Research progress on optimizing panicle characters and quality improvement of <italic>indica</italic> pedigree in northern <italic>japonica</italic> rice

Abstract

<p indent=0mm>Since the Eleventh Five Year Plan began, the northern <italic>japonica</italic> rice breeding team, headed by the Rice Research Institute of Shenyang Agricultural University, has undertaken an effort to improve the quality of northern <italic>japonica</italic> rice, with particular attention to panicle traits and regulating the <italic>indica</italic> pedigree. Remarkable progress has been made in the elucidation of the genetic and physiological basis of important agronomic traits, germplasm resource innovation, and the breeding of new varieties. (1) We found that erect panicle architecture is controlled by a dominant allele of <italic>EP1</italic>, which produces the shortened internode of the panicle, increases both the number of grains per panicle and the seed density, and eventually affects overall appearance and milling quality while improving the yield characters. We also found that the introgression of <italic>indica</italic> pedigree improved the yield potential of <italic>japonica</italic> rice, though there was a detectable quality penalty, especially in eating and cooking quality. (2) We proposed a model of ideal panicle architecture and identified effective selection indicators including panicle curvature, panicle type index, vascular bundle efficiency, primary branch efficiency, and <italic>indica</italic> pedigree introgression percentage. On this basis, we established the quantitative parameters for ideal panicle type of <italic>japonica</italic> rice in northern China (neck panicle curvature < 40°, panicle type index > 0.50, vascular bundle efficiency ≥ 0.75, primary branch efficiency ≈ 1.0, number of primary branches ≥ 15, brown rice rate ≥ 83%, head rice rate ≥ 68%, chalky rice rate ≤ 10%, taste value ≥ 80, the <italic>indica</italic> pedigree introgression at about 10%) to coordinate the balance between yield and quality, especially eating quality. (3) We have systematically summarized the results of our research, and we developed a strong theory for the improvement of the quality and technical systems of northern <italic>japonica</italic> rice by optimizing panicle traits and <italic>indica</italic> pedigree introgression. We first selected elite offspring derived from the cross between <italic>indica</italic> and <italic>japonica</italic>, and then backcrossed or double-crossed them with high-quality, high-yield <italic>japonica</italic> germplasm resources, optimized the eating and cooking quality using a directional selection of <italic>indica</italic> pedigree introgression, and narrowed the gap between superior and inferior grains through the increase in the number of panicle neck vascular bundles and primary branches to realize the synchronous improvement of milling, appearance, and eating and cooking quality on the basis of maintaining the grain yield. This research has led to a 16.60% increase in the per unit yield of <italic>japonica</italic> rice in northeastern China and a 96.88% increase in cultivation area since the beginning of the 21st century. The proportion of total yield in the country has increased from 9.51% to 17.91%, indicating a synchronous and significant improvement of cultivation area per unit yield, and grain quality. In this paper, we also discuss existing problems and directions for the development of related fields.