Downloaded from genesdev.cshlp.org on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press REVIEW Move on up, it’s time for change—mobile signals controlling photoperiod-dependent flowering Yasushi Kobayashi and Detlef Weigel1 Department of Molecular Biology, Max Planck Institute for Developmental Biology, D-72076, Tübingen, Germany Plants do not bloom randomly—but how do they know mining where plants could grow. Some 50 years later, when and where to make flowers? Here, we review mo- the French scientist Tournois (1912) found that both Hu- lecular mechanisms that integrate spatial and temporal mulus (hop) and Cannabis (hemp) flower precociously information in day-length-dependent flowering. Prima- when kept in winter in a greenhouse supplemented with rily through genetic analyses in two species, Arabidop- artificial light, in agreement with Henfrey’s postulate. sis thaliana and rice, we today understand the essentials The German Klebs made similar observations with Sem- of two central issues in plant biology: how the appropri- pervivum (house leek). Importantly, he realized that it ate photoperiod generates an inductive stimulus based was unlikely to be light quantity (as a nutritive factor, as on an external coincidence mechanism, and the nature he called it) but rather light duration (acting as a cata- of the mobile flowering signal, florigen, which relays lytic factor) that was critical in this process (summarized photoperiod-dependent information from the leaf to the in Klebs 1913). growing tip of the plant, the shoot apex. Despite Tournois’ and Klebs’ contributions, the dis- covery of day length as a crucial factor in flowering con- trol is generally accredited to Wightman Garner and We all love springtime, not least because flowers are ev- Harry Allard, scientists at the US Department of Agri- erywhere. But how do plants know it is the right time to culture. Garner and Allard (1920, 1923) were working on bloom? Just like us, plants can recognize the seasons. two practical problems. One was the question of why Among different indicators such as temperature and certain strains of soybeans often would initiate flowers light, the increase in day length is the most robust tell- more or less simultaneously, typically during the height tale sign that spring is upon us, at least for those of us of summer, even if farmers had spread out the sowing who do not live in the tropics. The ability to recognize dates. The other related to a tobacco variety that had and respond to changes in day length is known as pho- spontaneously arisen in the field. This strain, appropri- toperiodism and is common to both plants and animals. ately called “Maryland Mammoth,” would grow very In this review, we will focus on events downstream from tall, yielding nearly 100 leaves, whereas normal tobacco the circadian clock that relay the photoperiodic signal plants switch to making flowers with only ∼20 leaves. from the site of perception, the leaves, to the shoot apex, The issue here was that this variety did not live up to its where flowers are formed. Recent advances in this area full commercial potential, because the delay in flowering have finally provided a molecular underpinning for two often caused Maryland Mammoth plants to succumb to long-standing tenets of plant biology, the external coin- frost before they had produced any seeds, making it dif- cidence model and the florigen hypothesis. ficult to propagate the strain. The reason that Garner and Allard are remembered for being the first to clearly demonstrate the effect of pho- The discovery of photoperiodic control of flowering toperiod is that they performed controlled experiments. One of the first to recognize the importance of day length Starting with Biloxi soybean and Maryland Mammoth in flowering was the English botanist Arthur Henfrey tobacco plants raised in pots, they exposed one set to (1852), who suggested that summer day length, which natural outdoors conditions, but artificially shortened changes with latitude, is an important factor in deter- the day for the other group by moving them into a win- dowless shed in the afternoon, returning them to the field in the morning. This simple trick greatly acceler- [Keywords: Flowering; floral induction; photoperiod; CONSTANS; ated the flowering response of both soybean and tobacco, FLOWERING LOCUS T; Arabidopsis] leading Garner and Allard (1920, 1923) to deduce that 1Corresponding author. E-MAIL [email protected]; FAX 49-7071-601-1412. these strains only flowered when day length fell under a Article is online at http://www.genesdev.org/cgi/doi/10.1101/gad.1589007. certain threshold. This neatly explained why the exact GENES & DEVELOPMENT 21:2371–2384 © 2007 by Cold Spring Harbor Laboratory Press ISSN 0890-9369/07; www.genesdev.org 2371 Downloaded from genesdev.cshlp.org on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press Kobayashi and Weigel time point of sowing has not much influence on when interaction might occur. According to the Bünning hy- Biloxi soybeans start to make flowers or why Maryland pothesis, a 24-h day is divided into a photophile (light- Mammoth tobacco produce flowers only late in fall. sensitive) and scotophile (dark-sensitive) phase, and the It did not take much longer for scientists to realize rhythmic alternation of the two is prescribed by an en- that flowering plants generally belong to three catego- dogenous circadian oscillator. The presence or absence of ries: those in which short days promote flowering or long external light during either phase would tell a plant days prevent it, those in which long days promote flow- whether it was exposed to a short or a long day (Fig. 1). ering or short days prevent it, and a group that does not The circumstances of the time prevented the ideas laid care either way. These categories are known as short- out by Bünning, an avowed Social Democrat opposed to day, long-day, and day-neutral plants, and are often dis- the Nazis, from gaining immediate and wide acceptance. tinguished further by the adjectives obligate and faculta- From the late 1950s, Colin Pittendrigh, Professor at tive, depending on whether the corresponding day length Princeton and Stanford, became a great advocate of Bün- requirement is absolute (for review, see Thomas and ning’s work. Pittendrigh, who was working with insects, Vince-Prue 1997). Furthermore, the terms “short days” formalized and expanded the Bünning hypothesis, coin- and “long days” are relative—they merely indicate that ing the term “external coincidence,” since the model one plant will flower when day length falls below its posits that the photoperiodic signal is only generated critical photoperiod, while another flowers once it rises when a specific external phase (light or dark) coincides above its own specific threshold. Hence, the short day of with the appropriate internal phase of the 24-h cycle. one plant may actually not be different in length from Importantly, light has a dual role in this model: It en- the long day of another plant. trains the circadian oscillation of light- and dark-sensi- tive phases, and it is directly required for the production of the signal (Pittendrigh 1960). In contrast to a simple How plants (and other organisms) measure the length hourglass mechanism, what matters in the external co- of day and night The discovery of photoperiodism raised several ques- tions, including which molecules the plant uses to sense light and how these molecules help the plant to measure day length. Perhaps the simplest mechanism one can envision is modeled on an hourglass. For example, every time the light is switched on, a substance that prevents flowering above a certain level begins to accumulate. If this substance is in addition degraded again every night, it will only stay under the critical threshold and hence permit flowering if days are sufficiently short. Erwin Bünning was the first to propose that plants might use a more sophisticated mechanism. His model was based on the known fact that bean seedlings show a pronounced daily rhythm in leaf movement and that this rhythm is maintained even after plants have been moved to permanent darkness. Key insights of Bünning and his collaborator Kurt Stern (Bünning and Stern 1930) were that the red component of the light spectrum causes the movement of different individual plants to be synchro- nous and that without the red light cue the period would slowly diverge from the normal 24 h. Bünning correctly concluded that the “biological clock” of plants is par- tially independent of the daily light/dark cycle. He fur- thermore found that different strains of beans have en- dogenous rhythms that can be either shorter or longer Figure 1. Two models to explain photoperiodic responses in than 24 h, and, using crosses, he demonstrated a genetic short-day plants. In the external coincidence model, light en- basis for these differences. trains a periodic function (ochre). Light also causes the produc- Initially, Bünning thought that the movement of bean tion of a repressor (red) directly proportionate to this function. leaves had no adaptive value, but rather was just a con- Thus, even short night breaks, if given at the correct time, can venient indicator of what we call today the circadian lead to the repressor surpassing a critical threshold (dashed line). In the internal coincidence model, light signals (e.g., lights on/ clock, to reflect its role in time measurement on a 24-h off) entrain two different periodic functions. If one function scale. He did, however, believe that an interaction of the (ochre) leads to production of a repressor, but the other function endogenous rhythm with external rhythms would be im- (blue) inhibits it, it will depend on their relative overlap portant for plant development.