The roles of phytohormones in floral arrest and carpic dominance

The number of reproductive organs, and their positioning in both space and time (‘reproductive architecture’) are vital factors for the production of offspring in plants. We do not yet fully understand the mechanisms by which plants ‘decide’ how many reproductive organs to make, and where to make them, although elements are known to be controlled by long-distance hormonal signalling. Here, we have examined the control of reproductive architecture in Arabidopsis and the wider Brassicaceae; focusing on the control of inflorescence production, the quantity and positioning of fruits, and the mechanisms underlying floral arrest. We have shown that early signals control the number of inflorescences produced, and that these strongly predict the number of fruits that the plant will make. Fruit are distributed across these inflorescences in a highly predictable manner, with ~50% being supported by the secondary inflorescences, likely as a consequence of the timing of inflorescence arrest. Our examination of inflorescence arrest shows that arrest is a two-stage process, beginning with inflorescence meristem arrest, and followed by floral arrest. We clarify previous misconceptions around floral arrest, showing that it is in fact a local process, and is not regulated globally. Our data highlight the importance of both cytokinin and auxin in inflorescence arrest, with auxin export from developing fruits being required for normal arrest. In support of recent work, we have shown that cytokinin is a key promoter of inflorescence meristem activity, and that increasing cytokinin signalling can delay both inflorescence meristem and floral arrest. Development of this work highlights how the cytokinin signalling receptors ARABIDOPSIS HISTIDINE KINASE2 (AHK2) and AHK3 are differentially involved in the regulation of meristem activity in the inflorescence meristem and flowers respectively. Overall, this work provides a basis on which to develop future crop research; manipulation of cytokinin signalling and/or sensitivity shows excellent promise for yield increases without the need for increased inputs.