Maria D'Aloia

Cytokinin promotes flowering of Arabidopsis via transcriptional activation of the FT paralogue TSF

Cytokinins are involved in many aspects of plant growth and development, and physiological evidence also indicates that they have a role in floral transition. In order to integrate these phytohormones into the current knowledge of genetically defined molecular pathways to flowering, we performed exogenous treatments of adult wild type and mutant Arabidopsis plants, and analysed the expression of candidate genes. We used a hydroponic system that enables synchronous growth and flowering of Arabidopsis, and allows the precise application of chemicals to the roots for defined periods of time. We show that the application of N⁶-benzylaminopurine (BAP) promotes flowering of plants grown in non-inductive short days. The response to cytokinin treatment does not require FLOWERING LOCUS T (FT), but activates its paralogue TWIN SISTER OF FT (TSF), as well as FD, which encodes a partner protein of TSF, and the downstream gene SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1). Treatment of selected mutants confirmed that TSF and SOC1 are necessary for the flowering response to BAP, whereas the activation cascade might partially act independently of FD. These experiments provide a mechanistic basis for the role of cytokinins in flowering, and demonstrate that the redundant genes FT and TSF are differently regulated by distinct floral-inducing signals.

Integrating roots into a whole plant network of flowering time genes in Arabidopsis thaliana.

Molecular data concerning the involvement of roots in the genetic pathways regulating floral transition are lacking. In this study, we performed global analyses of the root transcriptome in Arabidopsis in order to identify flowering time genes that are expressed in the roots and genes that are differentially expressed in the roots during the induction of flowering. Data mining of public microarray experiments uncovered that about 200 genes whose mutations are reported to alter flowering time are expressed in the roots (i.e. were detected in more than 50% of the microarrays). However, only a few flowering integrator genes passed the analysis cutoff. Comparison of root transcriptome in short days and during synchronized induction of flowering by a single 22-h long day revealed that 595 genes were differentially expressed. Enrichment analyses of differentially expressed genes in root tissues, gene ontology categories, and cis-regulatory elements converged towards sugar signaling. We concluded that roots are integrated in systemic signaling, whereby carbon supply coordinates growth at the whole plant level during the induction of flowering. This coordination could involve the root circadian clock and cytokinin biosynthesis as a feed forward loop towards the shoot.

Stability of SaFLC repression in Sinapis alba: A link with quantitative effect of vernalization

In Arabidopsis thaliana, vernalization promotes flowering by repressing the floral inhibitor FLOWERING LOCUS C (AtFLC). This repression is mediated through epigenetic modifications at the AtFLC locus, leading to gene silencing. Whether the well-known quantitative effect of vernalization is due to the degree of AtFLC repression and/or its stability after return to normal temperature conditions has not been clarified. Here, we examine this question in white mustard, Sinapis alba, taking advantage of our recent cloning of the AtFLC ortholog SaFLC. Addendum to: D’Aloia M, Tocquin P, Périlleux C. Vernalization-induced repression of FLOWERING LOCUS C stimulates flowering in Sinapis alba and enhances plant responsiveness to photoperiod. New Phytol 2008; 178:755–65.