Polarity establishment and signalling mechanisms controlling patterning in Hibiscus Trionum petals

Abstract

Angiosperm diversity results in part from coevolution between flowering plants and their pollinators. Distinct cell shapes, cuticular ornamentation, and pigments combine with each other to pattern petals surfaces and these patterns can mediate plant-animal communication. Hibiscus trionum flowers produce a striking bullseye pattern by specifying cells with different pigmentation, shape, and texture in different regions of the adaxial petal epidermis. However, the mechanisms controlling cell differentiation across the petal surface to form such an elaborate and robust pattern are unclear. Positional information is required for cells to be able to differentiate correctly along the proximo-distal (PD) axis of the petal, but the establishment of tissue polarity is also not well understood. Through my PhD work I have started to address both aspects by characterising polarity establishment during petal development and by investigating one possible signalling mechanism controlling cell differentiation across the adaxial epidermis. A recently discovered group of proteins, named SOSEKI (‘cornerstone’ in Japanese) each have unique sub-cellular localisations in different corners of the cell, and unlike other polarity proteins, SOSEKIs are expressed in multiple types of tissues. I have used marker lines of SOSEKI proteins in A. thaliana to establish their expression profile during petal development (Results Chapter 1) and have further exploited SOSEKI proteins as a marker of proximo-distal polarity in H. trionum petals (Results Chapter 2). My results indicate that proximo-distal polarity is established at very early-stage petal primordia, well before the petal is pre-patterned. To pattern correctly, cells need to be able to engage with positional information as part of a tissue, but this positional information also need to be carried, by a mobile signal for instance. Our data suggest that the Hibiscus bullseye is pre-patterned by a diffusing signal from the petal base early in development. I identified three genes involved in GABA synthesis among those most preferentially expressed at the base of the Hibiscus petal early in development. GABA acts as a neurotransmitter in mammals and is also produced by plants. How GABA affects plant development is not yet well understood but a role in plant lateral organ polarity has been postulated. I have investigated the role of GABA signalling during petal organogenesis, and found that manipulating the expression of GABA- related genes (constitutive overexpression and knockout) induce effects on bullseye patterning (Results Chapter 3). My results indicate that the GABA pathway is important for the establishment of pigmentation in the petal and could act as one of the currently unknown upstream signalling processes regulating pigment production during flower development.