Plant Genetics 2003: Mechanisms of Genetic Variation

	In late October, Snowbird offered crystal clear mountain vistas but no snow.
	Promega was one of six exhibitors at the meeting.

In October 2003, ASPB sponsored its first special topics meeting on plant genetics in Snowbird, Utah. The meeting’s purpose was to stimulate new ideas and approaches for research focusing on mechanisms of genetic variation. The meeting included sessions on natural variation and processes of evolution, speciation and crop domestication, epigenetic mechanisms, RNA and protein movement, and genetic mechanisms influencing plant form and function. Top scientists from around the world came together to discuss the different aspects of plant genetics and a variety of species and approaches. Thanks go to organizers Venkatesan Sundaresan (University of California, Davis) and Rich Jorgensen (University of Arizona) for developing a schedule packed with excellent speakers and high-quality posters. This article presents a brief sampling of the many outstanding presentations given. Abstracts for speakers and posters can be accessed at http://www.aspb.org/meetings/pg-2003/.

QTL Analysis: Off and Running

One of the major themes to emerge from the meeting was the realization that analysis of quantitative trait loci (QTLs), coupled with the use of new genomic tools and genomewide resources in model systems, is rapidly moving to the forefront of plant functional genetics. The plant science literature is replete with QTL studies that have remained little more than unidentified and often vaguely defined genomic regions for years. QTL analysis is now progressing to rapid selection of candidate genes and identification of individual genes responsible for major QTL effects in many cases. Through his description of the isolation of a cryptochrome2 allele, which confers day-length insensitivity, Maarten Koornneef (Wageningen University, the Netherlands) demonstrated the feasibility of cloning individual QTL through map-based cloning with the use of near-isogenic introgression lines that are developed from naturally occurring populations of Arabidopsis. Richard Clark (working with John Doebley at the University of Wisconsin–Madison) discussed the use of QTL mapping to show that teosinte branched1 (tb1) corresponds to a major QTL that controls the difference between the long branches of teosinte and the short branches of maize. Sequence analysis of this gene region provided evidence that the tb1 promoter but not the coding sequence underwent selection during maize domestication and suggested that changes in regulation of tb1 expression underlie morphological evolution in maize. Kai-Yi Chen (working with Steve Tanksley at Cornell University) presented the results of high-resolution mapping of a major QTL in tomato, Se2.1, which controls stigma morphology associated with self-fertilization. Chen and Tanksley found that the Se2.1 locus comprises five tightly linked genes, one controlling style length, three controlling stamen length, and the fifth affecting anther dehiscence. These were just three of numerous examples presented of QTL analyses linking phenotype to a specific gene and facilitating the functional analysis of genetic loci.

	Daily afternoon poster sessions were lively and well attended.
	Ray Ming, a plant molecular geneticist at the Hawaii Agriculture Research Center, on poster duty.
	Participants enjoyed the views from the Snowbird Tram.

Helianthus: Going with the Flow

In a presentation on hybridization and ecological divergence in wild sunflowers (Helianthus spp.), Loren Rieseberg (Indiana University, Bloomington) observed that gene flow is underappreciated as a creative mechanism in plant evolution. Rieseberg has shown that hybridization allows colonization of extreme habitats in a number of cases. In other words, he found that a number of diploid hybrid sunflower species are specially adapted to extreme environments, such as salt marshes and desert sand dunes. He and his colleagues have also found that complementary gene action, as opposed to epistasis, appears to offer the best explanation for transgressive segregation (i.e., hybrid progeny outperforming either parent in a particular environment) in this genus.

HOTHEAD: “Hot” Locus Award

The prize for the most interesting and most unusual genetic locus was awarded to Bob Pruitt and Susan Lolle (Purdue University) for their work on a novel mechanism of genetic reversion of the HOTHEAD (HTH) locus in Arabidopsis. HTH encodes a glucose-methanol-choline oxidoreductase that is expressed in all tissues and that plays a role in regulating the proper separation and expansion of organs at the growing points of the plant. A fascinating attribute of this locus is that loss-of-function mutations are genetically unstable and undergo an apparently novel form of gene conversion to produce a heterozygous genotype (exhibiting a wild-type phenotype) in up to 15 percent of progeny from self-fertilization of numerous independent homozygous mutant lines. Pruitt convincingly and engagingly detailed a series of experiments that showed that genetic reversion at this locus cannot be explained by any known genetic or epigenetic mechanisms. He offered the intriguing hypothesis that HTH conversion occurs through an RNA template and further speculated that this might constitute another genetic mechanism for enhancing variability in self-fertilizing plant species.

Small Regulatory RNAs and RNA Silencing

The regulation of gene expression by small regulatory RNAs is a hot topic in genetics and development in both plants and animals. In plants, both transgene-induced, double-stranded short-interfering RNA (siRNA) and endogenous single-stranded microRNA (miRNA) activate cleavage of homologous RNA targets. Bonnie Bartel (Rice University) gave an excellent overview of this topic and described work demonstrating that similar mechanisms direct miRNA processing in plants and animals. Bartel highlighted evidence from her research and others that miRNAs represent an important genetic mechanism regulating key aspects of development in plants as well as in animals. Herve Vaucheret (Institut National de la Recherche Agronomique) presented results showing that different sets of genes are required for the production of siRNA and miRNA and that there appears to be limited connection between these two pathways in plants.

Vicki Vance (University of South Carolina, Columbia) has been investigating the suppression of RNA silencing in plants by the helper component proteinase (HC-Pro) of potyviruses for a number of years. Vance reported on new work showing that HC-Pro suppresses several classes of transgene-induced RNA silencing, in each case eliminating the accumulation of small RNAs without blocking either transgene methylation or systemic silencing. The level of endogenous microRNAs in transgenic plants overexpressing HC-Pro was greatly elevated, suggesting that HC-Pro affects multiple small regulatory RNA pathways, possibly by altering the regulation of one or more Dicer-like enzymes.

Concluding Remarks

Plant Genetics 2003 will be remembered as one of the highlights of the year for the more than 200 attendees, including many international participants. The venue, program, invited speakers, and attendees produced a meeting that stimulated new avenues of investigation for many researchers. During the conference breaks, attendees enjoyed the dramatic scenery and fresh mountain air by hiking or riding the famed Snowbird tram. Watch the ASPB News carefully for announcements of the fall 2005 meeting—it is sure to fill quickly.

Nan Eckardt
neckardt@aspb.org