We identified a mutant of Arabidopsis thaliana ectotype Col-O in which significantly reduced levels of expression of the gene for beta-amylase (AT beta-Amy) were detected in leaves in response to high concentrations of sucrose, glucose or fructose. Genetic studies, including a cross with transgenic plants that harbored the AT beta-Amy:GUS transgene with the promoter of AT beta-Amy, indicated that this phenotype was caused by a recessive mutation, Iba1, that affected expression of AT beta-Amy in trans. We also found a reduced level of sugar-induced expression of AT beta-Amy in the Landsberg erecta (Ler) ecotype compared with other ecotypes. This phenotype seemed to be due to a recessive trait, provisionally designated Iba2, that was linked to neither erecta nor Iba1. The Iba2 mutation also affected expression of AT beta-Amy:GUS transgene. Accumulation of starch and sugars after treatment of leaves with sucrose was not affected in the Iba1 mutant and Ler plants. However, both Iba1 mutant and Ler plants accumulated low levels of anthocyanin in response to sucrose, results that suggested the existence of some genetic linkage between regulation of the expression of AT beta-Amy and regulation of the accumulation of anthocyanin. Although the Iba1 and Iba2 mutations did not affect sugar-inducible gene expression in general, the expression of sugar-regulated genes other than the gene for beta-amylase was differentially affected in the Iba1 mutant and Ler plants. These results suggest that the sugar-regulated expression of many genes in plants might be mediated by multiple signal-transduction pathways.
SummaryThe low-beta-amylase1 (lba1) mutant of Arabidopsis thaliana has reduced sugar-induced expression of AtbAmy and shows pleiotropic phenotypes such as early flowering; short day-sensitive growth; and seed germination that is hypersensitive to glucose and abscisic acid and resistant to mannose. lba1 was a missense mutation of UPF1 RNA helicase involved in nonsense-mediated mRNA decay (NMD), which eliminates mRNAs with premature termination codons (PTCs), and replaces highly conserved Gly 851 of UPF1 with Glu. Expression of the wild-type UPF1 in lba1 rescued not only the reduced sugar-inducible gene expression, but also early flowering and altered seed-germination phenotypes. Sugar-inducible mRNAs were over-represented among transcripts decreased in sucrose-treated lba1 compared with Col plants, suggesting that UPF1 is involved in the expression of a subset of sugar-inducible genes. On the other hand, transcripts increased in lba1, which are likely to contain direct targets of NMD, included mRNAs for many transcription factors and metabolic enzymes that play diverse functions. Among these, the level of an alternatively spliced transcript of AtTFIIIA containing PTC was 17-fold higher in lba1 compared with Col plants, and it was reduced to the level in Col by expressing the wild-type UPF1. The lba1 mutant provides a good tool for studying NMD in plants.
The levels of P-amylase activity and of the mRNA for P-amylase in rosette leaves of Arabidopsis thaliana (1.) Heynh. increased significantly, with the concomitant accumulation of starch, when whole plants or excised mature leaves were supplied with sucrose. A supply of glucose or fructose, but not of mannitol or sorbitol, to plants also induced the expression of the gene for j?-amylase, and the induction occurred not only in rosette leaves but also in roots, stems, and bracts. These results suggest that the gene for P-amylase of Arabidopsis is subject to regulation by a carbohydrate metabolic signal, and expression of the gene in various tissues may be regulated by the carbon partitioning and sink-source interactions in the whole plant. P-Amylase (EC 3.2.1.2) is abundant in some starch-storing organs of plants, such as seeds and tuberous roots. In addition to these reproductive organs, 0-amylase is also present in various vegetative tissues (Beck and Ziegler, 1989). However, the physiological roles of p-amylase in plants in vivo are not known at present in spite of extensive enzymological studies of the breakdown of starch in vitro. Native starch granules are not attacked by plant P-amylases without the prior digestion by other enzymes or the solubilization by boiling (Beck and Ziegler, 1989). Precursors to P-amylases from plants ( Kreis et al., 1987;Monroe et al., 1991;Yoshida and Nakamura, 1991) do not contain the N-terminal transit peptide sequences that are responsible for targeting proteins to plastids. Indeed, results of most of the previous studies of vegetative tissues from various plant species indicate that P-amylase is present outside the '
The first intron of castor bean catalase gene, cat-1 was placed in the N-terminal region of the coding sequence of the beta-glucuronidase gene (gusA) and the intron-containing gusA was used with the cauliflower mosaic virus (CaMV) 35S promoter. Using this plasmid, pIG221, the effect of the intron on expression of beta-glucuronidase (GUS) activity was examined in transgenic rice calli and plants (a monocotyledon), and transgenic tobacco plants (a dicotyledon). The intron-containing plasmid increased the level of GUS enzyme activity 10 to 40-fold and 80 to 90-fold compared with the intronless plasmid, pBI221, in transgenic rice protoplasts and transgenic rice tissues, respectively. In contrast, the presence of the intron hardly influenced the expression of the GUS activity in transgenic tobacco plants. Northern blot analysis showed that the catalase intron was efficiently spliced in rice cells while transgenic tobacco plants contained both spliced and unspliced gusA transcripts in equal amounts. Furthermore, the level of the mature gusA transcript in transformed rice calli was greatly increased in the presence of the intron. The catalase intron was removed at the same splice junctions in transgenic rice and tobacco plants. These findings indicate that the stimulating effect of the intron on GUS expression is correlated with an efficient splicing of pre-mRNA and an increased level of mature mRNA.
An increase in the enzyme activity of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase induces the evolution of ethylene during the ripening of passion fruit. A much higher level of ethylene is produced in arils than in seeds or peels during ripening. The pattern of expression of two ACC synthase genes (PE-ACS1 and PE-ACS2), one ACC oxidase gene (PE-ACO1), and two ethylene receptor genes (PE-ETR1 and PE-ERS1) revealed that the expression of these genes is differentially regulated. Expression of PE-ACS1 and PE-ACO1 was enhanced during ripening and after ethylene treatment. However, prominent expression of PE-ACS1 was delayed compared to that of PE-ACO1. Much larger quantities of PE-ACS1 mRNA and PE-ACO1 mRNA were seen in arils than in seeds; this corresponds well with an increase in the amount of ethylene produced by the plant tissue itself. The level of PE-ACS2 mRNA was detectable in arils of the preclimacteric fruit, although it decreased during ripening. These results suggest that expression of PE-ACS1 and PE-ACO1 is required to increase the activity of ethylene biosynthetic enzymes during ripening. The level of expression of PE-ETR1 and PE-ERS1 did not significantly change over the course of ripening; however, the mRNA levels of PE-ETR1 and PE-ERS1 were much higher in arils than in seeds.
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