Development of hybrid rice with high yield and grain quality is a goal of rice breeding. To investigate the genetic mechanism of heterosis for rice milling and appearance quality in indica/xian rice, QTL mapping was conducted using 1061 recombinant inbred lines (RILs) derived from a cross of the xian rice cultivars Quan9311B (Q9311B) and Wu-shan-si-miao (WSSM), and a backcross F₁ (BC₁F₁) population developed by crossing the RILs with Quan9311A (Q9311A), combined with phenotyping in two environments. The F₁ hybrid (Q9311A × WSSM) showed various degrees of heterosis for milling and appearance quality. A total of 142 main-effect QTL (M-QTL) and 407 pairs of epistatic QTL (E-QTL) were identified for five milling and appearance quality traits and grain yield per plant (GYP) in the RIL, BC₁F₁ and mid-parental heterosis (H_MP) populations. Differential detection of QTL in three populations revealed that most additive loci detected in the RILs did not show heterotic effects, but some of them did contribute to BC₁F₁ trait performance. Unlike heterosis of GYP, single-locus overdominance and epistasis were the main contributors to heterosis for milling and appearance quality. Epistasis contributed more to the heterosis for milling quality than to that for appearance quality. Three (four) QTL regions harboring opposite (consistent) directions of favorable allele effects for GYP and grain quality were identified, indicating the presence of partial genetic overlaps between GYP and grain quality. Three strategies are proposed to develop hybrid rice with high yield and good grain quality: 1) pyramiding favorable alleles with consistent directions of gene effects for GYP and grain quality at the M-QTL on different chromosomes; 2) introgressing favorable alleles for GYP and grain quality into the parents and then pyramiding and fixing these additive effects in hybrids; and 3) pyramiding overdominant and dominant loci and minimizing or eliminating underdominant loci from the parents.

Salinity is one of the major abiotic stresses which impose constraints to plant growth and production. Rice (Oryza sativa L.) is one of the most important staple food crops and a model monocot plant. Its production is expanding into regions that are affected by soil salinity, requiring cultivars more tolerant to saline conditions. Understanding the molecular mechanisms of such tolerance could lay a foundation for varietal improvement of salt tolerance in rice. In spite of extensive studies exploring the mechanism of salt tolerance, there has been limited progress in breeding for increased salinity tolerance. In this review, we summarize the information about the major molecular mechanisms underlying salinity tolerance in rice and further discuss the limitations in breeding for salinity tolerance. We show that numerous gene families and interaction networks are involved in the regulation of rice responses to salinity, prompting a need for a comprehensive functional analysis. We also show that most studies are based on whole-plant level analyses with only a few reports focused on tissue- and/or cell-specific gene expression. More details of salt-responsive channel and transporter activities at tissue- and cell-specific level still need to be documented before these traits can be incorporated into elite rice germplasm. Thus, future studies should focus on diversity of available genetic resources and, particular, wild rice relatives, to re-incorporate salinity tolerance traits lost during domestication.

Iron and zinc are two trace elements that are essential for rice. But they are toxic at higher concentrations, leading to severe rice yield losses especially in acid soils and inland valleys. In this study, two reciprocal introgression line (IL) populations sharing the same parents were used with high-density SNP bin markers to identify QTL tolerant to iron and zinc toxicities. The results indicated that the japonica variety 02,428 had stronger tolerance to iron and zinc toxicities than the indica variety Minghui 63. Nine and ten QTL contributing to iron and zinc toxicity tolerances, respectively, were identified in the two IL populations. The favorable alleles of most QTL came from 02,428. Among them, qFRRDW2, qZRRDW3, and qFRSDW11 appeared to be independent of genetic background. The region C11S49–C11S60 on chromosome 11 harbored QTL affecting multiple iron and zinc toxicity tolerance-related traits, indicating partial genetic overlap between the two toxicity tolerances. Our results provide essential information and materials for developing excellent rice cultivars with iron and/or zinc tolerance by marker-assisted selection (MAS).

The backcross (BC) breeding strategy has been increasingly used for developing high yielding varieties with improved abiotic stress tolerances in rice. In this study, 189 Huang-Hua-Zhan (HHZ) introgression lines (ILs) developed from three different selection schemes were evaluated for yield related traits under drought stress and non-stress conditions in the target and off-season winter nursery environments to assess the selection efficiency of BC breeding for improving different complex traits, and led us to five important results. The first result indicated that the primary target traits should be selected first in the target environments (TEs) in order to achieve the maximum genetic gain. Secondly, BC breeding for drought tolerance (DT) in rice was almost equally effective by strong phenotypic selection in the main target environments and in the winter-season of Hainan. Thirdly, exploiting genetic diversity in the subspecific gene pools is of great importance for future genetic improvement of complex traits in rice. Fourthly, considerable genetic gain can be effectively achieved by selection for secondary target traits among the ILs with the primary traits. Finally, the developed ILs provide useful materials for future genetic/genomic dissection and molecular breeding of complex traits.