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

Heterologous expression and characterization of a thermoalkaliphilic SAM-synthetase from giant leucaena (Leucaena leucocephala subsp glabrata)

The cDNA encoding S-adenosylmethionine (SAM) synthetase was isolated from giant leucaena (Leucaena leucocephala subsp. glabrata) root tissue mRNA. Transcriptome data and 5'-RLM-RACE were used to obtain the transcript sequence and clone into the T₇-expression vector pEt14b. N-terminal Histidine-tagged recombinant protein was expressed highly in Escherichia coli, purified and characterized by activity assays. A straightforward method using isocratic reverse-phase HPLC analysis (mobile phase: 0.02M o-phosphoric acid) of enzyme assays determined optimal enzyme activity at pH 10.0, 55 °C and 200 mM KCl. In addition to thermophilic activity, giant leucaena SAM-synthetase remains highly active in solutions containing up to 4 M KCl and accepts Na⁺ to some extent as a substitute for K⁺, a known required cofactor for SAM-synthetases. The enzyme followed Michaelis-Menten kinetics (K_m = 1.82 mM, K_cat = 1.17 s^-1, V_max 243.9 μM. min^-1) and was not inhibited by spermidine, spermine or nicotianamine. Giant leucaena SAM-synthetase is a highly tolerant enzyme to extreme conditions, suggesting further studies on plant SAM-synthetases.

Calcium-dependent protein kinase CPK28 targets the methionine adenosyltransferases for degradation by the 26S proteasome and affects ethylene biosynthesis and lignin deposition in Arabidopsis

S-adenosylmethionine (AdoMet) is synthesized by methionine adenosyltransferase (MAT), and plays an essential role in ethylene biosynthesis and other methylation reactions. Despite increasing knowledge of MAT regulation at transcriptional levels, how MAT is post-translationally regulated remains unknown in plant cells. Phosphorylation is an important post-translational modification for regulating the activity of enzymes, protein function and signaling transduction. Using molecular and biochemical approaches, we have identified the phosphorylation of MAT proteins by calcium-dependent protein kinase (CPK28). Phenotypically, both MAT2-overexpressing transgenic plants and cpk28 mutants display short hypocotyls and ectopic lignifications. Their shortened hypocotyl phenotypes are caused by ethylene overproduction and rescued by ethylene biosynthesis inhibitor aminoethoxyvinylglycine treatment. Genetic evidence reveals that MAT2 mutation restores the phenotype of ectopic lignification in CPK28-deficient plants. We find that total MAT proteins and AdoMet are increased in cpk28 mutants, but decreased in CPK28-overexpressing seedlings. We also find that MATs in OE::CPK28 are degraded through the 26S proteasome pathway. Our work suggests that CPK28 targets MATs (MAT1, MAT2 and MAT3) for degradation by the 26S proteasome pathway, and thus affects ethylene biosynthesis and lignin deposition in Arabidopsis.

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 expression of the S-adenosyl-L-methionine synthase genes during pea development

Two genes coding for S-adenosyl-l-methionine synthase (SAMS, EC 2.5. 1.6) were previously isolated from pea (Pisum sativum) ovaries. Both SAMS genes were highly homologous throughout their coding regions but showed a certain degree of sequence divergence within the 5' and the 3' untranslated regions. These regions have been used as gene-specific probes to analyze the differential expression of SAMS1 and SAMS2 genes in pea plants. The ribonuclease protection assay revealed different expression patterns for each individual gene. SAMS1 was strongly expressed in nearly all tissues, especially in roots. SAMS2 expression was weaker, reaching its highest level at the apex. Following pollination, SAMS1 was specifically up-regulated, whereas SAMS2 was expressed constitutively. The up-regulation of SAMS1 during ovary development was also observed in unpollinated ovaries treated with auxins. In unpollinated ovaries an increase in SAMS1 expression was observed as a consequence of ethylene production associated with the emasculation process. In senescing ovaries both SAMS1 and SAMS2 genes showed increased expression. Ethylene treatment of unpollinated ovaries led to an increase in the SAMS1 mRNA level. However, SAMS2 expression remained unchangeable after ethylene treatment, indicating that SAMS2 induction during ovary senescence was not ethylene dependent. SAMS mRNAs were localized by in situ hybridization at the endocarp of developing fruits and in the ovules of senescing ovaries. Our results indicate that the transcriptional regulation of SAMS genes is developmentally controlled in a specific way for each gene.

The biosynthesis and metabolism of the aspartate derived amino acids in higher plants

The essential amino acids lysine, threonine, methionine and isoleucine are synthesised in higher plants via a common pathway starting with aspartate. The regulation of the pathway is discussed in detail, and the properties of the key enzymes described. Recent data obtained from studies of regulation at the gene level and information derived from mutant and transgenic plants are also discussed. The herbicide target enzyme acetohydroxyacid synthase involved in the synthesis of the branched chain amino acids is reviewed.

Hormonal regulation of S-adenosylmethionine synthase transcripts in pea ovaries

Two cDNA clones coding for S-adenosyl-L-methionine synthase (SAMs, EC 2.5.1.6) have been isolated from a cDNA library of gibberellic acid-treated unpollinated pea ovaries. Both cDNAs were sequenced showing a high degree of identity but coding for different SAMs polypeptides. The presence of two SAMs genes in pea was further confirmed by Southern analysis. Expression of the SAMs genes in the pea plant was found at different levels in vegetative and reproductive tissues. We characterized the expression levels of SAMs genes during the development or senescence of pea ovaries. Northern analysis showed that transcription of SAMs genes in parthenocarpic fruits was upregulated by auxins in the same manner as in fruits from pollinated ovaries. In both pollinated and 2,4-dichlorophenoxyacetic acid-treated ovaries, and benzyladenine, although able to induce parthenocarpic development, did not affect SAMs mRNA levels. These data are consistent with an active participation of auxins in the upregulation of SAMs during fruit setting in pea and suggest that, at the molecular level, parthenocarpic development of pea ovaries is different for gibberellin- and cytokinin-treated ovaries than for auxin-induced parthenocarpic biosynthesis since treatment of the ovaries with aminoethoxyvinylglycine resulted in a delay of senescence and prevention of SAMs mRNA accumulation. A possible mechanism for hormonal regulation of SAMs during ovary development is discussed.