Changes in levels of transcripts in endosperms of castor beans treated with exogenous gibberellic acid

Two classes of isocitrate lyase genes are expressed during late embryogeny and postgermination in Brassica napus L

We have analyzed the structure of genes encoding the glyoxylate cycle enzyme isocitrate lyase from Brassica napus L. and their expression during embryogeny and postgermination. Restriction mapping, nucleotide sequence, and DNA gel blot hybridization analyses of cDNA and genomic clones indicated that there are approximately six isocitrate lyase genes in the B. napus genome that can be divided into at least two subfamilies based upon their divergence in 5' and 3' untranslated regions. We showed previously that isocitrate lyase mRNA accumulates during late embryogeny and postgermination. Here, we present results which indicate that several isocitrate lyase genes are expressed at both stages of development. First, gene-specific probes were used to show that mRNAs encoded by representatives of both gene subfamilies accumulated in both late maturation stage embryos and in seedlings of B. napus. Second, a single B. napus isocitrate lyase gene, together with 3.5 kb and 1.4 kb of 5' and 3' flanking regions, respectively, was expressed in both embryos and seedlings of transgenic tobacco plants. The results indicated that accumulation of isocitrate lyase in late embryogeny and postgermination does not result from the alternate expression of distinct members of the gene family.

Translational or post-translational processes affect differentially the accumulation of isocitrate lyase and malate synthase proteins and enzyme activities in embryos and seedlings of Brassica napus

We have analyzed the accumulation of the glyoxylate cycle enzymes isocitrate lyase and malate synthase in embryos and seedlings of Brassica napus L. The two enzyme activities and proteins begin to accumulate during late embryogeny, reach maximal levels in seedlings, and are not detected in young leaves of mature plants. We showed previously that mRNAs encoding the two enzymes exhibit similar qualitative patterns of accumulation during development and that the two mRNAs accumulate to different levels in both embryos and seedlings (L. Comai et al., 1989, Plant Cell 1, 293-300). In this report, we show that the relative accumulation of the proteins and activities do not correspond to these mRNA levels. In embryos and seedlings, the specific activities of isocitrate lyase and malate synthase are approximately constant. By contrast, the ratio of malate synthase protein to mRNA is 14-fold higher than that of isocitrate lyase. Differences in the translational efficiencies of the two mRNAs in vitro do not appear to account for the discrepancy between mRNA and protein levels. Our results suggest that translational and/or post-translational processes affect differentially the accumulation of the proteins.

Nucleic acid (cDNA) and amino acid sequences of isocitrate lyase from castor bean

A cDNA library was constructed to mRNA enriched for isocitrate-lyase mRNA from castor-bean (Ricinus communis var. zanzibarensis) endosperms. Nine clones for isocitrate lyase (EC 4.1.3.1) were identified. The insert of 2.2 kb from clone ICL4 was sequenced and proved to contain the entire coding region, 1731 bp, for isocitrate lyase. The amino acid sequence of isocitrate lyase was deduced from the nucleic acid sequence. By analogy with muscle aldolase a lysine residue that possibly takes part in the binding of the substrate was identified. The 3' untranslated region contained three putative polyadenylation addition signals and two direct repeats.

Effect of abscisic and gibberellic acids on malate synthase transcripts in germinating castor bean seeds

Several clones complementary to malate synthase mRNA have been identified in a complementary-DNA library to mRNA from castor bean endosperm. One of these clones has been used as a probe to measure levels of transcripts during seed germination and the effects of gibberellic acid and abscisic acid on these levels have been examined.Malate synthase transcripts increased during germination and GA3 advanced their appearance in the endosperm. Exogenously applied ABA inhibited the accumulation of transcripts over a time course of germination but the addition of GA3 counteracted its inhibitory effects. The data confirmed previous reports which indicated that the action of both growth regulators was on transcript accumulation and that there is a coordinated induction of the enzymes involved in the lipid metabolism in oil seeds.

The action of exogenous abscisic acid on malate-synthase synthesis in germinating castor-bean seeds

The presence of 30 μM abscisic acid inhibited development of malate-synthase activity in the endosperm of germinating castor-bean seeds. Malate synthase was purified from castor-bean endosperms and an antibody to it was prepared from rabbit serum. This antibody was used to measure the amounts of malate-synthase mRNA using an in-vitro translation system. The effect of abscisic acid appeared to be greater on malate-synthase mRNA than on the bulk of mRNA, indicating some specificity of abscisic-acid action. The extent of the inhibition of malate-synthase activity and of malate-synthase mRNA accumulation were similar. This indicates that abscisic acid inhibits malate-synthase activity by lowering levels of translatable malate-synthase mRNA rather than by affecting the translation rate of this mRNA.

The action of exogenous abscisic and gibberellic acids on gene expression in germinating castor beans

Exogenously applied abscisic acid inhibits isocitrate-lyase activity of the endosperm during germination of castor-bean seeds. Amounts of isocitrate-lyase mRNA have been estimated by immunoprecipitation of in-vitro-translated polypeptide products. Exogenous abscisic acid leads to an inhibition of isocitrate lyase-mRNA accumulation. A large proportion of this effect of the growth factor may be accounted for by its action in inhibiting the overall accumulation of ribosomal RNA and total mRNA. However, the effect of abscisic acid on protein synthesis is not general, as the production of some mRNAs was stimulated. The major mRNA stored in the dry seed, coding for a 25600-Mr polypeptide that normally disappears within the first 12 h of germination, exhibited high levels in abscisic-acid-treated endosperms throughout the germination period. Three complementary DNA clones, of which two clones are complementary to isocitrate lyase, have been used to measure levels of transcripts during seed germination. The accumulation of both transcripts was inhibited by exogenous abscisic acid. The data strongly indicate that the action of abscisic acid on isocitrate lyase synthesis is either to inhibit the transcription, or to increase the transcript turnover. Exogenous gibberellic acid is able to counteract the inhibitory effects of abscisic acid.