Assessment Of Rice Seedling Heterosis And Research On The Relationship Between Gibberellin And The Biological Basis For Seedling Heterosis

Rice(Oryza sativa L.) is one of the most important food crops in the world. To solve the problem of the world food security, heterosis can be exploited to improve the yield potential of rice. A comprehensive understanding of the biological mechanism for heterosis will definitely benefit from knowledge at three levels:genetic, molecular and physiological. In rice, although many studies have been performed to contribute to a better understanding of the genetic and molecular basis of heterosis, little efforts have been made towards the physiological basis. Plant hormones play a vital role in the physiological regulation of plant growth and development, and it was proposed that gibberellins (GAs) provide a phytohormone basis for heterosis in plant. The study presented here is designed to investigate in depth the physiological role of GAs in the regulation of heterosis for rice seedling growth, or the hormonal basis for heterosis of rice.For this purpose, we developed a3×3incomplete diallel set and a matrix of all possible single-cross hybrids between them. Substantial and varying degree of heterosis for seedling growth was observed on the traits of SDW, ADW and TN. Mid-parent heterosis (MPH) ranged from-9%to61.7%for shoot dry weight,1.4%to73.3%for tiller number and3.7%to64.4%for axes dry weight.Then, the endogenous levels of GAs in these rice strains were measured unequivocally for the growing shoots of20DAS seedlings and the shoot plus root axes of4-d-old germinating seeds by GC-MS. It is worthy to note that all the molecules along the GA metabolic pathway, including not only bioactive GAs but also their precursors and catabolites, are included in the quantification analysis. To our knowledge, this might be the first study having such a full coverage of GA molecule species for heterosis analysis in plant. For20DAS seedlings, heterosis of GA contents was low and the contents in the hybrids were close to the means of the parents. In the shoot plus root axes of4-d-old germinating seeds, heterosis was relatively high.Furthermore, we also examined the expression profiles of a wide range of GA metabolism and signaling genes in rice using real time RT-PCR for the growing shoots of rice seedlings at20DAS. The cases of differential expression were frequently detected for most of the genes examined (13out of16), in the majority of which nonadditive modes of gene action were found, predominantly as the modes of overdominance and positive dominance.Through the combinative correlation analyses of the above data, we observed highly significant correlations of the contents of certain GA species with performance and heterosis of shoot dry mass and also with heterosis of tillering number. In accordance with this, strong correlations were detected between expression levels of certain GA-related genes and heterosis for shoot dry weight and tillering number, and also between heterotic expression effects of certain genes with heterosis of shoot dry weight. These strong correlations imply that GAs may play an important role at the physiologcial level in the regulation of heterosis for rice seedling growth.