Genome-wide Association Study (GWAS) of mesocotyl elongation based on re-sequencing approach in rice.

BACKGROUND: Mechanized dry seeded rice can save both labour and water resources. Rice seedling establishment is sensitive to sowing depth while mesocotyl elongation facilitates the emergence of deeply sown seeds. RESULTS: A set of 270 rice accessions, including 170 from the mini-core collection of Chinese rice germplasm (C Collection) and 100 varieties used in a breeding program for drought resistance (D Collection), was screened for mesocotyl lengths of seedlings grown in water (MLw) in darkness and in 5 cm sand culture (MLs). Twenty six accessions (10.53 %) have MLw longer than 1.0 cm. Eleven accessions had the highest mesocotyl lengths, i.e. 1.4 - 5.05 cm of MLw and 3.0 - 6.4 cm in 10 cm sand culture, including 7 upland landraces or varieties. The genotypic data of 1,019,883 SNPs were developed by re-sequencing of those accessions. A whole-genome SNP array (Rice SNP50) was used to genotype 24 accessions as a validation panel, giving 98.41 % of consistent SNPs with the re-sequencing data in average. GWAS based on compressed mixed linear model was conducted using GAPIT. Based on a threshold of -log(P) ≥8.0, 13 loci were associated to MLw on rice chromosome 1, 3, 4, 5, 6 and 9, respectively. Three associated loci, on chromosome 3, 6, and 10, were detected for MLs. A set of 99 associated SNPs for MLw, based on a compromised threshold (-log(P) ≥7.0), located in intergenic regions or different positions of 36 annotated genes, including one cullin and one growth regulating factor gene. CONCLUSIONS: Higher proportion and extension of elongated mesocotyls were observed in the mini-core collection of rice germplasm and upland rice landraces or varieties, possibly causing the correlation between mesocotyl elongation and drought resistance. GWAS found 13 loci for mesocotyl length measured in dark germination that confirmed the previously reported co-location of two QTLs across populations and experiments. Associated SNPs hit 36 annotated genes including function-matching candidates like cullin and GRF. The germplasm with elongated mesocotyl, especially upland landraces or varieties, and the associated SNPs could be useful in further studies and breeding of mechanized dry seeded rice.

Genome-wide association study identifies a gene conferring high physiological phosphorus use efficiency in rice.

Phosphate (Pi) is indispensable for the growth and development of plant, and low-Pi stress is a major limitation for crop growth and yield worldwide. The tolerance to low-Pi stress varied among rice germplasm resources. However, the mechanisms underlying the tolerance of rice to low-Pi stress, as a complex quantitative trait, are not clear. We performed a genome-wide association study (GWAS) through a diverse worldwide collection of 191 rice accessions in the field under normal-Pi and low-Pi supply in two years. Twenty and three significant association loci were identified for biomass and grain yield per plant under low-Pi supply respectively. The expression level of OsAAD as a candidate gene from a associated locus was significantly up-regulated after low-Pi stress treatment for five days and tended to return to normal levels after Pi re-supply in shoots. Suppression of OsAAD expression could improve the physiological phosphorus use efficiency (PPUE) and grain yields through affecting the expression of several genes associated with GA biosynthesis and metabolism. OsAAD would be a promising gene for increasing PPUE and grain yield in rice under normal- and low-Pi supply via genome editing.

Identification of Genetic Loci for Rice Seedling Mesocotyl Elongation in Both Natural and Artificial Segregating Populations.

Mesocotyl elongation of rice seedlings is a key trait for deep sowing tolerance and well seedling establishment in dry direct sowing rice (DDSR) production. Subsets of the Rice Diversity Panel 1 (RDP1, 294 accessions) and Hanyou 73 (HY73) recombinant inbred line (RIL) population (312 lines) were screened for mesocotyl length (ML) via dark germination. Six RDP1 accessions (Phudugey, Kasalath, CA902B21, Surjamkuhi, Djimoron, and Goria) had an ML longer than 10 cm, with the other 19 accessions being over 4 cm. A GWAS in RDP1 detected 118 associated SNPs on all 12 chromosomes using a threshold of FDR-adjusted p < 0.05, including 11 SNPs on chromosomes 1, 4, 5, 7, 10, and 12 declared by -log10(P) > 5.868 as the Bonferroni-corrected threshold. Using phenotypic data of three successive trials and a high-density bin map from resequencing genotypic data, four to six QTLs were detected on chromosomes 1, 2, 5, 6, and 10, including three loci repeatedly mapped for ML from two or three replicated trials. Candidate genes were predicted from the chromosomal regions covered by the associated LD blocks and the confidence intervals (CIs) of QTLs and partially validated by the dynamic RNA-seq data in the mesocotyl along different periods of light exposure. Potential strategies of donor parent selection for seedling establishment in DDSR breeding were discussed.

Dynamic transcriptome and phytohormone profiling along the time of light exposure in the mesocotyl of rice seedling.

Mesocotyl elongation is an important trait influencing seedling emergence and establishment in rice direct-seeding cultivation and is immediately inhibited after light exposure. Detailed researches on the molecular basis and biological processes underlying light repression of mesocotyl growth could probably provide useful information for key factors controlling this trait. Here we monitored the transcriptome and endogenous phytohormone changes specifically in the elongating mesocotyl in response to light exposure with a time-course. It was revealed that 974 transcripts were significantly differentially expressed (FDR < 0.05, |log2 (L/D) | ≥2) after light exposure. Most of the differential expression genes associated with the responses to hormone. Metabolic pathway analysis using the KEGG system suggested plant hormone signal transduction, α-linolenic acid metabolism and diterpenoid biosynthesis were critical processes of mesocotyl growth inhibited by light. Consistent with DEGs, the endogenous IAA, tZ and GA3 content was significantly reduced while JA level was dramatically increased, which indicated that light inhibited rice mesocotyl growth through decreasing IAA, tZ and GA3 content and/or increasing JA level. The present results enriched our knowledge about the genes and phytohormones regulating mesocotyl elongation in rice, which may help improve future studies on associated genes and develop new varieties tolerance to deep sowing.