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To branch or not to branch

The closest wild relative of maize, teosinte, does not look very promising as food. The ear is tiny compared to the domesticated one, and the grains are surrounded by hard fruitcases that are difficult to break open. Teosinte originated in Mexico, and, around 10,000 years ago, mutations in the wild population produced plants that attracted the attention of hunter gatherers looking for some starch in their diets. Saved seed was planted and desirable plants selected again in the next generation. Along with desirable traits, these early agriculturalists were selecting genes important for transforming a wild grass into a food plant. These same genes are being studied today to understand how maize and other crops in the grass family like rice, wheat, and sorghum produce grain. This knowledge is being used to create new varieties with better and consistent yield.

Dr. Erik Vollbrecht and his colleagues, Xiang Yang, Brandi Sigmon, Erica Unger-Wallace, and Zhuying Li, have studied mutations of some of the genes related to ear formation, among them, RAMOSA 1-3, which helped to transform the tiny teosinte ear with only 5-12 kernels into the large, massive corn cob we eat today. Dr. Vollbrecht, of the Department of Genetics, Development, and Cell Biology at Iowa State University, will be presenting this work at a symposium on Maize Biology at the annual meeting of the American Society of Plant Biologists in Mrida, Mexico (June 28, 10:40 AM).

The familiar spikes of grasses are the flower-bearing stems, or inflorescences, which produce tiny, wind-pollinated flowers. Different grass species, and especially the grain crops, differ in the number, length, and types of inflorescence branches, ranging from the straight spikes of wheat and barley to maize and sorghum with branched tassels. Maize has two separate inflorescencesthe male tassel or pollen-producing flowers, and the female flowers that produce the kernel-bearing ears. The terminal male inflorescence has long branches at its base and a central spike with shorter branches that carry the pollen-bearing flowers. The female inflorescences, or ears, are laterally positioned and have short branches, which is important for efficient packing and harvesting of seeds.

The differences in grass inflorescence architecture have important implications for grain yield. For example, the more branches in rice, the higher the grain yield. The opposite is true for maize, which puts its energy into the massive cobs that sit on short side branches. These different patterns of branching are determined by meristems, plant stem cells located at the tips of growing stems and at the bases of leaves (axils). The activity of these meristems is regulated by networks of genes expressed throughout plant development. Among these architectural genes so important in the domestication of maize are the RAMOSA genes and proteins studied by Dr. Vollbrecht and other researchers.

Through their analyses of these and other mutants,Vollbrecht and his co-workers have determined that the three RAMOSA genes (RA1-3) regulate inflorescence branching in maize. RA1 and RA2 are transcription factors, proteins that control the process in which a gene's DNA strands are read and rewritten as RNA strands. RA3 encodes a phosphatase that is important in the biosynthesis of a sugar, trehalose, thought to be an important developmental signaling molecule. Vollbrecht and his colleagues suggest that these three genes, along with others, act in a network unique to grasses, which controls the architecture of the maize inflorescence and, ultimately, grain yield. RA2 acts upstream of RA1, which is expressed at the boundary of meristems and forces the stem cells to produce short branches. RA2, in turn, regulates RA1; and RA3 may be involved in modifying a mobile signal that tells axillary meristems either to stop making branches or to continue growth. When the three genes are mutated, the mutant maize plants have more and longer branches and produce smaller and deformed cobs.

These scientists also studied the expression patterns of these genes in other species. RA1 appears to be absent in rice and is found only in the large tribe, Andropogoneae, which includes maize and sorghum. RA2 and RA3 appear to be conserved over many grass taxa suggesting that they are important in controlling inflorescence architecture in all grasses. Vollbrecht and his colleagues propose that all three of these genes have been important in the evolution of grass inflorescence architecture.

One of Charles Darwin's insights was that natural selection is the same as artificial selection. He formulated the theory of evolution, in part through his observations of the work of breeders of plants and animals. Domestication is a form of artificial selection, and it is thought that as many as 1200 genes were important in transforming maize into a major food crop. Maize is a good model plant for studying inflorescence and grain morphology because it has a complex genome and a rich genetic history with numerous developmental mutants. Mutant maize plants with more and longer branches have been known since the early 20th century, but the reasons for these architectural aberrations were unknown until recently. Studies of these mutants and the genes that were important in the domestication and evolution of grain crops are providing insights for the genetic engineering of crops to improve yield as well as resistance to pests and tolerance for difficult growing conditions such as poor soils, heat, and drought.


Contact: Erik Vollbrecht
American Society of Plant Biologists

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