Objective

• Combine conventional plant breeding with advances in molecular genetics.
• Techniques under investigation include using rice tissue culture to screen for rice herbicide resistance, using rice anther culture to produce pure breeding lines from hybrids, and a systematic search for rice lines that reproduce asexually.

The rice genetics project is conducted by the Agricultural Research Service of the U.S. Department of Agriculture in cooperation with the UC Department of Agronomy and Range Science and the Rice Experiment Station. It provides basic information and materials to U.S. rice breeders and advances the science and art of plant breeding. Past accomplishments include the development and extensive use by rice breeders of short stature genes, the discovery and use of improved resistance to stem rot and aggregate sheath spot diseases from wild species of rice, and progress, although slow, toward a possible system of hybrid rice production adaptable to the U. S.

Twin seedling rice plants are being tested genetically to determine if one of the pair has developed from mother tissue without pollination.

Twelve rice lines with potentially useful characteristics were jointly released to rice breeders by USDA-ARS, the University of California and the California Cooperative Rice Research Foundation. They contain such characteristics as early maturity, various plant heights, and sources of resistance to stem rot and the water weevil.

The genetic male sterile characteristic is being used to facilitate the production of hybrids without hand pollination. Two populations are under development that introduce genes from foreign sources. An Asian population of about 400 varieties from Japan, Korea, and Taiwan was wind pollinated with the genetic male sterile M-101. Nearly 4,000 hybrid plants were produced, and second generation selections produced 374 male sterile plants with desirable characteristics. Seeds have been provided to rice breeders. A similar program has been started by natural crossing of the genetic male sterile M101 with 200 African rice lines.

Genetic male sterile lines from other varieties are needed to take full advantage of the natural crossing system. These are being produced through the use of the chemical mutagen ethylmethyl sulfate (EMS). This chemical is very efficient for inducing male sterility. The project has produced genetic male sterile plants from the varieties M-201, L-202, M-202, Calmochi-101 and Quin Qun Wang, in addition to M-101. These additional male sterile lines will increase opportunities for genetic recombination among diverse parents.

The project is continuing work on genetic mechanisms that might make hybrid rice production easier. The current system requires the use of cytoplasmic male sterility, which is difficult to transfer into California rice varieties. Two antibiotics, streptomycin and mitomycin, were used in attempts to induce cytoplasmic male sterility; two sterile plants were found, but they do not appear to be cytoplasmic steriles and, therefore, are not useful for hybrid rice production.

Another possible system for developing hybrid rice, apomixis, a form of asexual seed production, is being investigated. Apomixis occurs naturally in several forage grasses but is not known in rice. Its application to rice would make possible the production of true-breeding F, hybrids with permanently fixed hybrid vigor. An intensive search is being made for apomictic rice from the world collection of cultivated rice and from weedy relatives of rice. Apomictic reproduction is being looked for among high frequency twin seedlings found in rice introductions from China. Efforts are also being directed to making intergeneric crosses between rice and known apomictic grasses. This project is supported in part by a grant from the Rockefeller Foundation.

Dr. John Chandler makes sterile transfer of rice tissue culture cells as one step in generating rice plants from cell tissue.

Two different biotechnology systems are being studied as possible methods for producing true breeding hybrids immediately from the first generation cross. One method, anther culture, is a process of regenerating a rice plant from anther tissue which contains only half the number of chromosomes of a normal plant. These plants, called haploids, spontaneously double to produce the normal complement of chromosomes. Progeny from these plants will be true breeding (not segregating) and, therefore, produce a stable variety immediately from a hybrid cross.

A search for genetic resistance in rice to the American Cyanamid grass-killing herbicide AC499 has been initiated. This herbicide could be useful in rice because it controls grasses and sedges at very low concentration, does not move after application to the soil, and has no mammalian or fish toxicity.

One part of the screening procedure uses mutagenized populations at the seedling stage. More intensive efforts are directed to screening for resistance to AC499 using somatic tissue culture. This procedure operates at the cellular level. Single cells or groups of cells from embryos are exposed to various concentrations of the herbicide, after which they are transferred to an herbicide-free regenerating medium. The purpose is to regenerate a living plant resistant to the herbicide. This same procedure has been successful in Minnesota to select corn plants resistant to a similar herbicide. Results with rice are encouraging.

In summary, the rice genetics project is trying to break new ground in rice breeding by facilitating large scale natural crossing as an adjunct to hand pollinations and by using several new approaches to possible hybrid rice production. Cell culture and mutagenesis (induced mutations) are techniques being used to discover genetic resistance in rice to a promising environmentally safe herbicide.