Date Published: September 16, 2014
Publisher: Public Library of Science
Author(s): Cristiana Paina, Stephen L. Byrne, Cristian Domnisoru, Torben Asp, Stefan Maas.
Vernalization is a key requirement for the induction of flowering in perennial ryegrass (Lolium perenne L.). The transcriptome of two genotypes with contrasting vernalization requirement was studied during primary (vernalization and short day conditions) and secondary induction (higher temperature and long day conditions) using an RNA-Seq approach. This revealed transcripts with expression profiles indicative of a role in floral induction, both in the promotion and repression of flowering. We observed similarities and specific differences between the two genotypes related to cold response, carbohydrate metabolism, and photoperiod regulation. Components of the photoperiod pathway showed regulation during vernalization, pointing to possible interactions between elements of the photoperiod and vernalization pathways. The results provide a global picture of the processes ongoing during the transition from vegetative to reproductive phase of perennial ryegrass genotypes with and without a vernalization requirement.
Plants from temperate regions have developed complex mechanisms for the change from vegetative to reproductive growth in order to time this transition to occur when conditions for pollination and seed development are optimal. The control over the timing of flowering is achieved through intricate regulatory networks in tight connection with environmental signals. As many other plants from temperate regions, perennial ryegrass developed a dual requirement for the induction of flowering . It requires a primary induction, represented by several weeks of low temperature (vernalization) accompanied by short days. This is followed by the secondary induction, which starts when the temperature and the day length increase.
This study set out to uncover the transcriptome profiles of perennial ryegrass through floral induction in genotypes with a contrasting requirement for vernalization. The genotype Falster, from Denmark, has a strong vernalization requirement, while the genotype Veyo, from Italy, has no vernalization requirement. This represents excellent material for studying the transcriptome through vernalization and the transition to long days. It was envisaged that we would be able to identify transcripts potentially involved in mediating the vernalization requirement of the Falster genotype by taking advantage of the resolution provided by RNA-Seq. The sample collection time points were chosen to allow differentiation between stress responses and processes specific to vernalization and the transition to long days. The plants were exposed to a period of 58 days of short day conditions before the start of vernalization treatment in order to differentiate between transcripts responding to short photoperiod and transcripts responding to low temperature, i.e. vernalization. In some temperate grasses, short day conditions were reported to substitute for vernalization , , . Although previous studies tested the requirements of the two genotypes and their flowering response to different primary and secondary induction treatments , , no record is available referring to their response solely to short day conditions with respect to flowering.
RNA-Seq enabled an untargeted profiling of the transcriptome of two perennial ryegrass genotypes with contrasting vernalization requirement. Expression profiling through vernalization identified a series of transcripts with patterns suggestive of a role in floral induction. Similarities were observed between the two genotypes, as well as differences related to cold acclimation, carbohydrate metabolism, and components of the floral induction pathways. Of particular interest was the differential regulation in the two genotypes of transcripts involved in the biosynthesis and degradation of trehalose-6-phosphate, suggestive of playing a role in the floral transition not only in Arabidopsis, but also in grasses.