The fus3 mutation of Arabidopsis thaliana affects several aspects of seed development. Mutant seeds are desiccation intolerant, viviparous and accumulate anthocyanins. Two major classes of storage proteins, the 12S cruciferins and the 2S albumins, are nearly absent, storage lipids are reduced and their composition is changed. The transcription of heterologous storage protein gene promoters in a fus3 genetic background is similarly affected. Our data suggest that the FUS3 gene is together with other genes like ABI3 and LEC1 central to the regulation of developmental processes during late embryogenesis.

SummaryThe Arabidopsis thaliana mutants fus3, lec1 and abi3 have pleiotropic defects during late embryogenesis. Mutant embryos fail to enter the maturation programme and initiate a vegetative germination pathway instead. Screening for genes which are differentially expressed in the fus3 mutant of Arabidopsis resulted in the isolation of several members of the MYB family. MYB domain proteins in plants represent an extended gene family of transcription factors, suggesting their participation in a variety of plant specific cellular functions. Here, the authors describe one of these genes, designated AtMYB13, representing a novel member of the MYB gene family. The structure of the gene as well as its genomic organisation and localisation are reported. The expression of the gene is regulated by dehydration, exogenous abscisic acid, light and wounding. A chimeric AtMYB13 promoter/GUS gene is tissuespecifically expressed in transgenic Arabidopsis plants. The GUS staining was predominantly detected in the shoot apex zone and at the basis of developing flowers. In addition, the AtMYB13 gene promoter is active at branching points of the inflorescence. Furthermore, ectopic expression of the AtMYB13 gene has a characteristic impact on the architecture of the inflorescence leading to peculiar hook structures at pedicel branching points. In addition, some transgenic plants exhibit a reversed order of first flowers and axillary buds. These data suggest a function of the AtMYB13 gene product in linking shoot morphogenic activity with environmental as well as intrinsic signals.

Two novel MYB genes (ATMYBR1 and ATMYBR2) were isolated from Arabidopsis thaliana. Binding to a conserved MYB recognition sequence is demonstrated for the ATMYBR1 protein. The expression of both genes is affected by the fus3, lec1 and abi3 mutations causing pleiotropic defects during late embryogenesis and seed maturation including the loss of dormancy and desiccation tolerance. The strong increase of the transcript levels of both MYB genes during very late stages of embryogenesis typically found in wild type is missing in the mutants. Furthermore, the expression of both MYB genes is developmentally regulated in vegetative tissues. The highly conserved repeats (R2 and R3) of the DNA binding MYB domain of both proteins represent chimeric structures combining features typical of plant and animal derived proteins. This demonstrates the existence of a distinct subfamily of animal-like MYB factors in plant genomes.

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