Submitted to: Plant Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 28, 1996
Publication Date: N/A
Interpretive Summary: Agrobacterium causes a "cancer" in plants known as Crown Gall Disease by inserting some of its own genes into the chromosomes of the infected host plants. This natural gene transformation system introduces one or few copies of the necessary bacterial genes into the plant's genome. Scientists can replace the natural bacterial genes with other genes so that cocultivation of plant cells with the altered Agrobacterium can be used to introduce desired genes into plants. Genetic engineering via Agrobacterium is almost routine for dicotyledons (tobacco, beans, etc.) which serve as natural hosts for the Agrobacterium. However, monocots (grasses) are not naturally infected by Agrobacterium. Most monocot transformation to date has been accomplished through other laboratory methods. It has been limited by the fact that although plant cells incorporate complete and working genes into their chromosomes, and even pass these genes intact to their sexual progeny, the genes are often not expressed in these progeny. Methylation is a natural process whereby organisms turn genes on and off. Examples of this are the expression of different genes in young and older organisms, or the transition from vegetative to reproductive stage in plants. Methods used for monocot transformation generally introduce multiple copies of the foreign gene into plant cells. It is believed that the presence of multiple gene copies is sensed as unnatural by the plants, which "rectify" the situation by turning the genes off via methylation. With new insight on the chemical processes associated with infection of dicots by Agrobacterium, it is now possible to synthetically infect and therefore transform monocots, like rice, using Agrobacterium. This paper presents two ways transformation of rice via Agrobacterium is enhanced.
The genetic basis for three grain yield components of rice - 1000 kernel weight (KW) grain number per panicle (GN), and grain weight per panicle(GWP) was investigated using 115 RFLP markers and F4 progeny testing from a cross between Lemont and Teqing. Following identification of 19 QTLs affecting these traits, we investigated the role of epistasis in genetic control of these phenotypes. Using both two-way ANOVA and multiple regression analyses, we were able to identify 63 markers throughout the genome which were involved in 79 highly significant interactions affecting the three traits. The majority of the putative epistatic loci did not appear to have main' effects on any of the traits, but influenced the traits in a predominantly complementary manner. The more complex traits GN and GWP appeared to be more greatly influenced by epistasis than the highly heritable trait KW. These results indicate that epistasis is an important genetic basis for complex traits such as yield components, especially GWP and GN. The identification of epistatic lock is a step toward resolution of discrepancies between QTL mapping and classical genetic dogma, contributes to better understanding of the persistence of genetic variation for quantitative traits in populations, and impels reconsideration of optimal mapping methodology and marker-assisted breeding strategies for improvement of complex traits.