A Critical Assessment of 60 Years of Maize Intragenic Recombination

Until the mid-1950s, it was believed that genetic crossovers did not occur within genes. Crossovers occurred between genes, the “beads on a string” model. Then in 1956, Seymour Benzer published his classic paper describing crossing over within a gene, intragenic recombination. This result from a bacteriophage gene prompted Oliver Nelson to study intragenic recombination in the maize Waxy locus. His studies along with subsequent work by others working with maize and other organisms described the outcomes of intragenic recombination and provided some of the earliest evidence that genes, not intergenic regions, were recombination hotspots. High-throughput genotyping approaches have since replaced single gene intragenic studies for characterizing the outcomes of recombination. These large-scale studies confirm that genes, or more generally genic regions, are the most active recombinogenic regions, and suggested a pattern of crossovers similar to the budding yeast Saccharomyces cerevisiae. In S. cerevisiae recombination is initiated by double-strand breaks (DSBs) near transcription start sites (TSSs) of genes producing a polarity gradient where crossovers preferentially resolve at the 5′ end of genes. Intragenic studies in maize yielded less evidence for either polarity or for DSBs near TSSs initiating recombination and in certain respects resembled Schizosaccharomyces pombe or mouse. These different perspectives highlight the need to draw upon the strengths of different approaches and caution against relying on a single model system or approach for understanding recombination

ELIGULUM-A regulates lateral branch and leaf development. Original figure files

TIFF and JPEG files for the photographs used in constructing figures and supplemental figures in the manuscript, "ELIGULUM-A regulates lateral branch and leaf development," submitted to Plant Physiology. The images document a mutation that alters most of the structures of the plant and how the ELIGULUM-A gene interacts with different developmental pathways. The Figure Legend files describe the images individually.Shoot development is controlled by the actions of the shoot apical and axillary meristems resulting in the development of lateral branches and leaves. The barley (Hordeum vulgare L.) uniculm2 (cul2) mutation blocks axillary meristem development, and mutant plants lack lateral branches, tillers, that normally develop from the crown. A genetic screen for cul2 suppressors recovered two recessive alleles of ELIGULUM-A (ELI-A) that partially rescued the cul2 tillering phenotype. Mutations in ELI-A produce shorter plants with fewer tillers, disrupt the leaf blade – sheath boundary resulting in a liguleless leaf, and secondary cell wall formation is reduced. ELI-A encodes a previously un-annotated plant gene that is conserved in land plants. ELI-A transcript accumulates at the preligule boundary, the developing ligule, leaf margins, cells destined to develop secondary cell walls, and cells surrounding leaf vascular bundles. Recent studies have identified commonalities in the genetic control of boundaries during leaf and lateral organ development. Interestingly, we observed ELI-A transcript at the preligule boundary, indicating a role in establishing the boundary between the blade and sheath. However, we did not observe ELI-A transcript at the axillary meristem boundary in leaf axils, indicating that it does not play a role in establishing the boundary for axillary meristem development. Our results provide a new player in the model for leaf and lateral branch development in which ELI-A acts as a boundary gene for ligule development but not during lateral branch development.Department of Agriculture-CSREES-NRI Plant Growth and Development program grant # 2004-03440Triticeae Coordinated Agricultural Project, US Department of Agriculture/National Institute for Food and Agriculture grant number 2011-68002-3002

The Barley <i>Uniculme4 </i>Gene Encodes a BLADE-ON-PETIOLE-Like Protein That Controls Tillering and Leaf Patterning

Tillers are vegetative branches that develop from axillary buds located in the leaf axils at the base of many grasses. Genetic manipulation of tillering is a major objective in breeding for improved cereal yields and competition with weeds. Despite this, very little is known about the molecular genetic bases of tiller development in important Triticeae crops such as barley (Hordeum vulgare) and wheat (Triticum aestivum). Recessive mutations at the barley Uniculme4 (Cul4) locus cause reduced tillering, deregulation of the number of axillary buds in an axil, and alterations in leaf proximal-distal patterning. We isolated the Cul4 gene by positional cloning and showed it encodes a BTB-ankyrin protein closely related to Arabidopsis BLADE-ON-PETIOLE1 (BOP1) and BOP2. Morphological, histological and in situ RNA expression analyses indicate that Cul4 acts at axil and leaf boundary regions to control axillary bud differentiation, as well as development of the ligule, which separates the distal blade and proximal sheath of the leaf. As the first functionally characterized BOP gene in monocots, Cul4 suggests partial conservation of BOP gene function between dicots and monocots, while phylogenetic analyses highlight distinct evolutionary patterns in the two lineages