The flowering transition and florigen

Abstract

What is the flowering transition? It marks the end of vegetative growth and the beginning of reproductive development in flowering plants. The physiological changes that cause it are referred to as floral induction.What is SAM? The shoot apical meristem — a group of undifferentiated progenitor cells that occupies the tip of the stem. It gives rise to both vegetative organs (leaves) and reproductive structures (inflorescence and flower).How is the flowering transition regulated? By environmental cues such as the photoperiod, temperature, and water and nutrient availability. Photoperiod is a measure of the season, and most plants time their reproductive development to the appropriate season. Most annuals flower in late summer and fall when daylight becomes short and are called short-day plants. Biennials or perennials have sufficient vegetative growth in spring and flower in spring or early summer to take advantage of the resourceful summer for reproductive development. These are called long-day plants. Some plants flower at a certain developmental stage regardless of the photoperiod, and are called autonomous-flowering or day-neutral plants.What is florigen? Florigen represents a signal that triggers flowering, and was thought to be a specific hormone. Early grafting experiments showed that photoperiod is sensed by leaves, yet affects flowering and the SAM. Therefore, a signal must be produced in the leaves and transported to the SAM. It remains a controversial proposition. More recently, others have proposed that substances are transmitted from leaves to the SAM and together trigger flowering. In fact, both plant hormones and chemical assimilates such as sugar can regulate flowering time.Is that all that's known? Recent molecular genetic studies in Arabidopsis have made considerable progress in understanding the control of floral induction. The perception of photoperiod depends on plant photoreceptors, phytochromes and cryptochromes, which detect red/far-red and blue/UV-A light, respectively. In particular the Arabidopsis phytochromes PHYA and PHYB play a role in regulating flowering time. And the Arabidopsis cryptochrome CRY2 is important for sensing photoperiod and promotes flowering.How do photoreceptors affect floral initiation? Photoreceptors regulate the plant circadian clock which regulates flowering. The photoreceptors regulate the expression of flowering time genes such as CONSTANTS (CO). CO promotes floral induction only under long-day conditions. Expression of CO is regulated by photoperiod and is decreased in cry2 mutants. CO likely acts between the circadian clock and the control of flowering.Are CO and florigen connected?CO is in leaves, yet can activate the expression of two genes, LEAFY and APETALA1, expressed in reproductive meristems, that directly control the initiation of flower development. Therefore CO could regulate the synthesis of a chemical that might fit the definition of florigen. Alternatively, CO could control the levels of several substances, which together regulate the flowering transition. Similar arguments can be made for a role of the maize gene indeterminate in regulating the synthesis of transportable substances because of its function in regulating floral initiation and its expression in leaves.Now the future… Recent studies provide no evidence to distinguish different models for substances that are transmitted from leaves to the SAM. Further studies are required to provide new insights into the area of flower transition, including understanding the properties of the proposed florigen.Where can I find out more?