Progress of collection, conservation and innovative utilization of citrus resources in the National Citrus Germplasm Repository (Chongqing)

Citrus is a subtropical evergreen fruit tree, mainly distributed south of the Yangtze River in China. Several wild species, including Ichang papeda (Citrus ichangensis Swingle) and Mangshan wild mandarin (C. mangshanensis S. W. He), originated in China. Cross-hybridization occurs easily among citrus genera and species, resulting in a wealth of germplasm resources. With over four thousand years of cultivation history and diverse ecological and geographical environments, China is widely known as the world’s citrus origin center. Currently, citrus is the most important economic fruit tree in China in terms of both acreage and production. The National Citrus Germplasm Repository in Chongqing (NCGRCQ) is the world’s largest citrus field genebank, having conserved 2175 citrus germplasm acces-sions by the end of 2024. Among them, 1517 accessions were collected from 22 provinces (municipality, and autonomous regions) and 658 accessions were introduced from 33 countries. These germplasms include 910 landraces (41.80%), 849 breeding varieties (39.00%), 224 genetic materials (10.30%), 179 wild accessions (8.23%), and 13 closely related plants (0.60%). A total of 1740 accessions (80%) have been cataloged based on their basic passport information, morphological characteristics, and agronomic traits. Through field evaluation trials, 234 elite accessions with desirable traits such as high total soluble solids, low acid content, seedlessness, early or late maturity, and tolerance to canker or Alternaria brown spot disease, etc. were identified. These materials have also been used in cross-breeding program. An efficient breeding strategy was established, using germplasm accessions with excellent fruit quality, seedlessness and monoembryonic characteristics- such as Nankou, Beni madona, Nishinokaori, Kiyomi and Asumi as female parents, combined with seedling top- grafting technology to shorten the juvenility stage. As a result, several excellent new varieties such as Dayagan, Shangpin No. 2, Huamei No. 7, Q mandarin, Xiamiyou, and Gold Autumn Shatangju have been developed. NCGRCQ also conducted citrus bud sport mutation selection activities in cooperation with government and nursery enterprises in major citrus-producing areas such as Xinping county in Yunnan province and Danling in Sichuan province. Several elite bud sports, such as Xinbing No. 30 (from Bintang Orange) and Jinle Gan (from Shirarui tangor) were selected. In recent years, NCGRCQ has implemented programs for accurate identification of citrus germplasm, phenotyping and genotyping 903 accessions using GBS and whole-genome resequencing technology, morphological, and metabolomics methods. Using GWAS and selective sweep analysis, researchers identified important candidate genes associated with fruit quality traits (such as fruit fresh color, fruit weight, mono-embryonic, seed numbers, citric acid content, etc.) as well as genes related to tolerance to diseases such as Alternaria brown spot and canker. Fifteen major regulatory genes including CcCCD4a, PEG, CcISAUR49, PDH-E2, ERF119, etc. were identified. In addition, 23 tightly linked molecular markers have been developed, which improves citrus breeding efficiency. Meanwhile, NCGRCQ has shared a total of 15 700 citrus germplasm accessions and associated data with various applicants. These materials have been widely used in scientific research, citrus breeding, orchard construction, and nursery propagation, playing an important role in removing rural poverty and promoting the sustainable development of the citrus industry. This review summarizes the progress made by NCGRCQ in the conservation and evaluation of citrus germplasm over the past decades and proposes future research priorities.