The timing of flowering.

The initiation of flowering is a critical life-history trait; plants have presumably evolved to flower at a time of year that ensures maximal reproductive success in a given region. Decades of physiological studies have revealed that flowering is initiated in response to both environmental cues and endogenous pathways. Commonly studied environmental cues include changes in temperature and daylength. Endogenous pathways function independently of environmental signals and are related to the developmental state of the plant; such pathways are sometimes referred to as “autonomous” to indicate the lack of environmental influence. The relative contributions of autonomous and environmental inputs to the flowering “decision” vary among, and even within, species. For example, flowering is considered entirely due to autonomous pathways in a variety of tobacco (Nicotiana tabacum) that forms a fixed number of nodes before flowering regardless of the environment in which it is grown (McDaniel and Hsu, 1976). Yet, a single-gene change can cause tobacco to require short days to flower (Allard, 1919), which indicates that the underlying biochemical differences between environment-sensing and endogenous pathways can be minimal. Also, endogenous and environmental pathways can interact. For example, some plants pass through a juvenile phase in which they are not responsive to environmental cues that promote flowering (Poethig, 1990); that is, the transition from the juvenile to adult phase is a type of endogenous pathway that is necessary to provide competence for environmental pathways to promote flowering. The recent addition of molecular genetics to the range of approaches used to study the initiation of flowering has provided some molecular insights into these endogenous and environmentsensing pathways and has revealed how inputs from multiple pathways are integrated into the flowering decision. (Due to the sustained efforts of a multitude of scientists working in many species, we have learned much about the timing of flowering that is worth celebrating. Unfortunately, only a small part of this extensive body of work can be covered in this article because of length and reference limits. Accordingly, we frequently refer readers to recent review articles for more in-depth discussions, and we apologize to our colleagues whose work was not cited due to these constraints.)