Integration of seasonal signals through a two gene mutual repression switch in flower buds

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Plants use the seasonal cues temperature and day-length to time their transition from vegetative to reproductive development. Weather and climate affects plant development primarily by acting on these signalling pathways, and thus this is a highly studied area of plant biology from which a simple paradigm has emerged, especially in the control of time to flowering. Firstly, long term temperature is sensed by the FLC locus, with temperature known to affect FLC mRNA transcription (at least in part through regulation of antisense transcript) and resulting in epigenetic changes to FLC that reflect seasonal progression. In leaves FLC represses the expression of a second key gene FT, which receives a separate signal at its promoter that reflects a measurement of day length. Thus, temperature and day-length information is believed to act additively at the FT promoter to modify plant behaviour. FT is a general phloem-mobile seasonal signal, acting as a transcriptional co-factor in target cells to promote developmental programmes associated with summer.We have been studying the same seasonal sensing pathway in the control of seed dormancy. This is a maternal process through which the mother uses her memory of seasonal information to affect the dormancy characters of her progeny seed. In this way we have previously shown that seasonal information is passed from mother to progeny in plants. The preliminary data we present here show that although FLC and FT are two of the most intensively-studied genes in plants, the above paradigm of stepwise additive integration of seasonal cues at the FT promoter cannot explain our observations on the maternal control of seed dormancy. Specifically, we show using genetics, gene expression analysis and chromatin immunoprecipitation that FLC is a previously unrecognised direct target of FT, and that FT can deliver day-length information to the antisense FLC promoter. Thus we propose a new model for plant seasonal sensing in which FT and FLC both inhibit each other in a classical two gene mutual inhibition switch, a very similar model to that believed to control cell cycle progression. Such a switch is likely to have non-linear properties, and integrate day-length and temperature information in non-obvious ways to control the effects of weather, seasons, and climate on plant development.The first Objective, split into three parts, aims to discover missing components of the FT/FLC seasonal switch, and how the switch operates in response to seasonal combinations of temperature and photoperiod cues. We focus on the timing and duration of winter temperature cues, and how their sensing and efficacy can be modified by occurring in days with different photoperiods. For instance, temperature cues experienced in longer days might affect the system in a different way to temperature cues experienced in very short winter days.The second part of the proposal deals with identifying the tissue in which the switch occurs to control post-flowering reproductive development. We show striking preliminary evidence that the FT/FLC switch has a dramatic spatial dimension in flowers, with environmental cues and FT causing FLC mRNA expression to switch from the female organ to the male organ. We hypothesise therefore that environmental cues controlling seed dormancy are sensed in developing flower buds, also the time at which development of the first seed structures begins. This is interesting because it aligns our understanding of the behaviour of annual plants much more closely to perennial plants, in which buds are set in summer but are dormant until cold winter conditions break this dormancy and enable bud break in the spring. We will examine the effect of expressing FLC heterologously in the male and female flower organs on bud and seed development and dormancy, and aim to uncover the molecular mechanism that enables FLC expression to switch dramatically between the two flower organs in response to seasonal cues.