Theoretical and experimental evidence indicates that there is no detectable auxin gradient in the angiosperm female gametophyte

Abstract

The plant life cycle alternates between a diploid sporophytic and a haploid gametophytic generation. The female gametophyte (FG) of flowering plants is typically formed through three syncytial mitoses, followed by cellularisation that forms seven cells belonging to four cell types. The specification of cell fates in the FG has been suggested to depend on positional information provided by an intrinsic auxin concentration gradient. The goal of this study was to develop mathematical models that explain the formation of this gradient in a syncytium. Two factors were proposed to contribute to the maintenance of the auxin gradient in Arabidopsis FGs: polar influx at early stages and localised auxin synthesis at later stages. However, no gradient could be generated using classical, one-dimensional theoretical models under these assumptions. Thus, we tested other hypotheses, including spatial confinement by the large central vacuole, background efflux and localised degradation, and investigated the robustness of cell specification under different parameters and assumptions. None of the models led to the generation of an auxin gradient that was steep enough to allow sufficiently robust patterning. This led us to re-examine the response to an auxin gradient in developing FGs using various auxin reporters, including a novel degron-based reporter system. In agreement with the predictions of our models, auxin responses were not detectable within the FG of Arabidopsis or maize, suggesting that the effects of manipulating auxin production and response on cell fate determination might be indirect.

The Yin-Yang of Hormones: Cytokinin and Auxin Interactions in Plant Development

“…In what is now a familiar theme, this gradient of cytokinin mirrors an auxin gradient present in the sporophytic tissue early in female gametophyte development (Pagnussat et al, 2009) that is likely formed through the action of PIN1 (Ceccato et al, 2013). This auxin gradient was suggested to define cell-type specification in the female gametophyte (Pagnussat et al, 2009), though Lituiev et al (2013) suggested, on theoretical grounds and an analysis of a degron-based auxin sensor, that there was no auxin gradient in the female gametophyte itself, though they did observe a gradient in the surrounding sporophytic tissue. This suggests that in the sporophytic tissues, there are opposing auxin and cytokinin gradients that play complementary roles during early stages of female gametophyte development, including the specification of the functional megaspore cell from the four products arising from meiosis of the megaspore mother cell (C.-Y.…”

Plant germline formation: common concepts and developmental flexibility in sexual and asexual reproduction

“…Abolishing the auxin gradient, by expressing the YUCCA1 (YUC1) auxin biosynthetic protein in the entire embryo sac or by modulating the auxin response by downregulating selected AUXIN RESPONSE FACTOR (ARF genes), led to the loss or, at low frequencies, the mis-expression of cell fate markers in the female gametophyte (Table 3; Pagnussat et al, 2009). However, theoretical models attempting to describe the auxin gradient in the female gametophyte showed that only very shallow auxin gradients can be established even when using the most favourable parameters (Lituiev et al, 2013). A sensitivity analysis demonstrated that the steepness of the obtained gradients is not sufficient to determine distinct cell fates (Lituiev et al, 2013).…”

“…However, theoretical models attempting to describe the auxin gradient in the female gametophyte showed that only very shallow auxin gradients can be established even when using the most favourable parameters (Lituiev et al, 2013). A sensitivity analysis demonstrated that the steepness of the obtained gradients is not sufficient to determine distinct cell fates (Lituiev et al, 2013). Furthermore, the reinvestigation of auxin signalling using various sensors failed to detect auxin in the female gametophyte but instead found auxin signalling to be restricted to the surrounding ovule tissues in a dynamic polar pattern (Ceccato et al, 2013;Lituiev et al, 2013).…”

“…A sensitivity analysis demonstrated that the steepness of the obtained gradients is not sufficient to determine distinct cell fates (Lituiev et al, 2013). Furthermore, the reinvestigation of auxin signalling using various sensors failed to detect auxin in the female gametophyte but instead found auxin signalling to be restricted to the surrounding ovule tissues in a dynamic polar pattern (Ceccato et al, 2013;Lituiev et al, 2013). This polar auxin pattern in sporophytic tissues may non-cell-autonomously influence cell specification in the female gametophyte and may have been affected by manipulating the expression of YUC1 and ARFs (Lituiev et al, 2013).…”

An EAR-Dependent Regulatory Module Promotes Male Germ Cell Division and Sperm Fertility in Arabidopsis

“…The cellularization and differentiation of cells in the embryo sac appear to be transcriptionally regulated (Rabiger and Drews, 2013), and mechanisms involving RNA splicing and auxin and peptide signaling specify female gametic cells (Gross-Hardt et al, 2007;Pagnussat et al, 2009;Lieber et al, 2011;Lituiev et al, 2013). For male germline development, a regulatory framework for cell cycle progression and gamete specification has been established Borg and Twell, 2010;Twell, 2011).…”