Legume seed development is characterized by progressive differentiation of organs and tissues resulting in developmental gradients. The whole process is prone to metabolic control, and distinct metabolite profiles specify the differentiation state. Whereas early embryo growth is mainly maternally controlled, the transition into maturation implies a switch to filial control. A signaling network involving sugars, ABA, and SnRK1 kinases governs maturation. Processes of maturation are activated by changing oxygen/energy levels and/or a changing nutrient state, which trigger responses at the level of transcription and protein phosphorylation. This way seed metabolism becomes adapted to altering conditions. In maturing cotyledons photoheterotrophic metabolism improves internal oxygen supply and biosynthetic fluxes and influences assimilate partitioning. Transgenic legumes with changed metabolic pathways and seed composition provide suitable models to study pathway regulation and metabolic control. At the same time, desirable improvements of seed quality and yield may be achieved.

SummaryIn order to understand sucrose transport in developing seeds of cereals at the molecular level, we cloned from a caryopses library two cDNAs encoding sucrose transporters, designated HvSUT1 and HvSUT2. Sucrose uptake activity was con®rmed by heterologous expression in yeast. Both transporter genes are expressed in maternal as well as ®lial tissues. In a series of in situ hybridizations we analysed the cell type-speci®c expression in developing seeds. HvSUT1 is preferentially expressed in caryopses in the cells of the nucellar projection and the endospermal transfer layer, which represent the sites of sucrose exchange between the maternal and the ®lial generation and are characterized by transfer cell formation. HvSUT2 is expressed in all sink and source tissues analysed and may have a general housekeeping role. The rapid induction of HvSUT1 gene expression in caryopses at approximately 5±6 days after fertilization coincides with increasing levels of sucrose as well as sucrose synthase mRNA and activity, and occurs immediately before the onset of rapid starch accumulation within the endosperm. Starch biosynthesis requires sucrose to be imported into the endosperm, as direct precursor for starch synthesis and to promote storage-associated processes. We discuss the possible role of HvSUT1 as a control element for the endospermal sucrose concentration.

Plant seeds prepare for germination already during seed maturation. We performed a detailed transcriptome analysis of barley (Hordeum vulgare) grain maturation, desiccation, and germination in two tissue fractions (starchy endosperm/aleurone and embryo/scutellum) using the Affymetrix Barley1 GeneChip. To aid data evaluation, Arabidopsis thaliana MapMan and PageMan tools were adapted to barley. The analyses allow a number of conclusions: (1) Cluster analysis revealed a smooth transition in transcription programs between late seed maturation and germination within embryo tissues, but not in the endosperm/aleurone. (2) More than 12,000 transcripts are stored in the embryo of dry barley grains, many of which are presumably activated during germination. (3) Transcriptional activation of storage reserve mobilization events occurs at an early stage of germination, well before radicle protrusion. (4) Key genes of gibberellin (GA) biosynthesis are already active during grain maturation at a time when abscisic acid peaks suggesting the formation of an endogenous store of GA in the aleurone. This GA probably acts later during germination in addition to newly synthesized GA. (5) Beside the well-known role of GA in gene activation during germination spatiotemporal expression data and cis-element searches in homologous rice promoters confirm an equally important gene-activating role of abscisic acid during this developmental period. The respective regulatory webs are linked to auxin and ethylene controlled networks. In summary, new bioinformatics PageMan and MapMan tools developed in barley have been successfully used to investigate in detail the transcriptome relationships between seed maturation and germination in an important crop plant.

To analyze sugar transport processes during seed development of fava bean, we cloned cDNAs encoding one sucrose and one hexose transporter, designated VfSUTl and VfSTPl, respectively. Sugar uptake activity was confirmed after heterologous expression in yeast. Gene expression was studied in relation to seed development. Transcripts were detected in both vegetative and seed tissues. In the embryo, VfSUTl and VfSTP 7 mRNAs were detected only in epidermal cells, but in a different temporal and spatial pattern. VfSTPl mRNA accumulates during the midcotyledon stage in epidermal cells covering the mitotically active parenchyma, whereas the VfSUTl transcript was specific to outer epidermal cells showing transfer cell morphology and covering the storage parenchyma. Transfer cells developed at the contact area of the cotyledonary epidermis and the seed coat, starting first at the early cotyledon stage and subsequently spreading to the abaxial region at the late cotyledon stage. Feeding high concentrations of sugars suppressed both VfSUT7 expression and transfer cell differentiation in vitro, suggesting a control by carbohydrate availability.

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We previously provided evidence that seed coat‐associated invertase is involved in controlling the carbohydrate state of developing seeds and, by this way, triggering developmental processes (Weber et al. (1995) Plant Cell, 7, 1835–1846). To verify our postulate, we compared seed development of two genotypes of Vicia faba differing in seed weight. The seed coat of the large‐seeded genotype formed a higher number of parenchymatous cell layers and matured later. VfCWINV1 encoding a cell wall‐bound invertase is expressed in the unloading zone of the seed coat. mRNA levels peaked later in ‘large’ coats and mRNA was present in more cell layers over a longer time period. Cell wall‐bound invertase activity revealed a similar accumulation pattern, obviously generating the high hexose conditions present in the endospermal cavity bathing the premature cotyledons and thus controlling their carbohydrate state. High hexose conditions were correlated with an extended mitotic activity of the ‘large’ cotyledons. In ‘large’ and ‘small’ cotyledons, sucrose levels rose when hexoses decreased apparently terminating cell divisions and initiating differentiation and storage activities. This developmental switch was delayed in ‘large’ embryos. To prove the outlined relationship, sucrose was added in vitro to mitotically active cotyledons. This treatment favoured nuclear expansion and starch accumulation over cell division. In contrast, a hexose‐based medium maintained cell divisions. We conclude that development of the embryo is coordinately regulated with that of the maternal seed coat which controls, by metabolic signals, the phase of cell division of the embryo and consequently also seed size.

SummaryGene expression patterns covering over 10 000 seed-expressed sequences were analyzed by macroarray technology in maternal tissue (mainly pericarp) and filial endosperm and embryo during barley seed development from anthesis until late maturation. Defined sets of genes showing distinct expression patterns characterized both tissue type and major developmental phases. The analysis focused on regulatory networks involved in programmed cell death (PCD) and abscisic acid (ABA)-mediated maturation. These processes were similar in the different tissues, but typically involved the expression of alternative members of a common gene family. The analysis of co-expressed gene sets and the identification of cis regulatory elements in orthologous rice gene 'promoter' regions suggest that PCD in the pericarp is mediated by distinct classes of proteases and is under the hormonal control of both jasmonic acid (JA) and ethylene via ethylene-responsive element binding protein (EREBP) transcription factors (TFs). On the other hand, PCD in endosperm apparently involves only the ethylene pathway, but employs distinct gene family members from those active in the pericarp, and a different set of proteases and TFs. JA biosynthetic genes are hardly activated. Accordingly, JA levels are high in the pericarp but low in the endosperm during middle and late developmental stages. Similarly, genes acting in the deduced ABA biosynthetic pathway and signaling network differ between endosperm and embryo. ABA in the endosperm appears to exert an influence over storage product synthesis via SNF1 kinase. In the embryo, ABA seems to influence the acquisition of desiccation tolerance via ABA response element binding factors, but the data also suggest the existence of an ABA-independent but interactive pathway acting via the dehydrationresponsive element binding (DREB) 2A TF.