Auxin response and cell identity in the early Arabidopsis embryogenesis

Abstract

In all developmental processes oriented cell division and cell fate acquisition are combined in order to generate patterns. A major coordinator of these decisions in plants is the phytohormone auxin. It seems to coordinate nearly all plant developmental processes: from the formation of the root and shoot meristems to fruit development. One case in particular has fascinated the scientific community: embryogenesis. How the basic organization of the plant body arises from a single cell in a very tightly controlled series of steps. The Arabidopsis embryo constitutes an attractive model to study embryogenesis thanks to its regularity. This allows scientists to track the development of the embryonic pattern in a single-cell resolution. The auxin polar transport is fundamental for the apical-basal axis formation and ultimately for the embryo patterning. After the asymmetric zygotic division, two transcriptionally distinct cells are generated. The smaller apical cell will vary the division plane to ultimately form the embryo proper. The larger basal cell divides horizontally and all but its uppermost cell (hypophysis) will remain extraembryonic. In the apical cell auxin accumulates and promotes embryonic identity. This auxin response is blocked in suspensor cells; nevertheless, they have the potential to become embryonic cells until the apical-basal axis is fully established, that is, the transition stage. Despite decades of research, it is not fully understood how auxin regulates these events, especially at the very early stages of the embryonic development. This work aims to widen our knowledge on this complex process through the study of a gain-of function mutant directly involved in the auxin response during early embryo development and of cell identity changes in a filament-like embryo system. The first chapter focuses on the early auxin response in the embryo. Here we describe the role of the Aux/IAA IAA8 through its gain-of-function phenotype. Aux/IAAs bind to auxin response factors (ARFs) and repress the transcription of auxin response genes. In the IAA8 mutant embryos we observe a horizontal division of the apical daughter cell followed by a series of aberrant cell divisions. Additionally, IAA8-specific blocking of the auxin response in the apical cell causes dramatic cell fate changes. The second chapter describes the formation of an embryo from an induced suspensor-like structure. This is achieved by a temporal ectopic activation of the YDA pathway (pS4:SSP). This pathway controls zygote polarization and promotes suspensor identity during the first steps of embryogenesis. Using fluorescent identity markers, we observe that the release of this activation triggers the transition from early basal cells to embryonic ones and this is likely promoted via an auxin response. In brief, the new system to study cell fate acquisition and the characterization of an auxin-insensitive mutant further our knowledge on how auxin might dictate the cell division and cell differentiation decisions to form the early embryo pattern.