Tracking Down Plant Genes: Paths, Patterns, and Footprints

Abstract

From November 13 to 18, 1988, approximately 1000 scientists from 32 countries experienced 243 oral presentations and 420 posters from 8:40 in the morning to 10:30 at night at the Second International Congress sponsored by the International Society for Plant Molecular Biology (ISPMB) in Jersualem, Israel. Despite a warm welcome by the organizers, the meeting opened with the coldest day in Jerusalem in 68 years, resulting in the consumption of 6800 shekel's worth ($4000) of coffee. The following dis? cussion touches on only a few of the highlights amidst the many concurrent sessions. The number of nuclear plant genes and promoters that have been cloned, sequenced, and subjected to develop? mental studies has grown greatly since the first ISPMB conference 3 years ago. Striking views of expression in situ by hybridization or the /5-glucuronidase (GUS) reporter method were provided by a number of investigators, who have collected genes expressed specifically in vascular tissue, lateral roots, wounds, petal epidermis or mesophyll, anther parenchyma, stigma, style, receptacle, tapetal layer, etc. One particularly intriguing pattern of expression was reported by Tom Guilfoyle (University of Missouri, Colum? bia), who found hybridization to an auxin-induced cDNA specifically on the faster elongating side of hypocotyls undergoing gravitropic responses (McClure and Guilfoyle, 1989). A word of caution about interpreting hybridization and histochemical stains for reporter gene activity was given by Richard Jefferson (AFRC Institute of Plant Research, Cambridge, United Kingdom). Tissue sections where cells are small and closely packed can give rise to potentially misleading visual signals. For example, phloem cells might contain the same number of GUS enzyme units or autoradiography grains as larger parenchyma cells, but the signal will appear more concentrated per unit area in the smaller phloem cells. This is readily misinterpreted as higher expression in the vascular tissue. Jefferson also discussed the prospect of producing substrates for GUS that could provide nondestructive selection or screening methods for GUS activity. Such a development could eliminate a major drawback to the GUS reporter system, the necessity to destroy the cells under assay. Technical achievements by several laboratories point the way toward simpler and/or more powerful means to analyze plant gene function. Robert Ferl (University of Florida, Gainesville) reported on the successful application of the genomic sequencing method for examining devel? pment changes in chromosomal configuration and protein factors binding to a maize alcohol dehydrogenase gene. Elaine Tobin (University of California, Los Angeles) re? ported some promising experiments aimed at developing the fms2 gene of Agrobacterium, which encodes an indole acetamide hydrolase into a suicide gene. Tobin's postdoctoral associate George Karlin-Neumann has observed that seedling progeny of a plant containing a phytochrome regulated promoter-fms2 fusion were more sensitive than wild-type to 1-napthaleneacetamide (NAM), which is converted to napthaleneacetic acid (NAA) by the fms2 gene product. A remarkable method developed by Smith et al. (1989) for visualizing DNA undergoing electrophoresis in field inversion gels was described by Arnold Bendich (University of Washington, Seattle), who is applying the method to organelle DNAs. A videotape recording of fluorescent microscopy provided extraordinary views of linear DNA mol? ecules weaving through the gel and circular molecules becoming trapped. This method may help establish the true configuration of chloroplast and mitochondrial genomes, which remains unknown. Although circular restric? tion maps containing repeated sequences that can recombine have been produced from organelle DNAs of many different plant species, the large size and potential fragility of many of the predicted circular molecules preclude unambiguous analysis by electron microscopy. Field inver? sion gel electrophoresis of chloroplast DNA molecules has already given a surprising result, reported by Wilhelm Gruissem (University of California, Berkeley) and Pim Zabel (Agricultural University, Wageningen, The Netherlands). Chloroplast DNA molecules with apparent molecular weights ranging from monomer to tetramer were observed in tomato, maize, spinach, and pea. In contrast, no labo? ratory has reported banding plant mitochondrial DNAs following field inversion gel electrophoresis, although yeast mitochondrial genomes have been found as a band of appropriate molecular weight. Whether the multiple circular molecules predicted from current models of the plant mitochondrial genome actually predominate in vivo re? mains to be demonstrated. Those curious about the microscopic method can order a videotape made by Steve Smith entitled Individual DNA Molecules Undergoing Gel Electro-