The ABC Model Explains the Determination of Floral Organ Identity

In 1991 the ABC model was proposed to explain how homeotic genes control organ identity. The ABC model postulates that organ identity in each whorl is determined by a unique combination of the three organ identity gene activities (see Figure 24.6) Activity of type A alone specifies sepals. Activities of both A and B are required for the formation of petals. Activities of B and C form stamens. Activity of C alone specifies carpels. The model further proposes that activities A and C mutually...

Grafting Studies Have Provided Evidence for a Transmissible Floral Stimulus

The production in photoperiodically induced leaves of a biochemical signal that is transported to a distant target tissue (the shoot apex) where it stimulates a response (flowering) satisfies an important criterion for a hormonal effect. In the 1930s, Mikhail Chailakhyan, working in Russia, postulated the existence of a universal flowering hormone, which he named florigen. The evidence in support of florigen comes mainly from early grafting experiments in which noninduced receptor plants were...

Plants Can Be Classified by Their Photoperiodic Responses

Numerous plant species flower during the long days of summer, and for many years plant physiologists believed that the correlation between long days and flowering was a consequence of the accumulation of photosynthetic products synthesized during long days. This hypothesis was shown to be incorrect by the work of Wightman Garner and Henry Allard, conducted in the 1920s at the U.S. Department of Agriculture laboratories in Beltsville, Maryland. They found that a mutant variety of tobacco,...

Phytochrome Is the Primary Photoreceptor in Photoperiodism

How Does Phytochrome Control Flowering

Night-break experiments are well suited for studying the nature of the photoreceptors involved in the reception of light signals during the photoperiodic response. The inhibition of flowering in SDPs by night breaks was one of the first physiological processes shown to be under the control of phytochrome Figure 24.22 . In many SDPs, a night break becomes effective only when the supplied dose of light is sufficient to saturate the photoconversion of Pr phytochrome that absorbs red light to Pfr...

Gibberellins and Ethylene Can Induce Flowering in Some Plants

Among the naturally occurring growth hormones, gib-berellins GAs see Chapter 20 can have a strong influence on flowering see Web Topic 24.8 . Recent studies suggest that gibberellin promotes flowering in Arabidopsis by activating expression of the LFY gene Blazquez and Weigel 2000 . Exogenous gibberellin can evoke flowering when applied either to rosette LDPs like Arabidopsis, or to dual-day length plants such as Bryophyllum, when grown under short days Lang 1965 Zeevaart 1985 . In addition,...

The Circadian Clock Is Involved in Photoperiodic Timekeeping

Photoperiodism Examples

The decisive effect of night length on flowering indicates that measuring the passage of time in darkness is central to photoperiodic timekeeping. Most of the available evidence favors a mechanism based on a circadian rhythm Bunning 1960 . According to the clock hypothesis, pho-toperiodic timekeeping depends on an endogenous circa-dian oscillator of the type involved in the daily rhythms described in Chapter 17 in relation to phytochrome. The central oscillator is coupled to various...

Competence and Determination Are Two Stages in Floral Evocation

The term juvenility has different meanings for herbaceous and woody species. Whereas juvenile herbaceous meristems flower readily when grafted onto flowering adult plants see Web Topic 24.3 , juvenile woody meristems generally do not. What is the difference between the two Extensive studies in tobacco have demonstrated that floral evocation requires the apical bud to pass through two developmental stages Figure 24.12 McDaniel et al. 1992 . One stage is the acquisition of competence. A bud is...

Evidence for Antiflorigen Has Been Found in Some LDPs

Grafting studies have implicated transmissible inhibitors in flowering regulation as well. Such inhibitors have been called antiflorigen, but like florigen antiflorigen may consist of multiple compounds. For example, grafting an unin-duced leafy shoot from the LDP Nicotiana sylvestris onto the day-neutral tobacco cultivar Trapezond suppressed flowering in the day-neutral plant under short days but not long-day conditions Figure 24.31 . On the other hand, when an uninduced donor from the SDP...

Three Types of Homeotic Genes Control Floral Organ Identity

Floral Homeotic Genes

Five different genes are known to specify floral organ identity in Arabidopsis APETALA1 API , APETALA2 AP2 , APETALA3 AP3 , PISTILLATA PI , and AGAMOUS AG Bowman et al. 1989 Weigel and Meyerowitz 1994 . The organ identity genes initially were identified through mutations that dramatically alter the structure and thus the identity of the floral organs produced in two adjacent whorls Figure 24.5 . For example, plants with the ap2 mutation lack sepals and petals see Figure 24.5B . Plants bearing...