Ethylene Interacts with Abscisic Acid to Regulate Endosperm Rupture during Germination: A Comparative Approach UsingLepidium sativumandArabidopsis thaliana

Ada Linkies,William E Finch-Savage,Gerhard Leubner-Metzger,Meike Wenk,FrançOise Corbineau,Karl Morris,Kerstin MüLler,Miroslav Strnad,Veronika TurečKová,Cassandra S.C Cadman,James R Lynn

doi:10.1105/tpc.109.070201

Abstract

The micropylar endosperm cap covering the radicle in the mature seeds of most angiosperms acts as a constraint that regulates seed germination. Here, we report on a comparative seed biology study with the close Brassicaceae relatives Lepidium sativum and Arabidopsis thaliana showing that ethylene biosynthesis and signaling regulate seed germination by a mechanism that requires the coordinated action of the radicle and the endosperm cap. The larger seed size of Lepidium allows direct tissue-specific biomechanical, biochemical, and transcriptome analyses. We show that ethylene promotes endosperm cap weakening of Lepidium and endosperm rupture of both species and that it counteracts the inhibitory action of abscisic acid (ABA) on these two processes. Cross-species microarrays of the Lepidium micropylar endosperm cap and the radicle show that the ethylene-ABA antagonism involves both tissues and has the micropylar endosperm cap as a major target. Ethylene counteracts the ABA-induced inhibition without affecting seed ABA levels. The Arabidopsis loss-of-function mutants ACC oxidase2 (aco2; ethylene biosynthesis) and constitutive triple response1 (ethylene signaling) are impaired in the 1-aminocyclopropane-1-carboxylic acid (ACC)-mediated reversion of the ABA-induced inhibition of seed germination. Ethylene production by the ACC oxidase orthologs Lepidium ACO2 and Arabidopsis ACO2 appears to be a key regulatory step. Endosperm cap weakening and rupture are promoted by ethylene and inhibited by ABA to regulate germination in a process conserved across the Brassicaceae.

Highlights

The first Arabidopsis thaliana hormone mutants were identified using seed germination and dormancy phenotypes (Koornneef et al, 1982)
The time-course analysis in Figure 1 shows that endosperm cap weakening precedes endosperm rupture of Lepidium seeds when imbibed both without (CON) or with abscisic acid (ABA) added to the medium
When the endosperm cap puncture force of the concentrations of whole seeds incubated without (CON) and ABA series was plotted against the percentage of endosperm rupture in the seed population, the same relationship was evident in both treatments (Figure 1B)

Summary

Introduction

The first Arabidopsis thaliana hormone mutants were identified using seed germination and dormancy phenotypes (Koornneef et al, 1982). These key seed traits are antagonistically regulated by abscisic acid (ABA) and gibberellins (GA). In contrast with the situation in seedlings, ethylene is known to promote seed germination, and an ABA-ethylene antagonism is evident for seeds of Arabidopsis (Beaudoin et al, 2000; Ghassemian et al, 2000) and other species (Kucera et al, 2005; Matilla and Matilla-Vazquez, 2008). Ethylene-insensitive ethylene response (etr1) mutant seeds germinate poorly, their

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Results

Conclusion