RNA Biology and Molecular Physiology
Flowering Time Control
The Role of SRR1 in the circadian system
SRR1 is a regulator of the circadian clock and esstential for flowering time control in non-inductive conditions
The endogenous clock not only serves as a daily timer but also as a calender.
Plants adjust the onset of flowering to the appropriate time of the year. For that, the endogenous clock keeps track of daylength that changes with seasons. Other important flowering signals are ambient temperature, length of the winter cold period, age of the plant and phytohormones such as gibberelic acid (GA). These environmental signals are mediated in a hierarchical manner through molecular signaling pathways that in the end either promotes or represses the expression of key floral integrator genes, called florigens.
SENSITIVITY TO RED LIGHT REDUCED (SRR1) is a protein that is involved in the mediation of red light signals as well as in regulation of the circadian clock. Core clock genes show a shortened period of expression in srr1-1 loss-of-function mutants, suggesting that the SRR1 protein is important for correct clock function.
In addition, srr1-1 mutants flower earlier than wt plants in a photoperiod-independent manner, meaning that they have a problem discriminating between non-inductive and inductive photoperiods.
Flowering in Arabidopsis is triggered by the expression of the florigen FT. Expression of FT is tightly controlled by proteins that act as promoters or repressors to ensure that it is only expressed when the surrounding environment is suitable for flowering.
The role of SRR1 in this process is to prevent flowering until photoperiodic conditions are suitable for reproduction, by promoting the expression of genes that code for direct repressors of FT. These target genes are involved in different pathways regulating flowering, suggesting that SRR1 may act as a focal point to synchronize the flowering response.
Current Work
We are currently interested in identifying new proteins that act in the same signaling pathways as SRR1. To achieve this, we have performed a second-site suppressor mutant screen with srr1-1 mutant plant. In short, we introduced random point mutations using the chemical EMS in srr1-1 mutant plants and screened for individuals that suppressed the srr1-1 early flowering phenotype, so called suppressor of srr1-1 mutants (ssm).
This screen has resulted in several mutant candidates with interesting phenotypes. We are now working on identifying the casual singe nucleotide polymorphisms (SNP) that give rise to the suppressor phenotypes in the different mutant candidates. This mutant identification is done by full genome resequencing, followed by a bulk segregant analysis (see figure). By crossing the ssm candidates with their parent line (srr1-1) we created segregating F1 populations. From these populations, a pool of DNA was generated by selecting only the plants retaining the suppressor phenotype. This leads to an enrichment of the casual SNP behind the suppressor phenotype. The DNA pool can then be submitted to high throughput sequencing, to identify all SNPs in the genome pool. By calculating the coverage for each SNP, one can then generate a candidate list of genes where the casual SNP is located. When a candidate is identified, the aim is to characterize the novel protein and understand its role in relation to SRR1, for example by studying localization or protein-protein interactions.
Project-Related Publications
Johansson M, Staiger D (2014). SRR1 is essential to repress flowering in non-inductive conditions in Arabidopsis thaliana.Journal of Experimental Botany 65, 5811-5822.
Johansson M, Staiger D (2015) Time to flower – Interplay between photoperiod and the circadian clock. Journal of Experimental Botany 66, 719-730.
Staiger D, Allenbach L, Salathia N, Fiechter V, Davis SJ, Millar AJ, Chory J, Fankhauser C (2003) The Arabidopsis SRR1 gene mediates phyB signaling and is important for normal circadian clock function. Genes and Development 17, 256-268.
Project is part of SPP1530 "Flowering time control: from natural variation to crop improvement" funded by DFG.
Program Committee
Prof. Dr. Christian Jung (Program Coordinator)
Christian-Albrechts-University Kiel, Plant Breeding Institute
Prof. Dr. Ivo Grosse
Martin-Luther-Universität Halle-Wittenberg Institute for Computer Sciences
Prof. Dr. Klaus Pillen
Martin-Luther-University Halle-Wittenberg, Institute of Agricultural and Nutritional Sciences
Prof. Dr. Dorothee Staiger
Bielefeld University, Molecular Cell Physiology
Jun-Prof. Dr. Maria von Korff
Max Planck Institute for Plant Breeding Research Cologne