One of the most significant developmental changes a plant experiences is the switch to reproductive growth from vegetative growth. This transition requires a wholesale change in the identity of the shoot apical meristem, and for most meristems it also means a shift from indeterminate to determinate growth, implying that any new leaf growth must come from another meristem. Given the biological significance of this transition and the horticultural interest in understanding and controlling the timing of flowering, the events that lead to flowering have been intensely studied.
As with so many other ‘decisions’ made by plants, the inputs that regulate the transition to flowering are made up mostly of cues from the environment. For most plants, the strongest influence over the transition to flowering is day length, and plants can be sorted into 3 classes based on the photoperiod required to induce flowering. Short-day plants transition to flowering when day length begins shortening and reaches a duration less than the critical period for that species. In the northern hemisphere, these are plants that flower after June 21, the longest day of the year. In contrast, long-day photoperiodic plants begin their transition to flowering as day length increases past a certain minimum length. In the northern hemisphere, these are plants that flower in spring and early summer, as day length is waxing. Finally, many plants are day-neutral and will transition to flowering regardless of light cues. Plants that have strict photoperiod requirements for flowering are known as obligate short- or long-day flowering plants, while those that will eventually flower regardless of day length but a particular photoperiod promotes flowering are known as facultative short- or long-day flowering plants.
There is a certain irony in all this talk of day length in promoting flowering, which is that the plant is not actually responsive to the day length at all, but rather to the length of the dark period. In order for obligate short-day flowering plants to begin flowering, they must experience a long, uninterrupted period of darkness. Likewise, obligate long-day flowering plants must experience a shorteningn period of darkness. The reason plants sense the duration of darkness lies in the biophysics of phytochrome, the red/far-red photoreversible photoreceptor. The pool of phytochrome molecules becomes activated by exposure to red light, such as that experienced by a plant during daytime. This pool of molecules slowly reverts back to the inactive red-absorbing form during the dark period, hence, the longer the duration of darkness, the greater the proportion of total phytochrome is inactive. For obligate short-day (or long night) flowering plants, if this dark period is interrupted by even a brief pulse of intense light, that is enough to disrupt the balance of phytochrome signaling that leads to floral transition.
A number of other signals besides photoperiod can influence the transition to flowering in some plants. Some plants flower after a given number of days since germination, regardless of the photoperiod. Related to this, some plants transition to flowering after attaining a minimum size — for example, many trees must be 5 or more years old before flowering, probably due to the high resource demands associated with seed and fruit production.
Temperature is another particularly important cue for some plants in the transition to flowering. Many plants require the exposure of the meristem to cold temperatures in order to complete the transition to flowering meristem identity. This process of cold treatment is known as vernalization because of its association with growing through the winter months. The cold treatment has been shown to modulate certain epigenetic factors associated with gene expression, such as DNA methylation. Some plants that have this requirement undergo a two year life cycle with the exposure to cold in the intervening winter. These plants are known as biennial plants, flowering in their second year of growth and typically dying thereafter.
In order to begin making flowers, shoot apical meristems must undergo a change in identity from vegetative to floral (or inflorescence) meristems. This change in meristem identity is influenced by numerous environmental cues, but how do these cues affect the identity of the meristem? Where do the signals come from that result in flower formation, and how do they form?
Through a series of clever experiments starting in the 1920’s, plant biologists have shown that, although the meristem is the site of floral transition, the signals that convert the meristem from leaf-making to flower-making come from the leaves. One of the key experiments to demonstrate this involved grafting a leaf from a plant induced to flower onto a plant that had not yet been induced, resulting in the induction of flowering. For many decades researchers attempted to isolate the diffusible substance causing flowering, but were unsuccessful. Around this same time the plant hormones auxin and gibberellin had been succesfully isolated and characterized, leading many physiologists to search for a similar small organic molecule as the “flowering hormone,” or florigen.
Recent work has revealed that the mRNA and/or protein product of FLOWERING LOCUS T (FT) acts as a positive regulator of flowering. Its expression is upregulated in response to photoperiodic induction in the leaves, and it trafficks through the phloem to the shoot apical meristem. FT expression in leaves is mediated by the expression of another gene, CONSTANS, which encodes a transcription factor that is expressed as an output of the photoperiodic pathway. The movement of FT protein and/or mRNA from the leaves to the meristem requires interaction with a partner called FTIP, or FT interacting protein.
Upon reaching in the meristem, FT interacts with the transcription factor FLOWERING LOCUS D (FD), and together they activate expression of genes that change the identity of the meristem to that of a flower. One example target of the FT-FD dimer is APETALA1 (AP1), a meristem identity gene; another example target is SOC1, which promotes the expression of another meristem identity gene called LEAFY (LFY). Both AP1 and LFY expression are repressed during vegetative growth, and this repression is removed upon arrival of FT in the meristem.