Juicy Stories on Female Reproductive Tissue Development: Coordinating the Hormone Flows

In the past 20-30 years, developmental biologists have made tremendous progress in identifying genes required for the specifica-tion of individual cell types of an organ and in describing how they interact in genetic networks. In comparison, very little is known about the mechanisms that regulate tissue polarity and overall organ patterning. Gynoecia and fruits from members of the Brassicaceae family of flowering plants provide excellent model systems to study organ patterning and tissue specification because they become partitioned into distinct domains whose formation is determined by polarity establishment both at a cellular and whole tissue level. Interactions among key regulators of Arabidopsis gynoecium and fruit development have revealed a network of upstream transcription factor activities required for such tissue differentiation. Regulation of the plant hormone auxin is emerging as both an immediate downstream output and input of these activities, and here we aim to provide an overview of the current knowledge regarding the link between auxin and female reproductive development in plants. In this review, we will also demonstrate how available data can be exploited in a mathematical modeling approach to reveal and understand the feedback regulatory circuits that underpin the polarity establishment, necessary to guide auxin flows.

Systems Biology Approach Pinpoints Minimum Requirements for Auxin Distribution during Fruit Opening

The phytohormone auxin is implied in steering various developmental decisions during plant morphogenesis in a concentration-dependent manner.Auxin maxima have been shown to maintain meristematic activity,for example,of the root apical meristem,and position new sites of outgrowth,such as during lateral root initiation and phyllotaxis.More recently,it has been demonstrated that sites of auxin minima also provide positional information.In the developingArabidopsis fruit,auxin minima are required for correct differentiation of the valve margin.It remains unclear,however,how this auxin minimum is generated and maintained.Here,we employ a systems biology approach to model auxin transport based on experimental observations.This allows us to determine the minimal requirements for its establishment.Our simulations reveal that two alternative processes-which we coin "flux-barrier" and "flux-passage"-are both able to generate an auxin minimum,but under different parameter settings.Both models are in principle able to yield similar auxin profiles but present qualitatively distinct patterns of auxin flux.The models were tested by tissue-specific inducible ablation,revealing that the auxin minimum in the fruit is most likely generated by a flux-passage process.Model predictions were further supported through 3D PIN localization imaging and implementing experimentally observed transporter localization.Through such an experimental-modeling cycle,we predict how the auxin minimum gradually matures during fruit development to ensure timely fruit opening and seed dispersal.