Phytohormonal regulation of S-adenosylmethionine synthetase and S-adenosylmethionine levels in dwarf pea epicotyls

Spatial specificity of metabolism regulation of abscisic acid-imposed seed germination inhibition in Korean pine (Pinus koraiensis sieb et zucc)

Introduction

Abscisic acid (ABA) can negatively regulate seed germination, but the mechanisms of ABA-mediated metabolism modulation are not well understood. Moreover, it remains unclear whether metabolic pathways vary with the different tissue parts of the embryo, such as the radicle, hypocotyl and cotyledon.

Methods

In this report, we performed the first comprehensive metabolome analysis of the radicle and hypocotyl + cotyledon in Pinus koraiensis seeds in response to ABA treatment during germination.

Results and discussion

Metabolome profiling showed that following ABA treatment, 67 significantly differentially accumulated metabolites in the embryo were closely associated with pyrimidine metabolism, phenylalanine metabolism, cysteine and methionine metabolism, galactose metabolism, terpenoid backbone biosynthesis, and glutathione metabolism. Meanwhile, 62 metabolites in the hypocotyl + cotyledon were primarily involved in glycerophospholipid metabolism and glycolysis/gluconeogenesis. We can conclude that ABA may inhibit Korean pine seed germination primarily by disrupting the biosynthesis of certain plant hormones mediated by cysteine and methionine metabolism and terpenoid backbone biosynthesis, as well as reducing the reactive oxygen species scavenging ability regulated by glutathione metabolism and shikimate pathway in radicle. ABA may strongly disrupt the structure and function of cellular membranes due to alterations in glycerophospholipid metabolism, and weaken glycolysis/gluconeogenesis in the hypocotyl + cotyledon, both of which are major contributors to ABA-mediated inhibition of seed germination. These results highlight that the spatial modulation of metabolic pathways in Pinus koraiensis seeds underlies the germination response to ABA.

Rice salT promoter is activated in Papaver somniferum and Nicotiana tabacum transgenic cells in the absence of exogenous ABA

With the aim of modifying secondary metabolism in Opium poppy (Papaver somniferum) and tobacco (Nicotiana tabacum) cells, gene transfer was performed using the sam1 gene from Arabidopsis thaliana under the control of the salT promoter. This promoter is induced by ABA in rice and in tobacco and we have shown that it is also induced in poppy cells (gus gene). Putatively transformed poppy and tobacco cell lines with the sam1 gene were obtained. In the absence of exogenous inducer we noticed the expression of the transgene resulting in a significant increase of SAM-S activity in all tested transformants of poppy and in half the transgenic tobacco cell lines tested. Addition of ABA to the culture medium failed to enhance the expression of the transgene in both species and resulted in a decrease of the sam1 gene expression in some cell lines. Since the salT promoter is induced by exogenous ABA in both species (gus reporter gene), we suggest a partial sam1 transgene inactivation in certain cell lines. These results show that the efficiency of a regulatory sequence may be different when fused with a reporter gene (gus) compared to fusion with a gene belonging to the housekeeping family (sam1).

Differential regulation of S-adenosylmethionine synthetase isozymes by gibberellic acid in dwarf pea epicotyls

Dwarf pea epicotyls contained a single activity peak of S-adenosylmethionine (AdoMet) synthetase (isozyme I). Gibberellic acid (GA3, 1 microM) induced two additional isozymes (II and III). Cycloheximide (20 microgram/ml) blocked the appearance of GA3-induced isozymes, suggesting that it is dependent on de novo protein synthesis. Conclusive proof was obtained by labelling the isozymes II and III with [35S]SO2-(4) in vivo. The purified 35S-labelled AdoMet synthetase isozymes (II, III) showed a single protein band that coincided with the single radioactive peak on SDS-PAGE. Molecular-sieve chromatography of the isozyme I from control dwarf pea epicotyls and three isozymes of GA3-treated epicotyls on Sepharose CL-6B showed a single activity peak with an identical molecular mass of 174 kDa for each isozyme. Analysis of purified AdoMet synthetase isozymes (I, II, III) on SDS-PAGE showed a single silver-stained protein band with a molecular mass of 87 kDa. This proved the dimeric nature of all the isozymes of AdoMet synthetase which could be physically separated by ion-exchange chromatography on DE-52. In vitro molecular hybridization of physically separated isozymes by NaCl-freeze-thaw treatment method revealed that the three isozymes (I, II, III) in GA3-treated dwarf pea epicotyls are formed through the random dimerization of two different species of enzyme subunits that differ in their net charge. Thus, the two flanking activity peaks (isozymes I, III) represent homodimers, while the middle activity peak (isozyme II) is a heterodimer. Apparently, the single isozyme I in control epicotyls is a product of one gene of AdoMet synthetase (SAM 1), while three isozymes in GA3-treated epicotyls are the product of two genes of AdoMet synthetase. We speculate that the differential regulation of AdoMet synthetase in GA3-treated epicotyls is achieved by the expression of an alternate gene of AdoMet synthetase (SAM 2).

Phytohormonal regulation of S-adenosylmethionine synthetase by gibberellic acid in wheat aleurones

Gibberellic acid (GA3) brought about a 3-fold stimulation of AdoMet synthetase activity in wheat aleurones. At the qualitative level, three isozymes of AdoMet synthetase were observed by DE-52 chromatography in GA3-treated wheat aleurones. In contrast, the control wheat aleurones showed a single isozyme. Thus the phytohormone (GA3, 1 microM) induced two additional isozymes of AdoMet synthetase in wheat aleurones. The activity of all the three isozymes in GA3-treated aleurones was considerably decreased by the simultaneous presence of abscisic acid (ABA, 10 microM). Cycloheximide (20 micrograms/ml) also significantly lowered the levels of the three isozymes of AdoMet synthetase in Ga3-treated aleurones, thereby suggesting the requirement of de-novo protein synthesis for the complete induction of isozymes. However, wheat aleurones excised from embryonated wheat seeds, did not require the application of GA3 for the induction of two additional isozymes of AdoMet synthetase. Apparently, the transport of GA3 from the embryo to aleurones induced two new isozymes of AdoMet synthetase. Three isozymes of AdoMet synthetase were also observed in wheat embryos excised from germinated wheat grains, without exogenous application of GA3. The molecular weight of all the three isozymes of AdoMet synthetase in wheat system is 181,000. The molecular weight of the subunit of the enzyme is 84,000. The dimeric nature of AdoMet synthetase was established by SDS-PAGE analysis of the purified enzyme. In-vitro hybridization of two flanking isozymic peaks I and III by NaCl-freeze-thaw method resulted in the appearance of an additional middle activity peak (isozyme II). However, no additional isozymic peaks were generated when isozymic peaks I and III were individually given a freeze-thaw treatment. Thus the flanking isozymic peaks I and III represent homodimers that differed in their net charge. In contrast, the middle isozymic activity peak II, when subjected to NaCl-freeze-thaw treatments yielded two additional isozymic peaks, I and III, thereby suggesting its heterodimeric nature. We envisage that the three isozymes in GA3-treated wheat aleurone layers are formed by the random dimerization of two classes of enzyme subunits. The two enzyme subunits which differ in their net charge could be the product of two genes of AdoMet synthetase (SAM1 and SAM2). Based on this assumption, we propose that a single isozyme I in water imbibed control wheat aleurones is the product of SAM1 gene of AdoMet synthetase. The occurrence of three isozymes in GA3-treated aleurones could be ascribed to the expression of an alternate gene of AdoMet synthetase (SAM2 gene).(ABSTRACT TRUNCATED AT 400 WORDS)