc downregulating plants. Induction of either stress-responsive genes or PCD was dependent on the level of Spd-derived apoplastic H 2 O 2 . Thus, in wild-type and A-pao plants, stress-responsive genes were efficiently induced, although in the latter at a lower rate, while S-pao plants, with higher H 2 O 2 levels, failed to accumulate stress-responsive mRNAs, inducing PCD instead. Furthermore, decreasing intracellular PAs, while keeping normal apoplastic Spd oxidation, as in samdc downregulating transgenic plants, caused enhanced salinity-induced PCD. These results reveal that salinity induces the exodus of Spd into the apoplast, where it is catabolized by PAO, producing H 2 O 2 . The accumulated H 2 O 2 results in the induction of either tolerance responses or PCD, depending also on the levels of intracellular PAs.

Perturbation of Polyamine Catabolism Can Strongly Affect Root Development and Xylem Differentiation

“…8) similarly to ZmPAO overexpression in the cell wall, suggesting that xylem differentiation and PCD induction are governed by a complex regulatory system in which intracellular PAs and apoplastic PAderived H 2 O 2 are likely integrated in coordinated signaling pathways; this is consistent with a previous hypothesis where salinity-induced cell death was proposed to be governed by the PAs/H 2 O 2 ratio (Moschou et al, 2008b). In this regard, Moschou et al (2008aMoschou et al ( , 2008b reported that the balance between the apoplastic PA catabolism versus the intracellular PA levels plays a central role in plant responses to salinity by governing the cell fate decision, leading to either induction of PCD or to tolerance mechanisms. Specifically, it was proposed that lowering the PA anabolism/PA catabolism ratio promotes PCD (S-ZmPAO and RNAi-SAMDC plants), while if PA anabolism predominates over catabolism, PCD fails to occur, allowing simultaneous induction of stress-responsive genes and plant tolerance (wild-type and A-ZmPAO plants).…”

“…Specifically, it was proposed that lowering the PA anabolism/PA catabolism ratio promotes PCD (S-ZmPAO and RNAi-SAMDC plants), while if PA anabolism predominates over catabolism, PCD fails to occur, allowing simultaneous induction of stress-responsive genes and plant tolerance (wild-type and A-ZmPAO plants). Indeed, the RNAi-SAMDC plants, while keeping normal apoplastic Spd exodus and oxidation, exhibited constitutively lower intracellular PA levels as well as enhanced H 2 O 2 production under salt stress in comparison to wildtype plants, with a PAs/H 2 O 2 ratio analogous to that shown by S-ZmPAO plants, which also exhibited lower Spd and Spm levels and higher H 2 O 2 production than the wild-type plants (Moschou et al, 2008b). In agreement with the hypothesis based on the PA/H 2 O 2 ratio, our results showed that lowering the PAs/H 2 O 2 ratio, by either increasing H 2 O 2 production (S-ZmPAO transgenic plants; Fig.…”

“…8), promoted root differentiation, resulting in a similar plant phenotype. Moreover, vascular terminal differentiation occurred in a similar fashion in A-ZmPAO plants down-regulating endogenous PAO (in which PA anabolism predominates over catabolism and salinity-induced PCD failed to occur; Moschou et al, 2008b) as compared to wild-type plants (not shown). Another possible explanation for the lack of phenotype shown by A-ZmPAO plants could be the involvement of additional and/or alternative H 2 O 2 sources, such as apoplastic CuAOs, that could cooperate with PAOs in H 2 O 2 delivery during tobacco xylem tissue differentiation.…”

“…In tobacco (Nicotiana tabacum), H 2 O 2 derived from apoplastic PA catabolism is a key molecule in PCD signaling during the cryptogein-induced hypersensitive response (HR; Yoda et al, 2006) as well as in the induction of HR marker genes by mitochondrial dysfunction and activation of mitogen-activated protein kinases (Takahashi et al, 2003;Kusano et al, 2007;Moschou et al, 2009). Moreover, it has been recently demonstrated that salinity stress induces the exodus of Spd into the apoplast, resulting in the induction of a PAO-mediated H 2 O 2 burst leading to either tolerance responses or PCD, depending on the levels of intracellular PAs (Moschou et al, 2008b).…”

Speculation: Polyamines are important in abiotic stress signaling

“…H 2 O 2 has long been known as a signal molecule. It is able to mediate various processes, such as stomatal closure, directly due to its ability to influence ion channels, while it can also activate specific stress response processes through the MAPK cascade [47]. Polyamines, especially spermidine, also induce superoxide anion (O 2˙-) production by the activation of NADPHoxidase.…”