The Arabidopsis COMATOSE locus regulates germination potential.
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Mutation of the COMATOSE locus in Arabidopsis results in a marked reduction in germination potential. Whilst the morphology of comatose (cts) embryos is not altered, physiological analysis reveals that mature cts seeds do not respond to gibberellin. Prolonged chilling of imbibed seeds only partially restores germination potential, and seeds do not after ripen. Genetic analysis shows that the cts phenotype is expressed in the embryo and phenotypic differences between wild-type and mutant plants were not observed during other stages of plant growth and development. Therefore cts represents a new class of mutant, with a specific lesion that results in severely impaired germination potential. Genetic interactions were analysed between cts and loci that regulate embryo maturation, and abscisic acid biosynthesis and perception. Results from these studies showed that the cts mutant phenotype required the wild-type action of these loci, and suggested that CTS exerts a repressive function on these loci. A model is presented postulating that CTS promotes increased germination potential, and represses embryo dormancy. These functions of CTS may result in the removal of embryo dormancy as a prerequisite to germination. (+info)
Differential expression of the Arabidopsis genes coding for Em-like proteins.
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Late embryogenesis abundant (lea) genes are a large and diverse group of genes highly expressed during late stages of seed development. Five major groups of LEA proteins have been described. Two Em genes (group I lea genes) are present in the genome of Arabidopsis thaliana L., AtEm1 and AtEm6. Both genes encode for very similar proteins which differ basically in the number of repetitions of a highly hydrophilic amino acid motif. The spatial patterns of expression of the two Arabidopsis Em genes have been studied using in situ hybridization and transgenic plants transformed with the promoters of the genes fused to the beta-glucuronidase reporter gene (uidA). In the embryo, AtEm1 is preferentially expressed in the pro-vascular tissues and in meristems. In contrast, AtEm6 is expressed throughout the embryo. The activity of both promoters disappears rapidly after germination, but is ABA-inducible in roots of young seedlings, although in different cells: the AtEm1 promoter is active in the internal tissues (vasculature and pericycle) whereas the AtEm6 promoter is active in the external tissues (cortex, epidermis and root hairs). The AtEm1 promoter, but not AtEm6, is also active in mature pollen grains and collapsed nectaries of young siliques. These data indicate that the two Em proteins could carry out at least slightly different functions and that the expression of AtEm1 and AtEm6 is controlled at, at least, three different levels: temporal, spatial and hormonal (ABA). (+info)
A germination-specific endo-beta-mannanase gene is expressed in the micropylar endosperm cap of tomato seeds.
(43/1498)
Endo-beta-mannanase (EC 3.2.1.78) is involved in hydrolysis of the mannan-rich cell walls of the tomato (Lycopersicon esculentum Mill.) endosperm during germination and post-germinative seedling growth. Different electrophoretic isoforms of endo-beta-mannanase are expressed sequentially in different parts of the endosperm, initially in the micropylar endosperm cap covering the radicle tip and subsequently in the remaining lateral endosperm surrounding the rest of the embryo. We have isolated a cDNA from imbibed tomato seeds (LeMAN2) that shares 77% deduced amino acid sequence similarity with a post-germinative tomato mannanase (LeMAN1). When expressed in Escherichia coli, the protein encoded by LeMAN2 cDNA was recognized by anti-mannanase antibody and exhibited endo-beta-mannanase activity, confirming the identity of the gene. LeMAN2 was expressed exclusively in the endosperm cap tissue of tomato seeds prior to radicle emergence, whereas LeMAN1 was expressed only in the lateral endosperm after radicle emergence. LeMAN2 mRNA accumulation and mannanase activity were induced by gibberellin in gibberellin-deficient gib-1 mutant seeds but were not inhibited by abscisic acid in wild-type seeds. Distinct mannanases are involved in germination and post-germinative growth, with LeMAN2 being associated with endosperm cap weakening prior to radicle emergence, whereas LeMAN1 mobilizes galactomannan reserves in the lateral endosperm. (+info)
A dual function alpha-dioxygenase-peroxidase and NAD(+) oxidoreductase active enzyme from germinating pea rationalizing alpha-oxidation of fatty acids in plants.
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An enzyme with fatty acid alpha-oxidation activity (49 nkat mg(-1); substrate: lauric acid) was purified from germinating pea (Pisum sativum) by a five-step procedure to apparent homogeneity. The purified protein was found to be a 230-kD oligomer with two dominant subunits, i.e. a 50-kD subunit with NAD(+) oxidoreductase activity and a 70-kD subunit, homolog to a pathogen-induced oxygenase, which in turn shows significant homology to animal cyclooxygenase. On-line liquid chromatography-electrospray ionization-tandem mass spectrometry revealed rapid alpha-oxidation of palmitic acid incubated at 0 degrees C with the purified alpha-oxidation enzyme, leading to (R)-2-hydroperoxypalmitic acid as the major product together with (R)-2-hydroxypalmitic acid, 1-pentadecanal, and pentadecanoic acid. Inherent peroxidase activity of the 70-kD fraction decreased the amount of the (R)-2-hydroperoxy product rapidly and increased the level of (R)-2-hydroxypalmitic acid. Incubations at room temperature accelerated the decline toward the chain-shortened aldehyde. With the identification of the dual function alpha-dioxygenase-peroxidase (70-kD unit) and the related NAD(+) oxidoreductase (50-kD unit) we provided novel data to rationalize all steps of the classical scheme of alpha-oxidation in plants. (+info)
Estimation of base temperatures for nine weed species.
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Experiments were conducted to test several methods for estimating low temperature thresholds for seed germination. Temperature responses of nine weeds common in annual agroecosystems were assessed in temperature gradient experiments. Species included summer annuals (Amaranthus albus, A. palmeri, Digitaria sanguinalis, Echinochloa crus-galli, Portulaca oleracea, and Setaria glauca), winter annuals (Hirschfeldia incana and Sonchus oleraceus), and Conyza canadensis, which is classified as a summer or winter annual. The temperature below which development ceases (Tbase) was estimated as the x-intercept of four conventional germination rate indices regressed on temperature, by repeated probit analysis, and by a mathematical approach. An overall Tbase estimate for each species was the average across indices weighted by the reciprocal of the variance associated with the estimate. Germination rates increased linearly with temperature between 15 degrees C and 30 degrees C for all species. Consistent estimates of Tbase were obtained for most species using several indices. The most statistically robust and biologically relevant method was the reciprocal time to median germination, which can also be used to estimate other biologically meaningful parameters. The mean Tbase for summer annuals (13.8 degrees C) was higher than that for winter annuals (8.3 degrees C). The two germination response characteristics, Tbase and slope (rate), influence a species' germination behaviour in the field since the germination inhibiting effects of a high Tbase may be offset by the germination promoting effects of a rapid germination response to temperature. Estimates of Tbase may be incorporated into predictive thermal time models to assist weed control practitioners in making management decisions. (+info)
The second step of the biphasic endosperm cap weakening that mediates tomato (Lycopersicon esculentum) seed germination is under control of ABA.
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The role of abscisic acid (ABA) in the weakening of the endosperm cap prior to radicle protrusion in tomato (Lycopersicon esculentum Mill. cv. Moneymaker) seeds was studied. The endosperm cap weakened substantially in both water and ABA during the first 38 h of imbibition. After 38 h the force required for endosperm cap puncturing was arrested at 0.35 N in ABA, whereas in water a further decrease occurred until the radicle protruded. During the first 2 d of imbibition endo-beta-mannanase activity was correlated with the decrease in required puncture force and with the appearance of ice-crystal-induced porosity in the cell walls as observed by scanning electron microscopy. Prolonged incubation in ABA resulted in the loss of endo-beta-mannanase activity and the loss of ice-crystal-induced porosity, but not in a reversion of the required puncture force. ABA also had a distinct but minor effect on the growth potential of the embryo. However, endosperm cap resistance played the limiting role in the completion of germination. It was concluded that (a) endosperm cap weakening is a biphasic process and (b) inhibition of germination by ABA is through the second step in the endosperm cap weakening process. (+info)
Development of beta-1,3-glucanase activity in germinated tomato seeds.
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Laminarin-hydrolysing activity developed in the endosperm of tomato (Lycopersicon esculentum) seeds following germination. The enzyme was basic (pI>10) and the apparent molecular mass was estimated to be 35 kDa by SDS-PAGE. It was specific for linear beta-1,3-glucan substrates. Laminarin was hydrolysed by the enzyme to yield a mixture of oligoglucosides, indicating that the enzyme had an endo-action pattern. Thus, the enzyme was identified as beta-1,3- endoglucanase (EC 3.2.1.39). The activity of the enzyme developed in the endosperm after radicle protrusion (germination) had occurred and the enzyme activity was localized exclusively in the micropylar region of the endosperm where the radicle had penetrated. When the lateral endosperm region, where no induction of the enzyme occurred, was wounded (cut or punctured), there was a marked enhancement of beta-1,3-glucanase activity. Thus the post-germinative beta-1, 3-glucanase activity in the micropylar endosperm portion might be brought about by wounding resulting from endosperm rupture by radicle penetration. (+info)
Comparison of globulin mobilization and cysteine proteinases in embryonic axes and cotyledons during germination and seedling growth of vetch (Vicia sativa L.).
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Vicilin and legumin, the storage globulins of mature dry vetch (Vicia sativa L.) seeds, are found in protein bodies which are present not only in the cotyledons, but also in the radicle, axis and shoot (together, for reasons of simplicity, here called axis). When at 24 h after the start of imbibition (hai) the radicle breaks through the seed coat a major part of the globulins in the axis has already been degraded, whereas in the cotyledons globulin breakdown cannot yet be detected. Globulin mobilization starts with the degradation of vicilin. At 48 hai when globulin mobilization in the cotyledons just begins, the axis is already nearly depleted of globulins. Mobilization of storage globulin is probably brought about by a complex of different cysteine proteinases (CPRs). The papain-like CPR2 and CPR4, and the legumain-like VsPB2, together with their mRNAs, are already present in axes and cotyledons of dry seeds. This means that they must have been formed during seed maturation. Additional papain-like CPRs are formed later during germination and seedling growth. CPR4 and VsPB2 together with their corresponding mRNAs become undetectable as germination and seedling growth proceed. VsPB2 and VsPB2-mRNA are substituted by the homologous legumain-like proteinase B and its mRNA. The composition of stored and newly formed CPRs undergoes developmental changes which differ between axes and cotyledons. It is concluded that storage globulin mobilization in germinating vetch seeds is started by stored CPRs, whereas the mobilization of the bulk of globulin is predominantly mediated by CPRs which are formed de novo. (+info)