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Zinc (Zn) malnutrition is a major public health issue. Genetic biofortification of Zn in rice grain can alleviate global Zn malnutrition. Therefore, elucidating the genetic mechanisms regulating Zn deprivation response in rice is essential to identify elite genes useful for breeding high grain Zn rice varieties. Here, a meta-analysis of previous RNA-Seq studies involving Zn deficient conditions was conducted using the weighted gene co-expression network analysis (WGCNA) and other in silico prediction tools to identify modules (denoting cluster of genes with related expression pattern) of co-expressed genes, modular genes which are conserved differentially expressed genes (DEGs) across independent RNA-Seq studies, and the molecular pathways of the conserved modular DEGs. WGCNA identified 16 modules of co-expressed genes. Twenty-eight and five modular DEGs were conserved in leaf and crown, and root tissues across two independent RNA-Seq studies. Functional enrichment analysis showed that 24 of the 28 conserved modular DEGs from leaf and crown tissues significantly up-regulated 2 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways and 15 Gene Ontology (GO) terms, including the substrate- specific transmembrane transporter and the small molecule metabolic process. Further, the well-studied transcription factors (OsWOX11 and OsbHLH120), protein kinase (OsCDPK20 and OsMPK17), and miRNAs (OSA-MIR397A and OSA-MIR397B) were predicted to target some of the identified conserved modular DEGs. Out of the 24 conserved and up-regulated modular DEGs, 19 were yet to be experimentally validated as Zn deficiency responsive genes. Findings from this study provide a comprehensive insight on the molecular mechanisms of Zn deficiency response and may facilitate gene and pathway prioritization for improving Zn use efficiency and Zn biofortification in rice.

Cadmium (Cd) is a non-essential toxic metal that is harmful to plants. To investigate the genetic mechanism of Cd tolerance in rice, quantitative trait loci (QTLs) associated with Cd tolerance at the seedling stage were analyzed using a recombinant inbred line (RIL) population derived from a cross between PA64s and 93-11. A total of 36 QTLs associated with shoot length, root length, shoot dry weight, root dry weight and total dry weight were detected in Hangzhou and Lingshui of China. Among them, 15 QTLs were identified under the control condition and 15 QTLs were identified under the Cd stress condition, and 6 QTLs for Cd tolerant coefficient were detected on chromosomes 1, 3, 7 and 9. The qCDSL1.1 and qCDSL1.2 were identified in Hangzhou and Lingshui, respectively, and had overlapping intervals on chromosome 1. To further confirm the effects of qCDSL1.1 and qCDSL1.2, we developed a chromosome segment substitution line (CSSL), CSSLqCDSL1, in 93-11 background harboring qCDSL1.1/qCDSL1.2 from PA64s. Compared to 93-11, CSSLqCDSL1 had increased shoot length under the Cd stress condition. These results pave the way for further isolation of those genes controlling Cd tolerance in rice and marker-assistant selection of rice elite varieties with Cd tolerance.

Aluminum (Al) toxicity in acid soils is a significant limitation to crop production worldwide, as 13% of the world's rice is produced in acid soil with high Al content. Rice is likely the most Al-resistant cereal and also the cereal, where Al resistance is the most genetically complex with external detoxification and internal tolerance. Many Al-resistance genes in rice have been cloned, including Al resistance transcription factor 1 (ART1) and other transcription factors, organic acid transporter genes, and metal ion transporter gene. This review summarized the recent characterized genes affecting Al tolerance in rice and the interrelationships between Al and other plant nutrients.

P1B-type heavy metal ATPases (HMAs) are transmembrane metal-transporting proteins that play a key role in metal homeostasis. We here reported the characterization of rice OsHMA6, a member of the P1B-type ATPase family. Phylogenetic tree analysis showed that OsHMA6 belonged to the Cu/Ag subgroup of the HMA family and had a close evolutionary relationship with OsHMA9. Amino acid sequence alignment showed 82.78% consistency between OsHMA6 and OsHMA9. OsHMA6 expressed in all organs at different growth stages, including spikelet, and abundant in leaf blades, however, OsHMA9 most strongly expressed in roots, but very low in spikelet. Excessive Cu2+ can up-regulate the expression of OsHMA6 and OsHMA9 in rice seedlings. The heterologous expression in yeast showed that OsHMA6 can significantly rescue the growth of yeast strain CM52 when supplied with 3 or 6 mmol/L Cu2+. Compared with the empty vector pYES2, the Cu concentration in OsHMA6-pYES2 decreased by 23.4% and 30.3% under 3 or 6 mmol/L Cu2+, respectively. Subcellular localization revealed that OsHMA6 was located in the plasma membrane. These results suggested that OsHMA6, similar to OsHMA9, is likely a copper efflux protein located in the plasma membrane. Keywords: OsHMA6, P1B-type ATPase, copper, cadmium, efflux, rice, subcellular localization

Summary: Upland rice is a distinctive drought‐aerobic ecotype of cultivated rice highly resistant to drought stress. However, the genetic and genomic basis for the drought‐aerobic adaptation of upland rice remains largely unclear due to the lack of genomic resources. In this study, we identified 25 typical upland rice accessions and assembled a high‐quality genome of one of the typical upland rice varieties, IRAT109, comprising 384 Mb with a contig N50 of 19.6 Mb. Phylogenetic analysis revealed upland and lowland rice have distinct ecotype differentiation within the japonica subgroup. Comparative genomic analyses revealed that adaptive differentiation of lowland and upland rice is likely attributable to the natural variation of many genes in promoter regions, formation of specific genes in upland rice, and expansion of gene families. We revealed differentiated gene expression patterns in the leaves and roots of the two ecotypes and found that lignin synthesis mediated by the phenylpropane pathway plays an important role in the adaptive differentiation of upland and lowland rice. We identified 28 selective sweeps that occurred during domestication and validated that the qRT9 gene in selective regions can positively regulate drought resistance in rice. Eighty key genes closely associated with drought resistance were appraised for their appreciable potential in drought resistance breeding. Our study enhances the understanding of the adaptation of upland rice and provides a genome navigation map of drought resistance breeding, which will facilitate the breeding of drought‐resistant rice and the "blue revolution" in agriculture. [ABSTRACT FROM AUTHOR]

SUMMARY: Mesocotyl length (ML) is a crucial factor in determining the establishment and yield of rice planted through dry direct seeding, a practice that is increasingly popular in rice production worldwide. ML is determined by the endogenous and external environments, and inherits as a complex trait. To date, only a few genes have been cloned, and the mechanisms underlying mesocotyl elongation remain largely unknown. Here, through a genome‐wide association study using sequenced germplasm, we reveal that natural allelic variations in a mitochondrial transcription termination factor, OsML1, predominantly determined the natural variation of ML in rice. Natural variants in the coding regions of OsML1 resulted in five major haplotypes with a clear differentiation between subspecies and subpopulations in cultivated rice. The much‐reduced genetic diversity of cultivated rice compared to the common wild rice suggested that OsML1 underwent selection during domestication. Transgenic experiments and molecular analysis demonstrated that OsML1 contributes to ML by influencing cell elongation primarily determined by H2O2 homeostasis. Overexpression of OsML1 promoted mesocotyl elongation and thus improved the emergence rate under deep direct seeding. Taken together, our results suggested that OsML1 is a key positive regulator of ML, and is useful in developing varieties for deep direct seeding by conventional and transgenic approaches. Significance Statement: Mesocotyl elongation (ML) is important for seedling emergence under dry direct seeding in rice. Here, we report the identification and functional validation of a gene, OsML1 (LOC_Os07g24090.1), which contributes greatly to the natural variation of ML, has been under selection during domestication and breeding, and functions mainly by affecting cell elongation via H2O2 homeostasis and the expression of expansin genes. [ABSTRACT FROM AUTHOR]

The multi-environment genomic selection enables plant breeders to select varieties resilient to diverse environments or particularly adapted to specific environments, which holds a great potential to be used in rice breeding. To realize the multi-environment genomic selection, a robust training set with multi-environment phenotypic data is of necessity. Considering the huge potential of genomic prediction enhanced sparse phenotyping on the cost saving of multi-environment trials (MET), the establishment of a multi-environment training set could also benefit from it. Optimizing the genomic prediction methods is also crucial to enhance the multi-environment genomic selection. Using haplotype-based genomic prediction models is able to capture local epistatic effects which could be conserved and accumulated across generations much like additive effects thereby benefitting breeding. However, previous studies often used fixed length haplotypes composed by a few adjacent molecular markers disregarding the linkage disequilibrium (LD) which is of essential role in determining the haplotype length. In our study, based on three rice populations with different sizes and compositions, we investigated the usefulness and effectiveness of multi-environment training sets with varying phenotyping intensities and different haplotype-based genomic prediction models based on LD-derived haplotype blocks for two agronomic traits, i.e., days to heading (DTH) and plant height (PH). Results showed that phenotyping merely 30% records in multi-environment training set is able to provide a comparable prediction accuracy to high phenotyping intensities; the local epistatic effects are much likely existent in DTH; dividing the LD-derived haplotype blocks into small segments with two or three single nucleotide polymorphisms (SNPs) helps to maintain the predictive ability of haplotype-based models in large populations; modelling the covariances between environments improves genomic prediction accuracy. Our study provides means to improve the efficiency of multi-environment genomic selection in rice. [ABSTRACT FROM AUTHOR]

Composite interval mapping (CIM) is the most commonly used method for mapping quantitative trait loci (QTL) with populations derived from biparental crosses. However, the algorithm implemented in the popular QTL Cartographer software may not completely ensure all its advantageous properties. In addition, different background marker selection methods may give very different mapping results, and the nature of the preferred method is not clear. A modified algorithm called inclusive composite interval mapping (ICIM) is proposed in this article. In ICIM, marker selection is conducted only once through stepwise regression by considering all marker information simultaneously, and the phenotypic values are then adjusted by all markers retained in the regression equation except the two markers flanking the current mapping interval. The adjusted phenotypic values are finally used in interval mapping (IM). The modified algorithm has a simpler form than that used in CIM, but a faster convergence speed. ICIM retains all advantages of CIM over IM and avoids the possible increase of sampling variance and the complicated background marker selection process in CIM. Extensive simulations using two genomes and various genetic models indicated that ICIM has increased detection power, a reduced false detection rate, and less biased estimates of QTL effects.