The protein phosphatase 2A catalytic subunit StPP2Ac2b acts as a positive regulator of tuberization induction in Solanum tuberosum L.

KEY MESSAGE: This study provides the first genetic evidence for the role of PP2A in tuberization, demonstrating that the catalytic subunit StPP2Ac2b positively modulates tuber induction, and that its function is related to the regulation of gibberellic acid metabolism. The results contribute to a better understanding of the molecular mechanism controlling tuberization induction, which remains largely unknown. The serine/threonine protein phosphatases type 2A (PP2A) are implicated in several physiological processes in plants, playing important roles in hormone responses. In cultivated potato (Solanum tuberosum), six PP2A catalytic subunits (StPP2Ac) were identified. The PP2Ac of the subfamily I (StPP2Ac1, 2a and 2b) were suggested to be involved in the tuberization signaling in leaves, where the environmental and hormonal signals are perceived and integrated. The aim of this study was to investigate the role of PP2A in the tuberization induction in stolons. We selected one of the catalytic subunits of the subfamily I, StPP2Ac2b, to develop transgenic plants overexpressing this gene (StPP2Ac2b-OE). Stolons from StPP2Ac2b-OE plants show higher tuber induction rates in vitro, as compared to wild type stolons, with no differences in the number of tubers obtained at the end of the process. This effect is accompanied by higher expression levels of the gibberellic acid (GA) catabolic enzyme StGA2ox1. GA up-regulates StPP2Ac2b expression in stolons, possibly as part of the feedback system by which the hormone regulates its own level. Sucrose, a tuber-promoting factor in vitro, increases StPP2Ac2b expression. We conclude that StPP2Ac2b acts in stolons as a positive regulator tuber induction, integrating different tuberization-related signals mainly though the modulation of GA metabolism.

Characterization of StPPI1, a proton pump interactor from Solanum tuberosum L. that is up-regulated during tuber development and by abiotic stress.

Plasma membrane proton pumps (PM H(+)-ATPases) are involved in several physiological processes, such as growth and development, and abiotic stress responses. The major regulators of the PM H(+)-ATPases are proteins of the 14-3-3 family, which stimulate its activity. In addition, a novel interaction partner of the AHA1 PM H(+)-ATPase, named PPI1 (proton pump interactor, isoform 1), was identified in Arabidopsis thaliana. This protein stimulates the activity of the proton pump in vitro. In this work, we report the characterization of an A. thaliana PPI1 homolog in potato (Solanum tuberosum L.) named StPPI1. The full-length coding sequence of StPPI1 was obtained. The open reading frame (ORF) encodes a protein of 629 amino acids showing 50% identity with A. thaliana PPI1 protein. The StPPI1 ORF is divided into seven exons split by six introns. Southern blot analysis suggests that StPPI1 belongs to a family of related genes. Recombinant StPPI1 stimulates H(+)-ATPase activity in vitro. Basal levels of StPPI1 transcripts are observed in all tissues, however, StPPI1 expression is higher in proliferative regions (shoot apex and flower buds), flowers and leaves than in shoots and roots. StPPI1 mRNA levels significantly increase during tuber development. StPPI1 is induced by salt stress and cold. Drought and mechanical wounding slightly increase StPPI1 transcript levels. In addition, the expression of SlPPI1, the tomato homolog of StPPI1, was determined under adverse environmental conditions in tomato plants. SlPPI1 mRNA levels are increased by drought and cold, but are unaffected by salt stress. Mechanical wounding slightly increases SlPPI1 expression.

Protein phosphatases type 2A mediate tuberization signaling in Solanum tuberosum L. leaves.

Tuber formation in potato (Solanum tuberosum L.) is regulated by hormonal and environmental signals that are thought to be integrated in the leaves. The molecular mechanisms that mediate the responses to tuberization-related signals in leaves remain largely unknown. In this study we analyzed the roles of protein phosphatase type 2A catalytic subunits (PP2Ac) in the leaf responses to conditions that affect tuberization. The responses were monitored by analyzing the expression of the "tuber-specific" genes Patatin and Pin2, which are induced in tubers and leaves during tuber induction. Experiments using PP2A inhibitors, together with PP2Ac expression profiles under conditions that affect tuberization indicate that high sucrose/nitrogen ratio, which promotes tuber formation, increases the transcript levels of Patatin and Pin2, by increasing the activity of PP2As without affecting PP2Ac mRNA or protein levels. Gibberellic acid (GA), a negative regulator of tuberization, down-regulates the transcription of catalytic subunits of PP2As from the subfamily I and decreases their enzyme levels. In addition, GA inhibits the expression of Patatin and Pin2 possibly by a PP2A-independent mechanism. PP2Ac down-regulation by GA may inhibit tuberization signaling downstream of the inductive effects of high sucrose/nitrogen ratio. These results are consistent with the hypothesis that PP2As of the subfamily I may positively modulate the signaling pathways that lead to the transcriptional activation of "tuber-specific" genes in leaves, and act as molecular switches regulated by both positive and negative modulators of tuberization.

Serine/threonine protein phosphatases type 2A and their roles in stress signaling.

Serine/threonine protein phosphatases are ubiquitous enzymes in all eukaryotes but many of their physiological roles in plants remain unknown. The available results have demonstrated critical functions for these enzymes in the regulation of adaptive stress responses, and recent studies have directed attention to the functional roles of Ser/Thr phosphatases type 2A (PP2A) as components of stress signaling pathways. This review is focused primarily on plant PP2As and their participation in the control of biotic and abiotic stress responses.

Characterization of potato (Solanum tuberosum) and tomato (Solanum lycopersicum) protein phosphatases type 2A catalytic subunits and their involvement in stress responses.

Protein phosphorylation/dephosphorylation plays critical roles in stress responses in plants. This report presents a comparative characterization of the serine/threonine PP2A catalytic subunit family in Solanum tuberosum (potato) and S. lycopersicum (tomato), two important food crops of the Solanaceae family, based on the sequence analysis and expression profiles in response to environmental stress. Sequence homology analysis revealed six isoforms in potato and five in tomato clustered into two subfamilies (I and II). The data presented in this work show that the expression of different PP2Ac genes is regulated in response to environmental stresses in potato and tomato plants and suggest that, in general, mainly members of the subfamily I are involved in stress responses in both species. However, the differences found in the expression profiles between potato and tomato suggest divergent roles of PP2A in the plant defense mechanisms against stress in these closely related species.

Wounding increases salt tolerance in tomato plants: evidence on the participation of calmodulin-like activities in cross-tolerance signalling.

Cross-tolerance is the phenomenon by which a plant resistance to a stress results in resistance to another form of stress. It has previously been shown that salt stress causes the accumulation of proteinase inhibitors and the activation of other wound-related genes in tomato plants (Solanum lycopersicum). However, very little is known about how different stresses interact with one another, and which are the signalling components that interrelate the responses triggered by different stress types. In the present work, it is shown that mechanical wounding increases salt-stress tolerance in tomato plants through a mechanism that involves the signalling peptide systemin and the synthesis of JA. Data are also provided indicating that calmodulin-like activities are necessary for the downstream signalling events that lead to cross-tolerance between wounding and salt stress. Finally, evidence was gathered supporting the hypothesis that LeCDPK1, a Ca2+ -dependent protein kinase from tomato previously described in our laboratory, could participate in this cross-tolerance mechanism interrelating the signalling responses to wounding and salt stress.