Citrus tristeza virus resistance in a core collection of sour orange based on a diversity study of three germplasm collections using QTL-linked markers

Seven markers linked to QTL involved in CTV accumulation, leafminer resistance and apomictic reproduction were used to characterize 64 sour orange ( Citrus aurantium L.) accessions from three national collections in order to identify a representative core in which the resistance behaviour against two Citrus tristeza virus (CTV) isolates was studied. Different degrees of apomixis were found between the foreign collections. Most of the C. aurantium accessions fall into three main groups based on three multilocus genotypes. The haplotype diversity at three CTV accumulation QTL-linked markers was further studied by sequence analysis of alleles. Genotypic and allelic diversity at one of them, tightly linked to Ctv-R ₂ in Poncirus trifoliata (L.) Raf. , match the plant–CTV interaction types reported among Poncirus and Citrus species. Only those selected accessions from the major group presented CTV resistance during 30 months of continuous growth, but later the resistance broke down in some plants. CTV tolerance appears related to slow growing genotypes. Certain micronutrients: Mn, and B depending on the accession, might play a relevant role in this host–pathogen interaction particularly in alkaline soils.     

Identification and gene cloning of a male‐sterile mutant Oswbc11 in rice

Commercial application of nuclear genetic male sterility has been improved recently by a novel technique, Seed Production Technology (SPT), which incorporates transgenic maintainer lines capable of propagating non‐transgenic nuclear male‐sterile lines for use as female parents in hybrid production. Here, we identified a rice nuclear male‐sterile mutant, Oswbc11, with abnormal pollen development and lipid transport. We finely mapped the Oswbc11 gene into a 12.5‐kb region on chromosome 10 and found one candidate gene, which had a base substitution (C to T) resulting in a premature stop codon and was functionally confirmed by CRISPR/Cas9 technology. Gene OsWBC11 encodes a sub‐family member of adenosine triphosphate‐binding cassette transporter, which participates in the active transport of a wide range of molecules across membranes. Moreover, the agronomic traits of Oswbc11 mutants showed no significant differences compared to the wild‐type control except for the seed setting rate. These results indicated that Oswbc11 gene could be used in rice hybrid breeding as a recessive nuclear male‐sterile gene combined with CRISPR/Cas9 and the SPT technology and applied in different rice varieties.     

Identification of a novel QTL for the number of spikelets per panicle using a cross between indica‐ and japonica‐type high‐yielding rice cultivars in Japan

Yield is a complex trait. To improve it, the accumulation of the favourable alleles of valuable genes is required for each yield‐related trait. In this study, we used two high‐yielding rice cultivars developed in Japan, indica‐type ‘Takanari’ and japonica‐type ‘Momiroman’, for a genetic analysis of the sink capacity‐related traits. An F₂ population showed transgressive segregation for the number of spikelets per panicle. Quantitative trait locus (QTL) analysis detected four QTLs for the trait. Two of the QTLs were most likely identical to previously cloned GN1a and APO1, and their Takanari alleles had positive effects. The Momiroman alleles of the other two QTLs had positive effects, and one of these QTLs was most likely identical to SPIKE/GPS. The QTL on the long arm of chromosome 3 appeared to be novel; it clustered with QTLs for grain length and days‐to‐heading. Substitution mapping revealed that the close linkage of QTLs caused the clustering. These results suggest that the combination of the favourable alleles of detected QTLs could lead to greater sink capacity than that of the parental cultivars.     

Development of AS‐PCR marker based on a key mutation confirmed by resequencing of Wx‐mp in Milky Princess and its application in japonica soft rice (Oryza sativa L.) breeding

Soft rice with low amylose content (AC) ranging by 5–15% is a unique type with special eating and appearance quality and has become popular in the rice market. We resequenced the Wx‐mp, a key allele from Milky Princess, a Japanese low AC variety, and found that the +473 mutation in exon 4 is the key mutation in both Wx‐mp and its ancestor allele, Wx‐mq from Milky Queen. Based on this functional mutation, an allele‐specific PCR (AS‐PCR) marker was developed and proven in a breeding population derived from a cross between a Chinese late variety Nan Keng 46 (Wx‐mp/Wx‐mp) and an early line Ning 63121(Wx‐b/Wx‐b). Based on the marker‐aided selection by our newly developed AS‐PCR marker for Wx‐mp and the known ST10 marker for Stvb‐i, a total of 12 Wx‐mp homozygotes were selected from 198 F₂ progenies, and four of them were immune to rice stripe virus (RSV) with averagely 11.3 days earlier heading than Nan Keng 46 without significant change in grain yield.