Seed germination is a complex trait of key ecological and agronomic significance. Few genetic factors regulating germination have been identified, and the means by which their concerted action controls this developmental process remains largely unknown. Using publicly available gene expression data from Arabidopsis thaliana, we generated a condition-dependent network model of global transcriptional interactions (SeedNet) that shows evidence of evolutionary conservation in flowering plants. The topology of the SeedNet graph reflects the biological process, including two state-dependent sets of interactions associated with dormancy or germination. SeedNet highlights interactions between known regulators of this process and predicts the germinationassociated function of uncharacterized hub nodes connected to them with 50% accuracy. An intermediate transition region between the dormancy and germination subdomains is enriched with genes involved in cellular phase transitions. The phase transition regulators SERRATE and EARLY FLOWERING IN SHORT DAYS from this region affect seed germination, indicating that conserved mechanisms control transitions in cell identity in plants. The SeedNet dormancy region is strongly associated with vegetative abiotic stress response genes. These data suggest that seed dormancy, an adaptive trait that arose evolutionarily late, evolved by coopting existing genetic pathways regulating cellular phase transition and abiotic stress. SeedNet is available as a community resource (http:// vseed.nottingham.ac.uk) to aid dissection of this complex trait and gene function in diverse processes.

Embryo and caryopsis dormancy, abscisic acid (ABA) responsiveness, after-ripening (AR), and the disorder pre-harvest sprouting (PHS) were investigated in six genetically related wheat varieties previously characterized as resistant, intermediate, or susceptible to PHS. Timing of caryopsis AR differed between varieties; AR occurred before harvest ripeness in the most PHS-susceptible, whereas AR was slowest in the most PHS-resistant. Whole caryopses of all varieties showed little ABA-responsiveness during AR; PHS-susceptible varieties were responsive at the beginning of the AR period whereas PHS-resistant showed some responsiveness throughout. Isolated embryos showed relatively little dormancy during grain-filling and most varieties exhibited a window of decreased ABA-responsiveness around the period of maximum dry matter accumulation (physiological maturity). Susceptibility to PHS was assessed by overhead misting of either isolated ears or whole plants during AR; varieties were clearly distinguished using both methods. These analyses allowed an investigation of the interactions between the different components of seed development, compartments, and environment for the six varieties. There was no direct relationship between speed of caryopsis AR and embryo dormancy or ABA-responsiveness during seed maturation. However, the velocity of AR of a variety was closely associated with the degree of susceptibility to PHS during AR suggesting that these characters are developmentally linked. Investigation of genetic components of AR may therefore aid breeding approaches to reduce susceptibility to PHS.

Highlight Phs-A1 confers resistance to sprouting in wheat by delaying the rate of seed dormancy loss and is distinct from the previously proposed PM19 candidate genes.

Potato has been genetically engineered for the production of commercially important ketocarotenoids including astaxanthin (3,3'-dihydroxy 4,4'-diketo-beta-carotene). To support the formation of 3-hydroxylated and 4-ketolated beta-carotene, a transgenic potato line accumulating zeaxanthin due to inactivated zeaxanthin epoxidase was co-transformed with the crtO beta-carotene ketolase gene from the cyanobacterium Synechocystis under a constitutive promoter. Plants were generated which exhibited expression of this gene, resulting in an accumulation of echinenone, 3'-hydroxyechinenone, and 4-ketozeaxanthin in leaves, as well as 3'-hydroxyechinenone, 4-ketozeaxanthin together with astaxanthin in the tuber. The amount of ketocarotenoids formed represent approximately 10-12% of total carotenoids in leaves and tubers. Negative effects on photosynthesis due to the presence of the ketocarotenoids in leaves could be excluded by the determination of variable fluorescence.

No abstract

Nicotiana glauca is a tobacco species that forms flowers with carotenoid-pigmented petals, sepals, pistil, ovary and nectary tissue. The carotenoids produced are lutein, ss-carotene as well as some violaxanthin and antheraxanthin. This tobacco species was genetically modified for ketocarotenoid biosynthesis by transformation with a cyanobacterial crtO ketolase gene under the 35S CaMV promoter. In the transformants, ketocarotenoids were detected in both leaves and flowers. Although astaxanthin was not detected other ketocarotenoids such as 4'-ketolutein, echinenone, 3'-hydroxyechinenone and 4-ketozeaxanthin were present. Accumulation of ketocarotenoids in leaves decreased their photosynthetic efficiency moderately. Under the green house conditions used no impairment of growth and development compared to the wild type was observed. In the crtO-transformants, an unexpected up-regulation of total carotenoid biosynthesis in leaves and especially in flower petals was observed. This led to a total ketocarotenoid concentration in leaves of 136.6 (young) or 156.1 (older) mug/g dry weight and in petals of 165 mug/g dry weight. In our engineered plants, the ketocarotenoid pathway is one step short of astaxanthin. Strategies are discussed to improve N. glauca flowers as a biological system for astaxanthin.

Ketocarotenoids and especially astaxanthin are high‐valued pigments used as feed additives. Conventionally, they are provided by chemical synthesis. Their biological production is a promising alternative. For the development of a plant production system, Nicotiana glauca, a species with carotenoid‐containing yellow pigmented flower petals, was transformed with a cyanobacterial ketolase gene. The resulting plants accumulated 4‐ketozeaxantin (adinoxanthin), which is the first ketocarotenoid synthesized in flower petals by genetic modification. Due to the very late flowering in this tobacco species, N. tabacum was used to optimize the yield and ketocarotenoid product pattern by metabolic engineering of the ketolation steps of carotenogenesis. The highly carotenogenic nectary tissue in the flowers represents a model of a flower chromoplast system. By expression of a ketolase gene, it was possible to engineer the biosynthetic pathway towards the formation of 3'‐hydroxyechinenone, 3‐hydroxyechinenone, 4‐ketozeaxanthin, 4‐ketozeaxanthin esters, 4‐ketolutein and 4‐ketolutein esters. Some of these ketocarotenoids were also formed in the leaves of the trangenic plants. In particular, by co‐expression of the ketolase gene in combination with a hydroxylase gene under an ubiquitous promoter, the formation of total carotenoids in nectaries increased by more than 2.5‐fold. In the nectaries of this type of transformants, more than 50% of the accumulating carotenoids were keto derivatives. In addition, the levels of ketocarotenoid esters were much lower and a higher percentage of the free ketocarotenoids accumulated. These results open new promising perspectives for a successful metabolic engineering of keto‐hydroxy carotenoid production in carotenogenic flowers.

No abstract