Morphological Characteristics And Genetic Mapping On A Twin-ovary Mutant In Rice

Rice is one of the most important food crops in the world. Rice production andquality depend on the development of the floral organ. As a gramineal model plant and adelegate of monocotyledon, genetic and molecular mechanism underlying the regulation ofthe rice floral organ development had become a hot spot of plant developmental biology.At present, most genes were isolated by screening the cDNA or DNA library with theprobes homologous to sequences of dicot plants, and their functions were identified by themorphology of floral organs in transgenic plant. But the method limited greatly theunderstanding of the regulation on floral development. The mutant of rice floral organ wasimportant for gene cloning and the studies on gene expression and its function. Researchon rice reproductive mechanism has significant theory worthiness.A rice floral mutant with twin-ovary was isolated from the progenies of thecombination of C2/Mianxiang5B.Floral malformed structures and its morphogenesis wereidentified by anatomical microscopy, paraffin slics, scanning electron microscopy andgenetic analysis of mutant traits, furthermore, related genes were preliminary molecularmapped. Main results were as below:1 Phenotype characteristic of TOR mutantCompared with wild type, the mutant showed dwarf plant-height, small panicle, theflag leaf curled and tied at the adaxial part, smaller grain, some floret without palea. Mostof florets were made up of normal paleola, more pistils, fewer stamens and 40.4% of themconsisted of three pistils, 45.25% four pistils. Most anthers were malformation and onestamen had two anthers in some florets. More than 97% ovarys were abnormal, 2/3 ofwhich consisted twin-ovary, and 2 to 4 stigmas were on the same ovary. 7.6% floretsdisplayed stamen- origin pistils. Malformation embryo sac, twin-ovule and twin-embryosac in an ovary were seen with paraffin slic. Another obvious character of the mutant wascomparatively lower seed setting rate, and the pollen fertility rate was about 10%.2 Development process of flora organ by SEMThe wild type and mutant were not different significantly at the beginning of floralprimordium differentation. But their appearance were obviously different at the beginningof stamen primordium. In mutant type florets, the stamen development was at the different stage. Two stamen primordiums at the cross of lemma and palea developed firstly, theybecame remarkably bigger than others and formed two carpel-like structures, the latter mayfurther develop into twin-ovary.3 Genetic analysis of TOR mutantSeven, eleven and eight lines were planted continuous three generation respectively.The segregation ratio of wild type plants to mutant plants in the population was accordedwith 3:1. It indicated that the mutant was controlled by a single recessive gene and the genewas named TOR tentatively.4 Molecular mapping of gene on TOR mutantThe mapping population were composed of 282 mutants from lines with mutant and20 wild type plants, and all the 8 lines were planted at Mianyang in 2006. Subsequently,the gene mapped on chr.12 with a distance of 24.7cM between the gene and RM101.5 PerspectiveAccording to the ABC model of floral organ, the traits were changed in the third andforth whorls of the mutant type florets. We deduced that the phenomenon of twin-ovaryand more stigmas in the forth whorls were from partly homeotic conversion of stamensinto pistils. It was very similar to the phenotype for loss of B functional genes inArabidopsis. Simultaneously, the mutant had malformation embryo sac and two ovules inthe same ovary, which implied that D functional genes were included.