Determining the effect of gene deletion is a fundamental approach to understanding gene function. Conventional genetic screens exhibit biases, and genes contributing to a phenotype are often missed. We systematically constructed a nearly complete collection of gene-deletion mutants (96% of annotated open reading frames, or ORFs) of the yeast Saccharomyces cerevisiae. DNA sequences dubbed 'molecular bar codes' uniquely identify each strain, enabling their growth to be analysed in parallel and the fitness contribution of each gene to be quantitatively assessed by hybridization to high-density oligonucleotide arrays. We show that previously known and new genes are necessary for optimal growth under six well-studied conditions: high salt, sorbitol, galactose, pH 8, minimal medium and nystatin treatment. Less than 7% of genes that exhibit a significant increase in messenger RNA expression are also required for optimal growth in four of the tested conditions. Our results validate the yeast gene-deletion collection as a valuable resource for functional genomics.

The functions of many open reading frames (ORFs) identified in genome-sequencing projects are unknown. New, whole-genome approaches are required to systematically determine their function. A total of 6925 Saccharomyces cerevisiae strains were constructed, by a high-throughput strategy, each with a precise deletion of one of 2026 ORFs (more than one-third of the ORFs in the genome). Of the deleted ORFs, 17 percent were essential for viability in rich medium. The phenotypes of more than 500 deletion strains were assayed in parallel. Of the deletion strains, 40 percent showed quantitative growth defects in either rich or minimal medium.

Plants distinguish among the pollen grains that land on the stigma, permitting only compatible pollen to fertilize egg cells. To investigate these cell-cell interactions, Arabidopsis mutations that affect pollen-pistil communication were isolated. A male-sterile mutation that disrupts pollen-pistil interactions by eliminating the extracellular pollen coat (tryphine) is described here. Stigma cells that contact the mutant pollen produce callose, a carbohydrate synthesized in response to foreign pollen. The mutant pollen fails to germinate because it does not absorb water from the stigma, yet germinates in vitro, indicating it is viable. The defect is also conditional; high humidity results in pollen hydration and successful fertilization. Analysis of mature, mutant pollen indicated that it is deficient in long-chain lipids and has none of the lipoidic tryphine normally present on its surface. Immature mutant pollen grains have aberrant tryphine that disappears during pollen development. The sterile plants also lack stem waxes, and pollen from other wax-defective (eceriferum) mutants with reduced fertility has few of the lipid droplets normally present in tryphine. These results demonstrate that tryphine is critical for pollen-stigma interactions and suggest that tryphine lipids are required for fertilization, either by directly signaling the stigma or by stabilizing other tryphine components.[Key Words: Pollen; Arabidopsis; compatibility; lipids; tryphine; eceriferum] Received February 22, 1993; revised version accepted April 7, 1993. Fertilization in flowering plants involves many cell-cell interactions, including adhesion of pollen grains to the -stigma surface, growth of pollen tubes through the pistil, migration of sperm cells within the pollen tube, and, ultimately, fusion of the sperm with egg cells; yet few of the molecules required for these interactions have been identified. As a first step in understanding these cellsignaling events, several sterile Arabidopsis mutants blocked in early steps of fertilization were identified. In this paper, one such mutant, defective in the interaction between pollen grains and stigma cells, is described.The signaling events that occur during fertilization presumably require direct contact between cells and rely on molecules present in low abundance. Few of these molecules have been identified through in vitro reconstitution experiments, in part because manipulation of reproductive tissues is technically difficult. In contrast, genetic approaches depend solely on the function of molecules, rather than on their abundance, and allow cellcell communication events to be identified and characterized in living plants. Because most plant species are ~Corresponding author.hermaphrodites (producing both sperm and eggs), mutant plants defective in either the male or the female reproductive process can be obtained and propagated readily.Arabidopsis thaliana is well-suited to a genetic approach to fertilization. The advantages of its short life cycle, small physical size, and sma...