The Transition to Flowering Involves Multiple Factors and Pathways

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It is clear that the transition to flowering involves a complex system of interacting factors that include, among others, carbohydrates, gibberellins, cytokinins, and, in the bromeliads, ethylene (see Web Topic 24.10). Leaf-generated transmissible signals are required for determination of the shoot apex in both autonomously regulated and photope-riodic species. Determining whether these transmissible signals consist of single or multiple components is a major challenge for the future.

Recent genetic studies have established that there are four genetically distinct developmental pathways that control flowering in the LDP Arabidopsis (Blazquez 2000). Figure 24.32 shows a simplified version of the four pathways:

1. The photoperiodic pathway involves phytochromes and cryptochromes. (Note that PHYA and PHYB have contrasting effects on flowering; see Web Topic 24.11.) The interaction of these photoreceptors with a circadian clock initiates a pathway that eventually results in the expression of the gene CONSTANS (CO), which encodes a zinc-finger transcription factor that promotes flowering. CO acts through other genes to increase the expression of the floral meristem identity gene LEAFY (LFY).

2. In the dual autonomous/vernalization pathway, flowering occurs either in response to internal signals—the production of a fixed number of leaves—or to low temperatures. In the autonomous pathway of Arabidopsis, all of the genes associated with the pathway are expressed in the meristem. The autonomous pathway acts by reducing the expression of the flowering repressor gene FLOWERING LOCUS C (FLC), an inhibitor of LFY (Michaels and Amasino 2000). Vernalization also represses FLC, but perhaps by a different mechanism (an epigenetic switch). Because the FLC gene is a common target, the autonomous and vernalization pathways are grouped together.

3. The carbohydrate, or sucrose, pathway reflects the metabolic state of the plant. Sucrose stimulates flowering in Arabidopsis by increasing LFY expression, although the genetic pathway is unknown.

Light

Photoperiodism

FIGURE 24.32 Four developmental pathways for flowering in Arabidopsis: the photoperiodism, autonomous/vernalization, sucrose, and gibberellin pathways. A transmissible floral stimulus ("florigen") from leaves is only involved in the photoperiodic pathway. (After Blazquez 2000.)

Leaf

Low

Sucrose

Gibberellins

number

temperature

AGAMOUS-LIKE 20 s.

Meristem

>

identity genes

LEAFY J

Light

Photoperiodism

Photoperiodism And Vernalization

FIGURE 24.32 Four developmental pathways for flowering in Arabidopsis: the photoperiodism, autonomous/vernalization, sucrose, and gibberellin pathways. A transmissible floral stimulus ("florigen") from leaves is only involved in the photoperiodic pathway. (After Blazquez 2000.)

Gibberellin Signal

4. The gibberellin pathway is required for early flowering and for flowering under noninductive short days.

All four pathways converge by increasing the expression of the key floral meristem identity gene AGAMOUS-LIKE 20 (AGL20). The role of AGL20, a MADS box-containing transcription factor, is to integrate the signals coming from all four pathways into a unitary output. Obviously the strongest output signal occurs when all four pathways are activated.

Figure 24.33 shows the level of AGL20 gene expression in the shoot apical meristem of an Arabidopsis plant after shifting from noninductive short days (8-hour day length) to inductive long days (16-hour day length). Note that an increase in AGL20 expression can be detected as early as 18 hours after the beginning of the long-day treatment (Borner et al. 2000). Thus it takes only 10 hours beyond an 8-hour short day for the meristem to begin responding to the floral stimulus from the leaves. This timing is consistent with pre vious measurements of the rates of export of the floral stimulus from induced leaves (discussed earlier in the chapter).

Although many pathways feed into AGL20, there must be some redundancy in the system because flowering is only delayed, but not completely blocked, in agl20 mutants. Thus, one or two other genes must be able to take over the role of AGL20 when it is mutated.

Once turned on by AGL20, LFY activates the floral homeotic genes—APETALA1 (API), APETALA3 (AP3), PISTILLATA (PI), and AGAMOUS (AG)—that are required for floral organ development. APETALA2 (AP2) is expressed in both vegetative and floral meristems and is therefore not affected by LFY. However, as discussed earlier in the chapter, AP2 exerts a negative effect on AG expression (see Figure 24.6).

Besides serving as a floral homeotic gene, AP1 functions as a meristem identity gene in Arabidopsis because it is involved in a positive feedback loop with LFY. Conse

Short days to long days at time 0

FIGURE 24.33 Increase in expression of the gene AGAMOUS-LIKE 20 (AGL20) during floral evocation in the shoot apical meristem of Arabidopsis. The times after shifting the plants from SDs to LDs are indicated. (From Borner et al. 2000.)

quently, once the transition to flowering has reached this stage, flowering is irreversible.

The existence of multiple flowering pathways provides angiosperms with maximum reproductive flexibility, enabling them to produce seeds under a wide variety of conditions. Redundancy within the pathways ensures that reproduction, the most crucial of all physiological functions, will be relatively insensitive to mutations and evo-lutionarily robust.

The details of the pathways undoubtedly vary among different species. In maize, for example, at least one of the genes involved in the autonomous pathway is expressed in leaves (see Web Topic 24.12). Nevertheless, the presence of multiple flowering pathways is probably universal among angiosperms.

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