Nathalie Gonzalez - Two models to understand the molecular regulation of plant organ growth: Arabidopsis leaf and tomato fruit

Arabidopsis leaves and tomato fruits are fundamentally different in shape, size, …. However, their growth is achieved through similar cellular processes, cell division and cell expansion, though coordinated differently. By using these two model organs and focusing on some of their specificity, we aim at understanding the molecular regulation of organ growth.

In the Arabidopsis leaf, meristemoids cells are small triangular stem-cell like cells scattered throughout the leaf surface giving rise to stomata. Meristemoids divide asymmetrically producing next to a new meristemoid, a larger sister cell that can differentiate to a pavement cell. Through their repeating asymmetric division, meristemoids are thus responsible for the production of a large proportion of the cells that form the leaf surface and are not only instrumental to determine the epidermal architecture, but also final leaf size and shape. In this context, we studied the PPD-signalling pathway to understand its role during meristemoid cell division and leaf growth.

In the Tomato fruit, and also in the Arabidopsis leaf, endoreduplication occurs simultaneously with cell expansion. Endoreduplication, during which cells increase their DNA content (ploidy) through successive rounds of full genome replication without cell division, is proposed to be a mean to produce large cells. In tomato fruit, ploidy levels can reach very high levels that are often correlated with very large cells. Our main objective is thus to understand the potential role of endoreduplication in cell size control and dissect the molecular regulation of this process. As a first step, we are determining the spatiotemporal distribution of gene expression based on the ploidy levels in the fleshy part of the fruit (pericarp) during development. Our major question is why plants produce cells with increased nuclear DNA content while they have a diploid genome.