Monogenic mutants of the early ecotype Landsberg erecta were selected on the basis of late flowering under long day (LD) conditions after treatment with ethyl methanesulphonate or irradiation. In addition to later flowering the number of rosette and cauline leaves is proportionally higher in all mutants, although the correlation coefficient between the two parameters is not the same for all genotypes. Forty-two independently induced mutants were found to represent mutations at 11 loci. The mutations were either recessive, intermediate (co locus) or almost completely dominant (fwa locus). The loci are located at distinct positions on four of the five Arabidopsis chromosomes. Recombinants carrying mutations at different loci flower later than or as late as the later parental mutant. This distinction led to the assignment of eight of the loci to three epistatic groups. In wild type, vernalization promotes flowering to a small extent. For mutants at the loci fca, fve, fy and fpa, vernalization has a large effect both under LD and short day (SD) conditions, whereas co, gi, fd and fwa mutants are almost completely insensitive to this treatment. SD induces later flowering except for mutants at the co and gi loci, which flower with the same number of leaves under LD and SD conditions. This differential response of the mutants to environmental factors and their subdivision into epistatic groups is discussed in relation to a causal model for floral initiation in Arabidopsis thaliana.

Genetic variation for seed dormancy in nature is a typical quantitative trait controlled by multiple loci on which environmental factors have a strong effect. Finding the genes underlying dormancy quantitative trait loci is a major scientific challenge, which also has relevance for agriculture and ecology. In this study we describe the identification of the DELAY OF GERMINATION 1 (DOG1) gene previously identified as a quantitative trait locus involved in the control of seed dormancy. This gene was isolated by a combination of positional cloning and mutant analysis and is absolutely required for the induction of seed dormancy. DOG1 is a member of a small gene family of unknown molecular function, with five members in Arabidopsis. The functional natural allelic variation present in Arabidopsis is caused by polymorphisms in the cis-regulatory region of the DOG1 gene and results in considerable expression differences between the DOG1 alleles of the accessions analyzed.natural variation ͉ seed germination ͉ abscisic acid ͉ gibberellin ͉ cis variation

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Timing of germination is presumably under strong natural selection as it determines the environmental conditions in which a plant germinates and initiates its postembryonic life cycle. To investigate how seed dormancy is controlled, quantitative trait loci (QTL) analyses has been performed in six Arabidopsis thaliana recombinant inbred line populations by analyzing them simultaneously using a mixed model QTL approach. The recombinant inbred line populations were derived from crosses between the reference accession Landsberg erecta (Ler) and accessions from different world regions. In total, 11 delay of germination (DOG) QTL have been identified, and nine of them have been confirmed by near isogenic lines (NILs). The absence of strong epistatic interactions between the different DOG loci suggests that they affect dormancy mainly by distinct genetic pathways. This was confirmed by analyzing the transcriptome of freshly harvested dry seeds of five different DOG NILs. All five DOG NILs showed discernible and different expression patterns compared with the expression of their genetic background Ler. The genes identified in the different DOG NILs represent largely different gene ontology profiles. It is proposed that natural variation for seed dormancy in Arabidopsis is mainly controlled by different additive genetic and molecular pathways rather than epistatic interactions, indicating the involvement of several independent pathways. recombinant inbred lines | quantitative trait loci analyses | near isogenic lines | transcriptome analyses S eed dormancy is an important adaptive trait that together with flowering time is a primary component of the different life history strategies of plants (1). Seasonal timing of germination might well be a stronger factor conditioning the flowering time of Arabidopsis in the field than variation in the genetic basis for flowering time itself (2). Seed dormancy controls the timing of germination by arresting growth and development, despite the presence of favorable environmental conditions to complete germination. Specific environmental and developmental triggers can overcome this arrest. Environmental factors can act during seed development on the mother plant, during seed storage (i.e., after-ripening; AR) and in mature imbibed seeds. The various aspects of seed dormancy and germination have been extensively reviewed recently (3-6). In addition, it has been shown that there is considerable variation for seed dormancy in nature (7-9). The identification of the genes underlying this natural variation for seed dormancy may help to further understand the mechanisms involved in this process. At the same time, it provides insight into the way nature shaped genetic variability for this trait during adaptive evolution. A common approach to discover genes that control quantitative traits is the use of whole-genome scans to identify quantitative trait loci (QTL). These analyses provide estimates of several genetic parameters that underlie phenotypic variation, including the number of loci, th...

We have analyzed two Arabidopsis strains differing in the mean seed size and seed number they produced. The accession Cape Verde Islands (Cvi) yielded on average about 40% fewer seeds than the laboratory strain Landsberg erecta (Ler), but Cvi seeds were almost twice as heavy. Maternal and nonmaternal genetic factors were involved in the seed size variation, and interactions between both types of factors presumably occurred. The Ler͞Cvi seed size difference increased through seed development from ovule maturation until seed desiccation, suggesting that multiple processes of seed development were affected. In addition, it involved changes in the final cell number and cell size of the seed coat and the embryo. Cell number variation was controlled mainly by maternal factors, whereas nonmaternal allelic variation mostly affected cell size. By using a recombinant inbred line population derived from Ler and Cvi, we mapped quantitative trait loci (QTLs) affecting 12 life history traits related to seed size, fruit size, seed number, and plant resources. Five of the seed size QTLs colocated with QTLs for other traits, suggesting that they control seed size via maternal components affecting ovule number and͞or carpel development, ovule development, or reproductive resource allocation in the mother plant. The six remaining putative seed size QTLs did not show a significant effect on any other trait, suggesting that this allelic variation may be involved specifically in seed development processes.

A dwarf mutant of Arabidopsis thaliana (L.) Heynh. was found to be less sensitive to applied gibberellins than the wild type, and this character was controlled by one partially‐dominant gene (denoted Gai) located on chromosome 1. This mutant resembled gibberellin‐deficient mutants since not only stem growth, but also apical dominanace and seed germination were reduced. However, in contrast to the latter mutants, gibberellin does not reverse these effects in the Gai mutant. The insensitivity of the mutant could be quantified in much more detail in the recombinant of this mutation with the GA deficient mutant ga‐1/ga‐1. Endogenous gibberellins of the Gai mutant did not differ from the wild type either in quantity or composition. The data suggest that the gene controls a step involved in gibberellin action.

In Arabidopsis recombinant inbred line (RIL) populations are widely used for quantitative trait locus (QTL) analyses. However, mapping analyses with this type of population can be limited because of the masking effects of major QTL and epistatic interactions of multiple QTL. An alternative type of immortal experimental population commonly used in plant species are sets of introgression lines. Here we introduce the development of a genomewide coverage near-isogenic line (NIL) population of Arabidopsis thaliana, by introgressing genomic regions from the Cape Verde Islands (Cvi) accession into the Landsberg erecta (Ler) genetic background. We have empirically compared the QTL mapping power of this new population with an already existing RIL population derived from the same parents. For that, we analyzed and mapped QTL affecting six developmental traits with different heritability. Overall, in the NIL population smaller-effect QTL than in the RIL population could be detected although the localization resolution was lower. Furthermore, we estimated the effect of population size and of the number of replicates on the detection power of QTL affecting the developmental traits. In general, population size is more important than the number of replicates to increase the mapping power of RILs, whereas for NILs several replicates are absolutely required. These analyses are expected to facilitate experimental design for QTL mapping using these two common types of segregating populations. Q UANTITATIVE traits are characterized by continuous variation. The establishment of the genetic basis of quantitative traits is commonly referred to as quantitative trait locus (QTL) mapping and has been hampered due to their multigenic inheritance and the often strong interaction with the environment. The principle of QTL mapping in segregating populations is based on the genotyping of progeny derived from a cross of distinct genotypes for the trait under study. Phenotypic values for the quantitative trait are then compared with the molecular marker genotypes of the progeny to search for particular genomic regions showing statistically significant associations with the trait variation, which are then called QTL (Broman 2001;Slate 2005). Over the past few decades, the field has benefited enormously from the progress made in molecular marker technology. The ease by which such markers can be developed has enabled the generation of dense genetic maps and the performance of QTL

In Arabidopsis thaliana, seed development in recombinants of the ABA-deficient aba mutant with the ABA response mutants abil or abi3 is compared to wild type and the monogenic parents. Aberrant seed development occurred in the aba,abi3 recombinant and was normal in aba,abil, abi3 and aba,abil seeds. Embryos of the recombinant aba,abi3 seeds maintained the green color until maturity, the seeds kept a high water content, did not form the late abundant 2S and 12S storage proteins, were desiccation intolerant, and often showed viviparous germination. Application of ABA, and particularly of an ABA analog, to the roots of plants during seed development partially alleviated the aberrant phenotype. Seeds of aba,abI3 were normal when they developed on a mother plant heterozygous for Aba. In contrast to seed development, the induction of dormancy was blocked in all monogenic mutants and recombinants. Dormancy was only induced by embryonic ABA; it could not be increased by matemal ABA or ABA applied to the mother plant. It is concluded that endogenous ABA has at least two different effects in developing seeds. The nature of these responses and of the ABA response system is discussed.It has been shown in a number of species that application of ABA to isolated seeds or embryos in an in vitro culture both inhibits precocious germination and stimulates the accumulation of certain late abundant reserve proteins (2,4,20). It has been suggested that these effects of applied ABA mimic a similar role of endogenous ABA during seed development. Mutants with a reduced capacity for ABA synthesis or reduced responsiveness to ABA have been used to test the validity of this hypothesis.In Arabidopsis thaliana mutants showing reduced levels of endogenous ABA were isolated (19). All mutations were found to be located at one locus denoted aba. Most notable among the several pleiotropic effects of the aba mutation were increased withering of stems, leaves, and fruits (siliques) and strongly reduced seed dormancy (11, 18). However, seed development was not affected in the mutants. Similar observations were described for the sit mutant of the tomato cultivar Moneymaker (8,12). In Arabidopsis, also mutants have been isolated with a reduced responsiveness to ABA by selecting for normal seedling growth on a medium containing 1O lM ABA (19). Developing seeds of these mutants contained slightly higher ABA levels than wild-type seeds. The so-called ABA-insensitive mutants (abi) represent three different loci that can be divided into two groups on account of their water relations. Mutants at the abil and abi2 loci transpire excessively and wilt like the aba mutants, whereas the abi3 mutant resembles wild type in showing normal water relations (19). Seeds of all mutants at all three loci failed to develop dormancy unless very late in development or after natural desiccation. All abi seeds developed normal seed weights (14). Thus, these observations on the hormone mutants did not support a role of ABA in seed development.However, it has to be rea...