A stress-response-related inter-compartmental signalling pathway regulates embryonic cuticle integrity in Arabidopsis.

Audrey Creff,Thomas Widiez,Jérôme Joubès,Gwyneth Ingram,Sophy Boeuf,Steven Moussu,Lysiane Brocard,Nicolas M Doll,Frédéric Domergue,Roberta Galletti,Anne-Charlotte Marsollier,Stéphanie Pascal,Ludivine Taconnat

doi:10.1371/journal.pgen.1007847

Abstract

The embryonic cuticle is necessary for normal seed development and seedling establishment in Arabidopsis. Although mutants with defective embryonic cuticles have been identified, neither the deposition of cuticle material, nor its regulation, has been described during embryogenesis. Here we use electron microscopy, cuticle staining and permeability assays to show that cuticle deposition initiates de novo in patches on globular embryos. By combining these techniques with genetics and gene expression analysis, we show that successful patch coalescence to form a continuous cuticle requires a signalling involving the endosperm-specific subtilisin protease ALE1 and the receptor kinases GSO1 and GSO2, which are expressed in the developing embryonic epidermis. Transcriptome analysis shows that this pathway regulates stress-related gene expression in seeds. Consistent with these findings we show genetically, and through activity analysis, that the stress-associated MPK6 protein acts downstream of GSO1 and GSO2 in the developing embryo. We propose that a stress-related signalling pathway has been hijacked in some angiosperm seeds through the recruitment of endosperm-specific components. Our work reveals the presence of an inter-compartmental dialogue between the endosperm and embryo that ensures the formation of an intact and functional cuticle around the developing embryo through an “auto-immune” type interaction.

Highlights

The Arabidopsis seed is a complex structure composed of three genetically distinct compartments, the maternally-derived seed coat, the embryo, and the endosperm
Microscopy and lipid analyses were respectively performed at the Bordeaux Imaging Center, and the Metabolome Facility of the Functional Genomic Center of Bordeaux
We found that the degree of phosphorylation of MITOGEN ACTIVATED PROTEIN KINASE6 (MPK6) was reduced by approximately 50% in gso1-1 gso2-1 double mutant seeds compared to wild-type, suggesting that a significant proportion of MPK6 phosphorylation in seeds depends on the activity of GSO1 and GSO2 (Fig 7D, S16 Fig)

Summary

Introduction

The Arabidopsis seed is a complex structure composed of three genetically distinct compartments, the maternally-derived seed coat, the embryo, and the endosperm. After fertilization the expansion of the endosperm drives the growth of the seed. The endosperm is an angiosperm innovation, thought to have arisen through the sexualisation of the central cell of the female gametophyte [2]. The ancestors of angiosperms probably had seeds more similar to those of gymnosperms, in which tissues of the female gametophyte proliferate independently of egg cell fertilization to produce a nutrient rich storage tissue. The peptide CLAVATA3/EMBRYO SURROUNDING REGION-RELATED8 (CLE8) may act non-cell autonomously to regulate early Arabidopsis embryogenesis [3]. Maternally-expressed peptides present in the central cell pre-fertilization, and subsequently in the early EMBRYO SURROUNDING REGION (ESR), were shown to regulate Arabidopsis suspensor development. Genetic analysis suggests that this regulation could be mediated by a pathway involving the Receptor-Like Cytoplasmic Kinase SHORT SUSPENSOR [4,5], the receptor involved remains unidentified

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