Bloom and bust: understanding the nature and regulation of the end of flowering

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The reproduction of flowering plants is an incredibly important process, both ecologically and economically. A huge body of work has examined the mechanisms by which flowering plants correctly time their entry into the reproductive phase (the ‘floral transition’). However, the corresponding mechanisms by which plants exit the reproductive phase remain relatively neglected. In this review, we identify four developmental processes that contribute to the end-of-flowering; floral arrest, inflorescence meristem arrest, inflorescence activation and ‘vegetative transition’. We highlight that, due to the highly divergent nature of reproductive systems among flowering plants, these processes are differently important for end-of-flowering in different species. For each of these processes, we examine recent advances in understanding the regulatory mechanisms that govern the process, and how these mechanisms determine the timing of end-of-flowering.

Section snippets

Introduction: a poorly understood but important process

In all plants, entry into the reproductive phase is carefully timed to maximize reproductive success. In flowering plants, it is essential that reproduction starts in the right season, allowing plants to utilize intermittent resources (e.g. water) and pollinator availability. After this ‘floral transition’, one or more vegetative shoot meristems becomes converted to an inflorescence meristem (IM), which initiates multiple floral meristems (FMs). The determinate FMs produce flowers with a

What is the end of flowering?

To understand how the end-of-flowering is regulated, we need to define exactly when and where it occurs, but unlike the floral transition, this is not straightforward. As a phenomenon, the most obvious definition for end-of-flowering would be the time-point when no further flowers open (‘floral arrest’). However, is flower-opening really a relevant regulatory checkpoint? Or do events earlier in the reproductive phase inevitably determine when end-of-flowering occurs? There is likely no single

The vegetative transition

The current consensus model for initiation of flowering is that a variety of environmental and developmental cues regulate the production of ‘florigen’, a mobile signal that triggers the floral transition in shoot meristems [8, 9, 10]. Over 200 genes regulating floral initiation have been identified [11] and much of the underlying genetic network has been characterized [12, 13, 14, 15], but the relative contribution of these genes can differ between species. Of particular note in Brassicaceae

Regulation of inflorescence number

As specialised shoot branches, activation of inflorescences is regulated comparably to vegetative branch activation. Indeed, since they are the only true branches Arabidopsis produces, a significant part of our knowledge of branching is derived from inflorescences. The current ‘hybrid’ model for shoot branching suggests that resource availability, communicated through hormonal signals including sugars, dictates the number of branches produced, with each shoot apex making a local ‘priming’

Regulation of IM arrest

As discussed above, once an inflorescence has been specified and activated, it may initiate an invariant or variable number of FMs. In this section, we specifically focus on the regulation of IM activity in variable IMs. One of the clear advantages of variable IMs is their ability to compensate for changes in status during flowering, and to produce more or less flowers depending on environmental conditions or past developmental events. Recent work in Arabidopsis and other Brassicaceae strongly

Regulation of flower maturation

While there has been much research into the initiation of flowers, we know less about regulation of their maturation, especially with regard to the numbers that are ultimately opened. Arabidopsis inflorescences typically arrest with a cluster of unopened buds (Figure 4a), so clearly the initiation of FMs does not result in their inevitable maturation into flowers. Thus, while IM activity regulates the number of flowers initiated, IM arrest is not sufficient to explain the timing of floral

Conclusions

This brief journey through the end of flowering highlights the diverse processes that contribute to bringing the reproductive phase to an end, and particularly the flexible manner in which these processes can be regulated to adjust the duration of flowering in response to environmental conditions. Deeper understanding of the regulatory mechanisms that underpin end-of-flowering will open up the prospect of precise manipulation of end-of-flowering to ‘push’ yields, or conversely to reduce

Conflict of interest statement

Nothing declared.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

The authors are grateful to Alexander Ware, Zoe Wilson and Laura Dixon for helpful discussions and insights that have helped to shape these ideas.

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