EXPRESSION OF HETEROLOGOUS PHYTOCHROME-A, PHYTOCHROME-B OR PHYTOCHROME-C IN TRANSGENIC TOBACCO PLANTS ALTERS VEGETATIVE DEVELOPMENT AND FLOWERING TIME

In this study, oat phytochrome A (phyA), Arabidopsis phytochrome B (ph yB) or Arabidopsis phytochrome C (phyC) were expressed in both day-neu tral and photoperiod-sensitive (short-day) tobacco (Nicotiana tabacum cv. Hicks). Introgression of the Maryland Mammoth (MM) gene into cv Hi cks was used to confer short-day photoperiodic sensitivity. Expression of oat phyA led to characteristic hypersensitivity of hypocotyls to r ed light (R) and far-red light (FR) and an overall dwarfing of the mat ure plant. Expression of Arabidopsis phyB enhanced the sensitivity of hypocotyls to R and caused even more marked dwarfing of the mature pla nt. In contrast, the expression of Arabidopsis phyC had no detectable consequences for the photocontrol of hypocotyl elongation. However, ph yC expression did lead to a R-dependent increase in cotyledon expansio n in de-etiolating seedlings and to a significant increase in leaf are a in mature plants. This provides the first experimental evidence that phyC is biologically active. The flowering time of cv Hicks plants gr own under 8 h photoperiods was virtually unaffected by a 30 min white light (W) night break given 8 h into the dark period. In contrast, cv Hicks MM plants responded to a night break with a delay in flowering. Expression of phyA or phyB led to a night break-dependent delay in flo wering in cv Hicks plants. For cv Hicks MM plants, the expression of a ny of phyA, phyB or phyC caused a marked enhancement of the flower-del aying effect of a night break. These observations indicate that transg enic phyA, phyB or phyC can interact with the endogenous mechanisms co ntrolling flowering time in tobacco.