Brian Schwartz (Oklahoma State University) described recent advances in the analysis of a class of embryo-defective mutants known as suspensor transformation (sus) mutants in which cells of the suspensor acquire characteristics normally restricted to the embryo proper. A T-DNA tagged allele of sus2 was used to recover flanking plant sequences and clone the corresponding wild-type gene. Sequence analysis revealed a striking similarity with the yeast PRP8 gene that encodes an essential protein component of the spliceosome. Mutants altered in PRP8 have not previously been described in a multicellular eukaryote. Molecular characterization of SUS2 should therefore provide a unique opportunity to dissect RNA processing in plants from a genetic perspective. The mutant phenotype is also consistent with existing models of developmental interactions between the embryo proper and suspensor.
Marilyn West (University of California) described the results of projects in three different laboratories associated with the Embryos for the 21st Century group. The first project (Harada lab) involved genetic and phenotypic characterization a T-DNA tagged allele of the lec1 mutant, described previously in the Meinke laboratory, that exhibits many interesting defects in embryonic maturation. One surprising observation was the discovery of twin embryos in double mutants involving lec1-2 and fus3. The second project (Goldberg lab) used in situ hybridization studies to provide evidence for the existence of regional domains within the globular embryo before other signs of apical-basal differentiation. The final project (Fischer lab) described a fascinating new class of mutants, similar to those described later in the session by Abed Chaudhury, in which seed development proceeds to a limited extent in the absence of fertilization. These results provided further evidence that dramatic new insights into the regulation of plant embryogenesis may be obtained even before the start of the 21st century.
Four poster talks were also included in this session. Ryuji Tsugeki (Carnegie Institution, Baltimore) described the isolation and characterization of a tagged embryo-defective mutant that contained a Ds element inserted into a ribosomal protein gene. The sequence of the gene product was most closely related to the nuclear-encoded S16 ribosomal protein localized in mitochondria of Neurospora. This work provided another example of what types of genes are likely to be disrupted in embryo-defective mutants, but even more importantly illustrated how rapidly sequence information can be obtained from Ds insertion lines using direct sequencing of TAIL-PCR products.
Wolfgang Lukowitz (Tubingen, Germany) presented the results of chromosome walking to the KNOLLE gene required for normal radial patterning in the developing embryo. KNOLLE encodes a protein 310 amino acids in length with similarity to vesicle docking proteins of the syntaxin family described in animal systems. KNOLLE function may be limited to specific stages of the cell cycle because transcripts are present only in isolated patches of cells in the developing embryo. The availability of multiple mutant alleles should facilitate further analysis of this interesting gene.
Abed Chaudhury (CSIRO, Australia) described the isolation and characterization of a fascinating collection of mutants that exhibit fertilization-independent seed development. These fis mutants were isolated as EMS-induced pseudorevertants of the mutant pistillata based on their ability to produce long siliques in the absence of stamens. At least three different loci have been identified and mapped. Seed enlargement and a limited amount of endosperm development were observed in the absence of fertilization. Embryo development arrested at the torpedo stage when heterozygous plants were pollinated. Further analysis of these mutants may provide valuable insights into the regulation of early stages of embryo and endosperm development.
Patrick Gallois (Perpignan, France) concluded the session by describing how a promoter trapping experiment designed to generate information on promoters active during embryogenesis resulted in the molecular isolation of an intriguing regulatory sequence that appears to drive GUS expression only in cells of the suspensor. This promoter may be particularly valuable in future experiments designed to investigate developmental interactions between the embryo proper and suspensor throughout seed development.