Auxin Import and Local Auxin Biosynthesis Are Required for Mitotic Divisions, Cell Expansion and Cell Specification during Female Gametophyte Development in Arabidopsis thaliana.

Aneesh Panoli,Monica Alandete-Saez,Li Yuan,Marissa Simon,Ranjan Swarup,Maria Victoria Martin,Christina Neff,Venkatesan Sundaresan,Gabriela C Pagnussat,Andrés Bellido

doi:10.1371/journal.pone.0126164

Abstract

The female gametophyte of flowering plants, called the embryo sac, develops from a haploid cell named the functional megaspore, which is specified after meiosis by the diploid sporophyte. In Arabidopsis, the functional megaspore undergoes three syncitial mitotic divisions followed by cellularization to form seven cells of four cell types including two female gametes. The plant hormone auxin is important for sporophytic developmental processes, and auxin levels are known to be regulated by biosynthesis and transport. Here, we investigated the role of auxin biosynthetic genes and auxin influx carriers in embryo sac development. We find that genes from the YUCCA/TAA pathway (YUC1, YUC2, YUC8, TAA1, TAR2) are expressed asymmetrically in the developing ovule and embryo sac from the two-nuclear syncitial stage until cellularization. Mutants for YUC1 and YUC2 exhibited defects in cell specification, whereas mutations in YUC8, as well as mutations in TAA1 and TAR2, caused defects in nuclear proliferation, vacuole formation and anisotropic growth of the embryo sac. Additionally, expression of the auxin influx carriers AUX1 and LAX1 were observed at the micropylar pole of the embryo sac and in the adjacent cells of the ovule, and the aux1 lax1 lax2 triple mutant shows multiple gametophyte defects. These results indicate that both localized auxin biosynthesis and auxin import, are required for mitotic divisions, cell expansion and patterning during embryo sac development.

Highlights

The plant life cycle alternates between a diploid (2n) sporophytic and a haploid (n) gametophytic generation
We previously found that the auxin biosynthetic YUCCA genes, YUC1 and YUC2, were expressed sporophytically, and subsequently gametophytically, at the micropylar pole of the developing embryo sac, at FG1-FG2 stages [5]
For the 11 YUC genes studied, we found only three genes showing consistent visible expression in the developing embryo sac: YUC1, YUC2 and YUC8

Summary

Introduction

The plant life cycle alternates between a diploid (2n) sporophytic and a haploid (n) gametophytic generation. The development of the female gametophyte (embryo sac) follows a tightly regulated program, which initiates with meiosis and terminates upon fertilization The haploid functional megaspore undergoes mitosis to generate a 2-nucleate coenocyte (FG2), which is followed by migration of nuclei to opposite poles of the cell and formation of a central vacuole (FG3). A second round of mitosis produces a 4-nucleate embryo sac (FG4) with a large central vacuole and a pair of nuclei at either pole. A final round of mitosis, followed by coordinated nuclear migration, produces an 8-nucleate and highly polarized embryo sac, composed by 3 nuclei occupying the micropylar pole, 3 at the chalazal pole, and 2 lying close to the micropylar end of the central vacuole (FG5). Cellularization results in acquisition of distinct cell fates and the formation of a 7-celled, 8-nucleate embryo sac, composed of 2 synergids, 1 egg cell, 1 central cell with 2 nuclei called polar nuclei, and 3 antipodal cells (S1 Fig, FG6), while the two polar nuclei of the central cell fuse to form the diploid central cell (S1 Fig, FG7) [2,3]

Methods

Results

Conclusion