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ARS Home » Southeast Area » Fort Pierce, Florida » U.S. Horticultural Research Laboratory » Subtropical Plant Pathology Research » Research » Publications at this Location » Publication #378666
Research Project: Mitigating High Consequence Domestic, Exotic, and Emerging Diseases of Fruits, Vegetables, and Ornamentals

Location: Subtropical Plant Pathology Research

Title: Selection and evaluation of a thornless and HLB-tolerant bud-sport of Pummelo citrus with an emphasis on molecular mechanisms

Author
item WU, BO - Clemson University
item LI, NA - Hunan Agricultural University
item DENG, ZHANAO - University Of Florida
item LUO, FENG - Clemson University
item Duan, Ping

Submitted to: Horticulture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2021
Publication Date: 8/31/2021
Citation: Wu, B., Li, N., Deng, Z., Luo, F., Duan, Y. 2021. Selection and evaluation of a thornless and HLB-tolerant bud-sport of Pummelo citrus with an emphasis on molecular mechanisms. Horticulture Research. 12:739108. https://doi.org/10.3389/fpls.2021.739108.
DOI: https://doi.org/10.3389/fpls.2021.739108

Interpretive Summary: A bud sport is a phenotypically distinct part of a plant. Many bud sports have been selected as new cultivars in some important horticultural plants, such as Citrus. Bud sports are not only an important source of new cultivars but also add to the phenotypic diversity of the plants, which may be vital to their survival in adverse circumstances, such as a disease. In this study, we discovered and evaluated a thornless pummelo bud-sport (TL) that grew more vigorously, and was more tolerant to Huanglongbing (HLB) than the original thorny seedling (wild-type, W). We also found the similar thornless mutants in sour orange seedling and Duncan grapefruit seedling. By carrying out whole genome sequencing of W, and transcriptome comparisons of W, TL, and partially recovered thorny ‘mutants’ (T), we were able to reveal great variations of gene expression, allelic expression, and alternative splicing among the "theoretically similar" genotypes of W, TL and partially recovered thorny ‘mutants’ (T) . The most remarkable finding is that two polar auxin transporter genes, PIN7 and LAX3, were expressed at a significantly lower level in TL than in both W and T. Therefore, we hypothesize that the alternation of polar auxin transport in TL may be responsible for the vigorous growth and thornless phenotype. In addition, we identified 73 and 46 disease resistance related genes that were significantly upregulated and downregulated, respectively, in TL and T compared with W. We conclude that the phenotype changes of the thornless bud-sport were associated with tremendous transcriptome alterations. These findings provide new germplasm and targets for conventional breeding and gene editing for citrus improvement.

Technical Abstract: Selection of elite bud-sports is an important breeding approach in horticulture. In this study, we discovered and evaluated a thornless pummelo bud-sport (TL) that grew more vigorously, and was more tolerant to Huanglongbing (HLB) than the original thorny seedling (wild-type, W). To reveal the underlying molecular mechanisms, we carried out whole genome sequencing of W, and transcriptome comparisons of W, TL, and partially recovered thorny ‘mutants’ (T). The results showed W, TL, and T varied greatly on gene expression, allelic expression, and alternative splicing. Although a few genes/pathways in T recovered to the W expression patterns, a majority of differentially expressed genes (DEG) between W and TL remained unchanged. Pathway and gene set enrichment analysis revealed that the expression of genes in multiple pathways, including photosynthesis, cell wall biosynthesis and, modification, and photosynthesis, were altered among the three genotypes. Importantly, two polar auxin transporter genes, PIN7 and LAX3, were expressed at a significantly lower level in TL than in both W and T, implying alternation of polar auxin transport in TL may be responsible for the vigorous growth and thornless phenotype. In addition, 73 and 46 disease resistance related genes were significantly upregulated and downregulated, respectively, in TL and T compared with W. These genes may be involved in enhanced SA-dependent defense and suppression of defense-related callose deposition and programmed cell death. Overall, these results indicated that the phenotype changes of the thornless bud-sport were associated with tremendous transcriptome alterations, providing new clues and targets for breeding and gene editing for citrus improvement.