Abstract
Fruits and seeds are the major food source on earth. Both derive from the gynoecium and, therefore, it is crucial to understand the mechanisms that guide the development of this organ of angiosperm species. In Arabidopsis, the gynoecium is composed of two congenitally fused carpels, where two domains: medial and lateral, can be distinguished. The medial domain includes the carpel margin meristem (CMM) that is key for the production of the internal tissues involved in fertilization, such as septum, ovules, and transmitting tract. Interestingly, the medial domain shows a high cytokinin signaling output, in contrast to the lateral domain, where it is hardly detected. While it is known that cytokinin provides meristematic properties, understanding on the mechanisms that underlie the cytokinin signaling pattern in the young gynoecium is lacking. Moreover, in other tissues, the cytokinin pathway is often connected to the auxin pathway, but we also lack knowledge about these connections in the young gynoecium. Our results reveal that cytokinin signaling, that can provide meristematic properties required for CMM activity and growth, is enabled by the transcription factor SPATULA (SPT) in the medial domain. Meanwhile, cytokinin signaling is confined to the medial domain by the cytokinin response repressor ARABIDOPSIS HISTIDINE PHOSPHOTRANSFERASE 6 (AHP6), and perhaps by ARR16 (a type-A ARR) as well, both present in the lateral domains (presumptive valves) of the developing gynoecia. Moreover, SPT and cytokinin, probably together, promote the expression of the auxin biosynthetic gene TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1 (TAA1) and the gene encoding the auxin efflux transporter PIN-FORMED 3 (PIN3), likely creating auxin drainage important for gynoecium growth. This study provides novel insights in the spatiotemporal determination of the cytokinin signaling pattern and its connection to the auxin pathway in the young gynoecium.
Highlights
Angiosperms are the most successful group of land plants on earth
The Arabidopsis gynoecium has two fused carpels, with a medial domain between them, and a lateral domain consisting of the carpel walls
This work demonstrates that the transcription factor SPATULA enables cytokinin signaling at the medial domain, while cytokinin signaling repressors are present in the lateral domain
Summary
Angiosperms (flowering plants) are the most successful group of land plants on earth In these species, flowers are formed, which normally produce a pistil or gynoecium, the female reproductive part of the flower, in their inner floral whorl. The CMM gives rise to all medial tissues, including the replum, placenta, ovules, septum and transmitting tract (Fig 1A) [1,2,3]. All these tissues are crucial for the reproductive success of the plant; our knowledge on the early events controlling CMM activity and medial tissue formation is fragmentary [1,4,5,6]
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