A mutant of Arabidopsis which is defective in seed development and storage protein accumulation is a new abi3 allele

In order to investigate the role of the plant hormones gibberellin (GA) and abscisic acid (ABA) in seed development and germination the GA biosynthetic inhibitor, Uniconazol, was used to isolate mutants with abnormal germination profiles. In one of these mutants, the ability to germinate on Uniconazol is due to a mutation in the ABI3 gene. However, unlike the previously reported abi3 mutant, this line displays an array of seed-specific developmental defects. The accumulation of seed reserve proteins is dramatically reduced due to reduced levels of the storage protein mRNA. The embryos remain green throughout development and are desiccation intolerant. However, immature seeds are completely non-dormant and grow normally. These results suggest the ABI3 gene is essential for the synthesis of seed storage proteins and for the protection of the embryo during desiccation.     

In vitro morphogenesis of arrested embryos from lethal mutants of Arabidopsis thaliana

Summary Arrested embryos from lethal (emb) mutants of Arabidopsis thaliana were rescued on a nutrient medium designed to promote plant regeneration from immature wild-type cotyledons. The best response was observed with mutant embryos arrested at the heart to cotyledon stages of development. Embryos arrested at a globular stage produced callus but failed to turn green or form normal shoots in culture. Many of the mutant plants produced in culture were unusually pale with abnormal leaves, rosettes, and patterns of reproductive development. Other plants were phenotypically normal except for the presence of siliques containing 100% aborted seeds following self-pollination. These results demonstrate that genes with essential functions during plant embryo development differ in their pattern of expression at later stages of the life cycle. Most of the 15 genes examined in this study were essential for embryogenesis but were required again for subsequent stages of development. Only EMB24 appeared to be limited in function to embryo development. These differences in the response of mutant embryos in culture may facilitate the classification of embryonic lethals and the identification of genes with developmental rather than housekeeping functions.     

Lanthanide ions are agonists of transient gene expression in rice protoplasts and act in synergy with ABA to increase Em gene expression

Previous work has shown that in rice suspension cells, NaCl at 0.4 M can induce Em gene expression and act synergistically with ABA, possibly by potentiating the ABA response pathway through a rate-limiting intermediate (R.M. Bostock and R.S. Quatrano (1992) Plant Physiol., 98, 1356–1363). Since calcium is an intermediate in ABA regulation of stomatal closure, we tested the effect of calcium changes on ABA-inducible Em gene expression in transiently transformed rice protoplasts. We show that calcium is required for ABA-inducible Em-GUS expression and can act in synergy with ABA. The trivalent ions lanthanum, gadolinium, and aluminum, which are known to interact with calcium- and other signaling pathways, can act at sub-millimolar concentrations to increase GUS reporter gene expression driven by several promoters in transiently transformed rice protoplasts. This effect is not specific for the ABA-inducible Em promoter, but is synergistic with ABA. The lanthanum synergy with ABA does not require calcium. In rice suspension cells, lanthanum alone does not induce Em gene expression, in contrast to transiently transformed protoplasts, yet can act synergistically with ABA to effectively increase the sensitivity to ABA greater than tenfold. Trivalent ions may be a useful tool to study the regulation of gene expression. The possible effects of trivalent ions on ABA signal transduction and gene expression are discussed.     

S-Nitrosoglutathion Reductase Activity Modulates the Thermotolerance of Seeds Germination by Controlling ABI5 Stability under High Temperature

Seed germination or dormancy status is strictly controlled by endogenous phytohormone and exogenous environment signals. Abscisic acid (ABA) is the important phytohormone to suppress seed germination. Ambient high temperature (HT) also suppressed seed germination, or called as secondary seed dormancy, through upregulating ABI5, the essential component of ABA signal pathway. Previous result shows that appropriate nitric oxide (NO) breaks seed dormancy through triggering S-nitrosoglutathion reductase (GSNOR1)-dependent S-nitrosylation modification of ABI5 protein, subsequently inducing the degradation of ABI5. Here we found that HT induced the degradation of GSNOR1 protein and reduced its activity, thus accumulated more reactive nitrogen species (RNS) to damage seeds viability. Furthermore, HT increased the S-nitrosylation modification of GSNOR1 protein, and triggered the degradation of GSNOR1, therefore stabilizing ABI5 to suppress seed germination. Consistently, the ABI5 protein abundance was lower in the transgenic line overexpressing GSNOR1, but higher in the gsnor mutant after HT stress. Genetic analysis showed that GSNOR1 affected seeds germination through ABI5 under HT. Taken together, our data reveals a new mechanism by which HT triggers the degradation of GSNOR1, and thus stabilizing ABI5 to suppress seed germination, such mechanism provides the possibility to enhance seed germination tolerance to HT through genetic modification of GNSOR1.     

Differential regulation of two ABA-inducible genes from Craterostigma plantagineum in transgenic Arabidopsis plants

In Craterostigma plantagineum the CDeT-6-19 and CDeT-27-45 genes are expressed following desiccation and/or ABA treatment. Their promoters were fused to the -glucuronidase reporter gene (GUS) and tested in transgenic Arabidopsis. GUS activity was measured in mature Arabidopsis seeds, and the responsiveness to ABA in vegetative tissue was found to be limited to the early developmental stages. When transgenic plants were crossed with plants over-expressing the ABI3 gene, it was observed that ABI3 is not required for ABA induction of the CDeT-6-19 promoter, whereas it is crucial for expression of the CDeT-27-45 promoter.     

Differential molecular responses to abscisic acid and osmotic stress in viviparous maize embryos

Substantial quantities of mRNA encoding the abundant Em polypeptide accumulate, in planta, in developing embryos of maize (Zea mays L.). By contrast, accumulation of Em mRNA is only barely detectable in embryos with the vp-5/vp-5 genotype [an abscisic acid (ABA)-deficient viviparous phenotype]. Em mRNA is not detectable within viviparous embryos of the vp-1/vp-1 genotype that are non-responsive to ABA. Culture of immature wild-type and vp-5/vp-5 embryos in the presence of exogenous ABA or of an osmotically active agent prevents precocious germination and results in expression of the Em genes. When vp-1/vp-1 embryos are cultured under similar conditions, only the application of osmotic stress prevents precocious germination. However, Em mRNA does not accumulate either in ABA-treated or stressed, arrested embryos, indicating a requirement for ABA perception through a VP-1-mediated mechanism for Em gene expression. Nevertheless, vp-1/vp-1 embryos do show both ABA and stress responses at the molecular level. Treatment with ABA causes the accumulation of mRNA encoding a polypeptide of approx. 30 kDa, whilst osmotic stress induces the accumulation both of a 30-kDa polypeptide and a set of approx. 20-kDa polypeptides. This indicates the existence of discrete, parallel ABA and stress response pathways in developing maize embryos.Abbreviations ABA abscisic acid - cDNA copy-DNA - DAP days after pollination - kDa kilodaltons - MS Murashige and Skoog medium - LEA late embryogenesis abundant - NEpHGE non-equilibrium pH gradient gel electrophoresis - SDS-PAGE sodium dodecyl sulphate-polyacrylamide gel electrophoresis     

Abscisic acid-insensitive mutations provide evidence for stage-specific signal pathways regulating expression of an Arabidopsis late embryo genesis-abundant (lea) gene

An Arabidopsis homolog of the abscisic acid (ABA)-inducible cotton D19 and wheat Em genes was cloned and its expression assayed at two developmental stages in wild-type, ABA-deficient (aba) and three ABA-insensitive (abi) lines of Arabidopsis thaliana. Expression of this gene was reduced slightly in seeds of aba mutants and approximately ten-fold in abi3 mutants, but seed expression was not decreased in either abi1 or abi2 monogenic mutants. In contrast, the abi1 and abi2 mutants showed a very slight reduction of ABA inducibility in 8-day-old plants, while the responses of aba and abi3 mutants were comparable to that of wild type. Although previous studies have shown that none of the abi mutations show completely stage-specific effects, the results reported here indicate that the importance of each of the ABI loci in regulating this single gene is stage-dependent. Furthermore, the fact that none of the abi mutations show more than minor effects on exogenous ABA inducibility of the Arabidopsis D19/Em homolog in young plants suggests that an additional ABA signalling pathway may be operating during vegetative growth.     

Cloning, mapping, and analyses of expression of the Em-like gene family in soybean [Glycine max (L). Merr.]

 The entire Em-like Group-1 late embryogenesis abundant (Lea) gene family from soybean was cloned and characterized. The five Group-1 Lea genes (Sle1-5) were divided into two classes based on sequence identity. Sle1-4 were genetically mapped to four different linkage groups. Nucleotide sequencing indicated that Sle1, Sle2, Sle3, and Sle5 encode polypeptides differing primarily by the presence of a repeated 20-amino acid motif. Sle1 and Sle5 were shown by Northern analysis to be expressed in developing embryos weeks earlier than Sle2 and Sle3. Sle4 was shown to be a pseudogene. Maximal levels of mRNA for all functional Sle genes accumulated in maturation-phase seeds, before significant desiccation had occurred, and declined rapidly upon seed imbibition. Desiccation did not induce Sle expression in seeds or vegetative tissue. Sle expression was confined to embryo tissues and Sle mRNA accumulated at similar levels in both the embryo axis and in the cotyledons. Received: 25 September 1996 / Accepted: 27 September 1996     

The ABSCISIC ACID-INSENSITIVE 3 (ABI3) gene is expressed during vegetative quiescence processes in Arabidopsis

The ABSCISIC ACID-INSENSITIVE 3 ( ABI3 ) gene of Arabidopsis thaliana (L.) Heynh is known to play an important role during seed maturation and dormancy. Here, we present evidence suggesting an additional role for ABI3 during vegetative quiescence processes. During growth in the dark, ABI3 is expressed in the apex of the seedlings after cell division is arrested. The 2S seed storage protein gene, a target gene of ABI3 in seeds, is also induced in the arrested apex under similar darkness conditions. In addition, β -glucuronidase expression under the control of the ABI3 promoter is abolished by treatments that provoke leaf development in the dark [sucrose and abscisic acid (ABA) biosynthesis inhibitors] and induced by treatments that prevent leaf development (darkness and ABA). Furthermore, ABI3 expression is absent in apices of dark-grown de-etiolated ( det 1 ) and abi3 mutants, both known to develop leaves or leaf primordia in the dark. The fact that the expression of the ABI3 gene is only observed in a fraction of the analysed plants suggests that ABI3 is probably only one of the components of a molecular network underlying quiescence. In addition to the expression of ABI3 in apices of dark-grown seedlings, the ABI3 promoter confers expression in other vegetative organs as well, such as the stipules and the abscission zones of the siliques. In conclusion, apart from its role in seed development, ABI3 might have additional functions.