A physiological and genetic analysis of the role of phytochrome in photoperiodic induction of flowering in Arabidopsis Thaliana

Abstract

An analysis of the photoperiodic induction of flowering in Arabidopsis thaliana (L.) Heyne, Landsberg erecta ecotype, was carried out. This revealed that 4 day old seedlings, at which time the cotyledons where expanding and greening, could differentiate between a LD and a SD. At this stage the critical daylength was between 8 and 10 hours. Plants grown in daylengths of 8 hours, a short day, flowered after 50-70 days and when grown in 16 hour daylengths, a long day, flowered after 24-27 days. At 7 days seedlings required five long days to fully induce flowering and as the seedlings aged in short days they became more sensitive to interposed long days, so that by day 20 one long day was fully inductive. It was found that there were two different photomorphogenic responses shown by plants grown in short days, firstly flowering was delayed and secondly further leaves were induced. The short days delay of flowering occurred in newly germinated seedlings older than 4 days and further leaves were induced in plants older than 10 days.
From light quality experiments it was concluded that both a blue light photoreceptor and phytochrome promoted flowering. The induction of flowering by phytochrome was through a HIR mode. Three of the photoreceptor mutants, hy, isolated by Koornneef et al. (1980) were used. In daylength transfer experiments all the hy mutants studied showed delay in flowering by short days and all responded to long days by flowering earlier. Both hy2 and hy3 produced far fewer leaves than Ler when grown in short days. The hy4 mutants flowered later in both long days and short days than Ler and had an increased leaf number. A scheme is proposed in which photoperiodic induction depends on the ability of the plant to sense photoperiod, the stage of development and the photobiological input. It also proposes that phyA or C and the blue light photoreceptor promote flowering whereas phyB promotes vegetative development.
Two screens were set up to isolate novel photoperiodic mutants. Six mutants were isolated, from ethtylmethane
sulphonate mutated seed, which all flowered earlier than Ler in SD. They were called FUN 1-6, flowering pjcoupled. Genetic analysis showed that all were non allelic and that they were recessive except .tun4 which was semi-dominant. Physiological studies showed that there were two types of mutants: firstly funl and 2 whose flowering was not significantly delayed by SD in comparison to long day treatments and flowered early in the dark; and secondly fun3-6 which all showed a delay in flowering when grown in SD. The .funl, and 2 mutants had poorly developed leaves as did fun5. The other mutants did not show any other clearly defined phenotypes. These results suggest that funl and 2 mutants are constitutive flowering mutants and the
remaining mutants are transduction chain mutants.