Abstract
BackgroundCell specification and differentiation in the endosperm of cereals starts at the maternal-filial boundary and generates the endosperm transfer cells (ETCs). Besides the importance in assimilate transfer, ETCs are proposed to play an essential role in the regulation of endosperm differentiation by affecting development of proximate endosperm tissues. We attempted to identify signalling elements involved in early endosperm differentiation by using a combination of laser-assisted microdissection and 454 transcriptome sequencing.Principal Findings454 sequencing of the differentiating ETC region from the syncytial state until functionality in transfer processes captured a high proportion of novel transcripts which are not available in existing barley EST databases. Intriguingly, the ETC-transcriptome showed a high abundance of elements of the two-component signalling (TCS) system suggesting an outstanding role in ETC differentiation. All components and subfamilies of the TCS, including distinct kinds of membrane-bound receptors, have been identified to be expressed in ETCs. The TCS system represents an ancient signal transduction system firstly discovered in bacteria and has previously been shown to be co-opted by eukaryotes, like fungi and plants, whereas in animals and humans this signalling route does not exist. Transcript profiling of TCS elements by qRT-PCR suggested pivotal roles for specific phosphorelays activated in a coordinated time flow during ETC cellularization and differentiation. ETC-specificity of transcriptionally activated TCS phosphorelays was assessed for early differentiation and cellularization contrasting to an extension of expression to other grain tissues at the beginning of ETC maturation. Features of candidate genes of distinct phosphorelays and transcriptional activation of genes putatively implicated in hormone signalling pathways hint at a crosstalk of hormonal influences, putatively ABA and ethylene, and TCS signalling.SignificanceOur findings suggest an integral function for the TCS in ETC differentiation possibly coupled to sequent hormonal regulation by ABA and ethylene.
Highlights
Crop seeds develop in distinct successive phases of tissue-specific differentiation [1]
During further differentiation characteristic cell wall thickenings became apparent in endosperm transfer cells (ETCs) at 7 days after flowering (DAF) (Fig. 1D)
In this report we present results gained from 454 transcriptome sequencing of microdissected ETCs at the syncytial stage, after cellularization and during further ETC maturation
Summary
Crop seeds develop in distinct successive phases of tissue-specific differentiation [1]. Cell fate specification occurs already in the endosperm coenocyte [2] and is proposed to be determined by surface position rather than signalling from maternal tissues [3]. Differentiation of the endosperm starts in the outermost cell row adjacent to NP generating the highly specific endosperm transfer cells (ETC) and will be completed after differentiation of the aleurone cells. During this time (5 to 10 DAF) transcriptional reprogramming indicates transition of the caryopsis into a storage product accumulating organ. Besides the importance in assimilate transfer, ETCs are proposed to play an essential role in the regulation of endosperm differentiation by affecting development of proximate endosperm tissues. We attempted to identify signalling elements involved in early endosperm differentiation by using a combination of laserassisted microdissection and 454 transcriptome sequencing
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