Gene expressions associated with in vitro floral transition in an orchid hybrid (Dendrobium grex Madame Thong-In) were investigated by differential display. One clone, orchid transitional growth related gene 7 (otg7), encoding a new MADS-box gene, was identified to be specifically expressed in the transitional shoot apical meristem (TSAM). Using this clone as a probe, three orchid MADS-box genes, DOMADS1, DOMADS2, and DOMADS3, were subsequently isolated from the TSAM cDNA library. Phylogenetic analyses show that DOMADS1 and DOMADS2 are new members of the AGL2 subfamily and SQUA subfamily, respectively. DOMADS3 contains the signature amino acids as with the members in the independent OSMADS1 subfamily separated from the AGL2 subfamily. All three of the DOMADS genes were expressed in the TSAM during floral transition and later in mature flowers. DOMADS1 RNA was uniformly expressed in both of the inflorescence meristem and the floral primordium and later localized in all of the floral organs. DOMADS2 showed a novel expression pattern that has not been previously characterized for any other MADS-box genes. DOMADS2 transcript was expressed early in the 6-week-old vegetative shoot apical meristem in which the obvious morphological change to floral development had yet to occur. It was expressed throughout the process of floral transition and later in the columns of mature flowers. The onset of DOMADS3 transcription was in the early TSAM at the stage before the differentiation of the first flower primordium. Later, DOMADS3 transcript was only detectable in the pedicel tissues. Our results suggest that the DOMADS genes play important roles in the process of floral transition.

The influence of irradiance on photosynthesis under natural conditions was studied in aseasonal Singapore using three Heliconia taxa: H. rostrata, H. psittacorum × H. spathocircinata cv. Golden Torch and H. psittacorum cv. Tay. When grown under full sunlight, all three heliconias exhibited reduced phatosynthetic capacities and lowered chlorophyll content per leaf area as compared with those grown under intermediate and deep shade. A marked decrease in the chlorophyll fluorescence Fv/Fm ratio and an increase in photochemical quenching (1‐ qp) and non‐photochemical quenching (qN) were observed in upper leaves of plants grown under full sunlight. Increases in qN suggest that ‘photoinhibition’ (decreases in Fv/Fm) in Heliconia grown under natural tropical conditions are probably due to photoprotective energy dissipation processes. The quantum yield, the maximum photosynthetic rate, Fv/Fm and the chlorophyll content of upper leaves were lower than those of lower leaves on the same plants grown under full sunlight. Similarly, lower values were obtained for the tip (sun) portion than for the base (shaded) portion of the leaves. The changes in Fv/Fm and in the levels of (1 –qp) in leaves grown under intermediate and deep shade were negligible in plants during the course of day. However, there was a steep decrease in Fv/Fm and an increase in the levels of (1 –qp), along with an increase in incident light in the sun leaves. The lowest Fv/Fm and the highest level of (1 –qp) indicated minimum PSII efficiency at midday in full sun. These results indicate that, in Heliconia, the top leaves (particularly leaf tips) experienced sustained decreases in PSII efficiency upon exposure to full sunlight. Although all three taxa exhibited sustained decreases in photosynthetic capacity in full sunlight, the sun leaves of ‘Tay’ showed higher photosynthetic capacity than those of the other two taxa. This could be due, at least in part, to the vertical leaf angle and smaller lamina area. When the upright leaves of ‘Tay’ were constrained to a horizontal angle, they exhibited lower PSII efficiency (FvIFm ratio), while horizontal leaves of ‘Rostrata’ and ‘Golden Torch’ inclined lo near‐vertical angles showed increased efficiency. Thus, an increase in leaf angle helps to achieve a reduction in the sustained decrease in PSII efficiency by decreasing the levels of incident sunlight and subsequently the leaf temperature.

The promotive effect of AgNO3 and aminoethoxyvinylglycine (AVG) on in vitro shoot regeneration from cotyledons of Brasska campestris ssp. peklnensls in relation to endogenous 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, ACC, and ethylene production was investigated. AgNO3 enhanced ACC synthase activity and ACC accumulation, which reached a maximum after 3 to 7 days of culture. ACC accumulation was concomitant with increased emanation of ethylene which peaked after 14 days. In contrast, AVG was inhibitory to endogenous ACC synthase activity and reduced ACC and ethylene production. The promotive effect of AVG on shoot regeneration was reversed by 2-chloroethylphosphonic acid at 50 micromolar or higher concentrations, whereas explants grown on AgNO3 medium were less affected by 2-chloroethylphosphonic acid. The distinctive effect of AgNO3 and AVG on endogenous ACC synthase, ACC, and ethylene production and its possible mechanisms are discussed.Plants usually exhibit enhanced ethylene production when wounded or under environmental stresses or pathogen attack (31). Ethylene biosynthesis in plants occurs via SAM' and ACC intermediates using methionine as a precursor (31), but the molecular mechanism of its action is not clear. It has been known for some time that cultured plant cells evolve ethylene (1 1 Cotyledonary explants excised from 3-d-old aseptically germinated seedlings of Brassica campestris ssp. pekinensis cv Shantung and cv Wong Bok were used. Because our preliminary investigations showed that explants cultured in 50-mL Erlenmyer flasks and 100-x 25-mm Petri dishes exhibited the similar capacity of shoot regeneration, both culture containers were used in this study. Explants were cultured either in a flask containing 25 mL medium or in a Petri dish containing 30 mL medium consisting of all constituents of Murashige and Skoog's medium (16)

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The plant hormone cytokinin plays a major role in regulating plant growth and development. Here we generated cytokinin-reduction Arabidopsis plants by overexpressing a heterologous cytokinin oxidase gene DSCKX1 from Dendrobium orchid. These transgenic plants exhibited reduced biomass, rapid root growth, decreased ability to form roots in vitro, and reduced response to cytokinin in growing calli and roots. Furthermore, the expression of KNAT1, STM, and CycD3 genes was signi¢cantly reduced in the transgenic plants, suggesting that cytokinin may function to control the cell cycles and shoot/ root development via regulation of these genes.