Regulation of seed germination requires coordinate action by the embryo and surrounding endosperm. We used Arabidopsis thaliana to establish the relative roles of embryo and endosperm in the control of seed germination and seedling establishment. We previously showed that endospermic oil reserves are used postgerminatively via gluconeogenesis to fuel seedling establishment and that lipid breakdown is repressed by abscisic acid (ABA) in embryo but not endosperm tissues. Here, we use RNA amplification to describe the transcriptome of the endosperm and compare the hormone responses of endosperm and embryo tissues. We show that the endosperm responds to both ABA and gibberellin but that ABA in particular regulates nuclear but not plastid-encoded photosynthetic gene expression in the embryo. We also show that ABA INSENSITIVE4 (ABI4) expression is confined to the embryo, accounts for the major differences in embryo response to ABA, and defines a role for ABI4 as a repressor of lipid breakdown. Furthermore, ABI5 expression in the endosperm defines a second region of altered ABA signaling in the micropylar endosperm cap. Finally, embryo and endosperm ABA signaling mutants demonstrate the spatial specificity of ABA action in seed germination. We conclude that the single cell endosperm layer plays an active role in the regulation of seed germination in Arabidopsis.

We conclude that SPT and PIL5 form part of a regulatory network coupling seed germination and GA3ox expression to light and temperature signaling in the seed.

Arabidopsis thaliana is used as a model system to study triacylglycerol (TAG) accumulation and seed germination in oilseeds. Here, we consider the partitioning of these lipid reserves between embryo and endosperm tissues in the mature seed. The Arabidopsis endosperm accumulates significant quantities of storage lipid, and this is effectively catabolized upon germination. This lipid differs in composition from that in the embryo and has a specific function during germination. Removing the endosperm from the wild-type seeds resulted in a reduction in hypocotyl elongation in the dark, demonstrating a role for endospermic TAG reserves in fueling skotomorphogenesis. Seedlings of two allelic gluconeogenically compromised phosphoenolpyruvate carboxykinase1 ( pck1) mutants show a reduction in hypocotyl length in the dark compared with the wild type, but this is not further reduced by removing the endosperm. The short hypocotyl phenotypes were completely reversed by the provision of an exogenous supply of sucrose. The PCK1 gene is expressed in both embryo and endosperm, and the induction of PCK1:b-glucuronidase at radicle emergence occurs in a robust, wave-like manner around the embryo suggestive of the action of a diffusing signal. Strikingly, the induction of PCK1 promoter reporter constructs and measurements of lipid breakdown demonstrate that whereas lipid mobilization in the embryo is inhibited by abscisic acid (ABA), no effect is seen in the endosperm. This insensitivity of endosperm tissues is not specific to lipid breakdown because hydrolysis of the seed coat cell walls also proceeded in the presence of concentrations of ABA that effectively inhibit radicle emergence. Both processes still required gibberellins, however. These results suggest a model whereby the breakdown of seed carbon reserves is regulated in a tissue-specific manner and shed new light on phytohormonal regulation of the germination process.

). † These authors contributed equally to this work. SummaryThe Arabidopsis acyl-CoA oxidase (ACX) family comprises isozymes with distinct fatty acid chain-length specificities that together catalyse the first step of peroxisomal fatty acid b-oxidation. We have isolated and characterized T-DNA insertion mutants in the medium to long-chain (ACX1) and long-chain (ACX2) acyl-CoA oxidases, and show that the corresponding endogenous activities are decreased in the mutants. Lipid catabolism during germination and early post-germinative growth was unaltered in the acx1-1 mutant, but slightly delayed in the acx2-1 mutant, with 3-day-old acx2-1 seedlings accumulating long-chain acyl-CoAs. In acx1-1 and acx2-1, seedling growth and establishment in the absence of an exogenous supply of sucrose was unaffected. Seedlings of the double mutant acx1-1 acx2-1 were unable to catabolize seed storage lipid, and accumulated long-chain acyl-CoAs. The acx1-1 acx2-1 seedlings were also unable to establish photosynthetic competency in the absence of an exogenous carbon supply, a phenotype that is shared with a number of other Arabidopsis mutants disrupted in storage lipid breakdown. Germination frequency of the double mutant was significantly reduced compared with wild-type seeds. This was unaffected by the addition of exogenous sucrose, but was improved by dormancy-breaking treatments such as cold stratification and after-ripening. We show that the acx1-1, acx2-1 and acx1-2 acx2-1 double mutants and the ketoacyl-CoA thiolase-2 (kat2) mutant exhibit a sucrose-independent germination phenotype comparable with that reported for comatose (cts-2), a mutant in a peroxisomal ABC transporter which exhibits enhanced dormancy. This demonstrates an additional role beyond that of carbon provision for the b-oxidation pathway during germination or in dormant seeds.

SUMMARYTrehalose and associated metabolites are part of the sugar signalling system in plants and have profound effects on development. Disruption of the TREHALOSE 6-PHOSPHATE SYNTHASE (TPS1) gene in Arabidopsis results in delayed embryo growth, altered cell wall morphology and carbon metabolism and abortion at the torpedo stage. Here we investigate the role of the TPS1 gene in post-embryonic development using two approaches. In the first we use the seed-specific ABI3 promoter to drive the TPS1 cDNA during embryo development, resulting in rescue of the embryo-lethal tps1 phenotype. Lack of expression from the ABI3::TPS1 transgene in post-germinative tps1 seedlings results in severe growth arrest, accumulation of soluble sugars and starch and leads to an increase in expression of genes related to ABA signalling. In the second approach we use TILLING (targeted induced local lesions in genomes) to generate three weaker, non-embryo-lethal, alleles (tps1-11, tps1-12 and tps1-13) and use these to demonstrate that the TPS1 protein plays a key role in modulating trehalose 6-phosphate (T6P) levels in vegetative tissues of Arabidopsis. All three weaker alleles give a consistent phenotype of slow growth and delayed flowering. Germination of tps1-11, tps1-12 and tps1-13 is hypersensitive to ABA with the degree of hypersensitivity correlating with the decrease in T6P levels in the different alleles. Stomatal pore aperture is regulated by ABA, and this was found to be affected in tps1-12. Our results show that the TPS1 gene product plays an essential role in regulating the growth of vegetative as well as embryogenic tissue in a mechanism involving ABA and sugar metabolism.