Distinct and dynamic auxin activities during reproductive development

Abstract

Flowering plants have evolved sophisticated and complicated reproductive structures to ensure optimal conditions for the next generation. Successful reproduction relies on careful timing and coordination of tissue development, which requires constant communication between these tissues. Work on flower and fruit development over the last decade places the phytohormone auxin in a key role as a master of patterning and tissue specification of reproductive organs. Although many questions still remain, it is now clear that auxin mediates its function in flowers and fruits through an integrated process of biosynthesis, transport, and signaling, as well as interaction with other hormonal pathways. In addition, the knowledge obtained so far about auxin function already allows researchers to develop tools for crop improvement and precision agriculture.

Figure 1.

Schematic picture of pollen and anther development. Diploid pollen mother cells surrounded by a tapetum layer goes through meiosis in phase 1. Auxin responses (yellow) are detected in the tapetum layer, the theca and the vasculature (V) from late stage 10. Callase released from the tapetum subsequently frees the microspore from the tetrads, and during phase 2, the microspore goes through mitosis. During these stages, auxin responses are detected in the pollen grains. The anthers dehisce and release the dehydrated pollen grains. St, stomium; CCC, circular cell cluster; V, vascular bundle; C, connective tissue; E, endothecium.

Figure 2.

Gynoecium morphology and the auxin gradient theory. The gynoecium promordia develops as an open cylinder that fuses apically to form the style and stigma, and internally to form the septum (gray) and placentae with ovules (gray). The apical–basal patterning has been suggested to depend on an auxin gradient (Nemhauser et al. 2000), and according to the proposed model, high auxin (yellow) apically results in style and stigma proliferation, whereas intermediate auxin level is required for ovary formation, and low level for gynophore formation. Reduction in PAT would result in trapping of apically produced auxin and thus in a steeper auxin gradient, resulting in boundary shifts. An alternative model proposed by Østergaard (2009) suggests that the ovary is formed in a low auxin (yellow) and cytokinin (orange) region, whereas high auxin induces style and stigma, and high cytokinin the gynophore.

Figure 3.

Model for hormonal activities on fertilization. The figure shows schematic cross sections of an Arabidopsis gynoecium. Before fertilization (top), DELLA proteins repress growth and elongation of the ovary. On fertilization (bottom), auxin (IAA) is produced in the ovules, inducing GA₃ production in the valves. GA₃ then mediates DELLA degradation and fruit growth.

Figure 4.

Formation of auxin minimum at the separation layer. IND directly represses PINOID (PID) and activates WAG2 to relocalize PIN auxin efflux carriers. PIN relocalization creates an auxin minimum in the cells, where separation will take place. Green lines indicate PIN localization, whereas cells with DR5::GFP signal are yellow. Small arrows indicate direction of auxin flow.