The plant hormone abscisic acid (ABA) is a key regulator of seed maturation and germination and mediates adaptive responses to environmental stress. In Arabidopsis, the ABI1 gene encodes a member of the 2C class of protein serine/threonine phosphatases (PP2C), and the abi1-1 mutation markedly reduces ABA responsiveness in both seeds and vegetative tissues. However, this mutation is dominant and has been the only mutant allele available for the ABI1 gene. Hence, it remained unclear whether ABI1 contributes to ABA signaling, and in case ABI1 does regulate ABA responsiveness, whether it is a positive or negative regulator of ABA action. In this study, we isolated seven novel alleles of the ABI1 gene as intragenic revertants of the abi1-1 mutant. In contrast to the ABA-resistant abi1-1 mutant, these revertants were more sensitive than the wild type to the inhibition of seed germination and seedling root growth by applied ABA. They also displayed increases in seed dormancy and drought adaptive responses that are indicative of a higher responsiveness to endogenous ABA. The revertant alleles were recessive to the wild-type ABI1 allele in enhancing ABA sensitivity, indicating that this ABA-supersensitive phenotype results from a loss of function in ABI1. The seven suppressor mutations are missense mutations in conserved regions of the PP2C domain of ABI1, and each of the corresponding revertant alleles encodes an ABI1 protein that lacked any detectable PP2C activity in an in vitro enzymatic assay. These results indicate that a loss of ABI1 PP2C activity leads to an enhanced responsiveness to ABA. Thus, the wild-type ABI1 phosphatase is a negative regulator of ABA responses.

We screened for mutations that either enhanced or suppressed the abscisic acid (ABA)–resistant seed germination phenotype of the Arabidopsis abi1-1 mutant. Alleles of the constitutive ethylene response mutant ctr1 and ethylene-insensitive mutant ein2 were recovered as enhancer and suppressor mutations, respectively. Using these and other ethylene response mutants, we showed that the ethylene signaling cascade defined by the ETR1, CTR1, and EIN2 genes inhibits ABA signaling in seeds. Furthermore, epistasis analysis between ethylene- and ABA-insensitive mutations indicated that endogenous ethylene promotes seed germination by decreasing sensitivity to endogenous ABA. In marked contrast to the situation in seeds, ein2 and etr1-1 roots were resistant to both ABA and ethylene. Our data indicate that ABA inhibition of root growth requires a functional ethylene signaling cascade, although this inhibition is apparently not mediated by an increase in ethylene biosynthesis. These results are discussed in the context of the other hormonal regulations controlling seed germination and root growth.

Background: As in other eukaryotes, plant mitogen-activated protein kinase (MAPK) cascades are composed of three classes of hierarchically organized protein kinases, namely MAPKKKs, MAPKKs, and MAPKs. These modules rapidly amplify and transduce extracellular signals into various appropriate intracellular responses. While extensive work has been conducted on the post-translational regulation of specific MAPKKs and MAPKs in various plant species, there has been no systematic investigation of the genomic organization and transcriptional regulation of these genes. Results: Ten putative poplar MAPKK genes (PtMKKs) and 21 putative poplar MAPK genes (PtMPKs) have been identified and located within the poplar (Populus trichocarpa) genome. Analysis of exon-intron junctions and of intron phase inside the predicted coding region of each candidate gene has revealed high levels of conservation within and between phylogenetic groups. Expression profiles of all members of these two gene families were also analyzed in 17 different poplar organs, using gene-specific primers directed at the 3'-untranslated region of each candidate gene and real-time quantitative PCR. Most PtMKKs and PtMPKs were differentially expressed across this developmental series. Conclusion: This analysis provides a complete survey of MAPKK and MAPK gene expression profiles in poplar, a large woody perennial plant, and thus complements the extensive expression profiling data available for the herbaceous annual Arabidopsis thaliana. The poplar genome is marked by extensive segmental and chromosomal duplications, and within both kinase families, some recently duplicated paralogous gene pairs often display markedly different patterns of expression, consistent with the rapid evolution of specialized protein functions in this highly adaptive species.

A membrane-associated lipoxygenase from breaker-stage fruit of tomato (Lycopersicon esculentum Mill.) was purified and partially sequenced. Using degenerate oligonucleotides corresponding to portions of this sequence, a cDNA was amplified by PCR and used to screen a breaker fruit cDNA library. Two clones, tomloxA and tomloxB, were isolated and one of these (tomloxA) corresponded to the isolated protein. Genomic clones were isolated and sequence data from these were used to obtain the 5' ends of the cDNAs. The 2.8-kb cDNAs encode proteins that are similar in size and sequence to each other and to other plant lipoxygenases. DNA blot analysis indicated that tomato contains three or more genes that encode lipoxygenase. RNA blot analysis showed that tomloxA is expressed in germinating seeds as well as in ripening fruit, where it reached its peak during breaker stage. tomloxB appears to be fruit specific and is at its highest level in ripe fruit.