75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

371 Natural Variation for Seed Dormancy in Arabidopsis thaliana
Chunlao Tang, Keyan Zhao, Magnus Nordborg
University of Southern California
Seed dormancy is an important adaptive trait, primarily due to its effect on seedling survival. However, little is known
about natural variation for this trait, both because of complex genetics and because of difficulties in obtaining reliable
phenotypic measurements. Here we report the results of a survey of primary seed dormancy in a world-wide collection
of 96 Arabidopsis thaliana accessions, for which genome-wide polymorphism data are also available.
We found extensive variation of seed dormancy among accessions and demonstrated that the requirement for afterripening
for seed germination (specifically, the number of days of dry storage required to reach 50% germination ratio)
is a feasible index of this trait in a large-scale survey as long as experimental conditions are strictly controlled. Overall,
no significant association was found between seed dormancy and latitude, temperature or flowering time. However,
the geographic distribution of seed dormancy was evidently non-random. The strongest seed dormancy was found in
accessions from regions with dry summers and/or harsh winters such as the Mediterranean or Central Asia. At the other
extreme, all seeds were found to be non-dormant in accessions from northern Sweden, where prompt germination might
be advantageous due to the very short growing season. In general, large variation in seed dormancy was found between
accessions from very close origins, suggesting considerable micro-geographic heterogeneity.
372 Sucrose Specific Induction of Anthocyanin Biosynthesis in Arabidopsis Requires the
MYB75/PAP1 Gene
Sheng Teng 1 , Joost Keurentjes 2 , Leonie Bentsink 1 , Maarten Koornneef 2 , Sjef Smeekens 1
1
Department of Molecular Plant Physiology, Utrecht University, the Netherlands, 2 Laboratory of Genetics,
Wageningen University, the Netherlands
Sugar-induced anthocyanin accumulation has been observed in many plant species. We observed that sucrose is the
most effective inducer of anthocyanin biosynthesis in Arabidopsis seedlings. Other sugars and osmotic controls are either
less effective or ineffective. Analysis of sucrose induced anthocyanin accumulation in 43 Arabidopsis accessions shows that
considerable natural variation exists for this trait. The Cvi accession essentially does not respond to sucrose, whereas Ler
is an intermediate responder. The existing Ler/Cvi recombinant inbred population was used in a QTL analysis for sucrose
induced anthocyanin accumulation. A total of four quantitative trait loci for sucrose induced anthocyanin accumulation
(SIAA) were identified in this way. The locus with the largest contribution to the trait, SIAA1, was fine mapped and
using a candidate gene approach, it was shown that the MYB75/PAP1 gene encodes SIAA1. Genetic complementation
studies and analysis of a laboratory-generated knockout mutation in this gene confirmed this conclusion. Sucrose, in a
concentration dependent way, induces MYB75/PAP1 mRNA accumulation. Moreover, MYB75/PAP1 is essential for
the sucrose-mediated expression of the DFR gene.
The SIAA1 locus in Cvi probably is a weak or loss of function MYB75/PAP1 allele. The C24 accession similarly
shows a very weak response to sucrose induced anthocyanin accumulation encoded by the same locus. Sequence analysis
showed that the Cvi and C24 accessions harbor mutations both inside and downstream of the DNA binding domain of
the MYB75/PAP1 protein, which most likely result in loss of activity.