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6. Climate change and plants. Is vegetation part of the problem or the solution?
Antonis Giakountis (Max Planck Institute, Germany)

In the natural world, seasonal variation in environmental parameters affecting flowering, such as day length, exists. Arabidopsis flowering time is accelerated under long days (LD) of 16 h while it is delayed under short days (SD) of 10 h. Genetic variability within species, called natural variation, has been used to identify the genetic components that regulate flowering in response to changing day length. Among the components of the photoperiodic pathway are the floral integrators FLOWERING LOCUS T (FT) and SUPPRESSOR OF CONSTANS OVEREXPRESSOR 1 (SOC1). A caveat in the existing approaches for Arabidopsis is the exclusive utilization of LD of 16 h under inductive conditions. Such photoperiods are considered to be extreme since they are restricted to a limited number of locations within the geographical distribution of the species.
We selected two approaches to better understand how day length regulates flowering in Arabidopsis. First, we screened the flowering time responses of a collection of genetically divergent Arabidopsis accessions under six different day lengths. The same accessions were also transformed with a promoter fusion of GIGANTEA (GI), a clock-regulated gene associated with flowering, to the firefly luciferase gene. Using this transgene as a molecular marker and performing in vivo imaging of bioluminescence, we were able to identify natural variation operating specifically within the photoperiod pathway.
We report the presence of extensive natural genetic variation in day length perception of long or short days of similar duration. Quantitative Trait Locus (QTL) analysis in five different mapping populations suggests that in nature, phenotypes with an enhanced response to changing day length can be created in more than one way. In addition, we report extensive epistatic interactions operating on a genome-wide level that contribute to the observed genetic variability. Currently, we are focusing our efforts on fine mapping with the aim of cloning the causal genetic factors in one of our varieties.
In an attempt to molecularly characterize the consequences of our observed genetic variations for flowering time, we performed reciprocal shift experiments from noninductive SD to various inductive LD of similar duration. We report the presence of natural variation in the speed of floral commitment between the accessions. This variation is linked to molecular variability in the kinetics of FT mRNA upregulation in the leaves of different accessions under the same day length. Interestingly, a second layer of molecular variability in the kinetics of FT mRNA exists between day lengths of similar duration for the same accessions. The expression of flowering time genes operating in the apex, such as SOC1, tightly follows the observed molecular variability in the leaves, highlighting the strong cross talk between these two important sites of plant development.

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Frontiers of Plant Science in the 21st Century

Report on the Panel Discussion