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Scholarly Works (3 results)

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  • Peer Reviewed
UC Davis Electronic Theses and Dissertations (2024)

Tomato is an important commodity worldwide, consumed both as fresh fruit or processed products (such as juice, ketchup, and others), separating the industry into fresh and processing. For fresh-market tomatoes, fruit shelf-life and flavor (influenced by the rate of fruit softening), are important quality attributes. For processing tomatoes, major quality attributes include fruit color, total soluble solids, acidity, and juice consistency. The overarching goal of this Ph.D. dissertation was to investigate the role of pectin modifying enzymes in tomato quality for both fresh and processed markets, as the composition and structure of this polysaccharide are known to impact fruit texture and rheological properties. I followed a multidisciplinary approach that integrated the study of physiological, genetic, and biochemical factors during ripening.In Chapter 1, I prepared a literature review that provides a synthesis and analysis of the current knowledge surrounding the genetics of key quality traits in processing tomatoes, which influence both product quality and production efficiency. While processing and fresh-market tomatoes share the same genome and many biochemical pathways, they have developed distinct phenotypes tailored to their specific market needs. This review highlights these differences and examines how these traits have been shaped through breeding and selection. Additionally, I identify critical areas where further research is needed to advance the field. In Chapter 2, I investigated the combined effect on fruit quality and shelf-life of two important pectin-degrading enzymes: SlPG2a (Solyc10g080210 ) and SlPL(Solyc03g111690 ) by knocking them out simultaneously. Based on previous evidence, I hypothesized that these two genes work cooperatively. Therefore, their simultaneous knockout would reduce pectin breakdown, translating into significantly firmer fruit with a longer shelf-life. I first generated the double knockout CRISPR PGPL tomato line in the fresh-market AC variety. I evaluated the gene expression of both genes, and I then assessed different quality attributes such as firmness, color, sugar, volatiles, pH, and TA, as well as the fruit’s postharvest performance. I demonstrated that knocking out SlPG2a and SlPL together improves tomato shelf-life and firmness while maintaining other quality attributes. This work advances our current understanding of plant science as it presents an alternative way to extend tomato fruit shelf-life and reduce fruit waste without hindering consumer-based fruit quality attributes as traditional breeding does. Lastly, in Chapter 3, I aimed to elucidate the major genes or biological processes that influence tomato juice consistency. I studied 11 commercial processing varieties grown under different conditions across California. I first characterized fruit physiological attributes and juice consistency of all lines. I also evaluated the sugar content of water-soluble pectins to gather further information about pectin composition in the fruit and juice. I then performed a whole-genome transcriptome analysis on the 11 lines at the turning and red-ripe ripening stages. I ran a WGCNA using the transcriptomic data to correlate gene expression patterns with the juice consistency data. Ultimately, I determined the expression patterns of the major genes involved in physiological processes during ripening, and I identified potential candidate genes that could be driving consistency, including some related to pectin depolymerization. The results of this dissertation offer valuable information to guide breeding strategies aimed at improving tomato texture, extending fruit shelf-life, reducing decay, and enhancing the processing quality of tomato products. These findings could ultimately contribute to increased crop efficiency and profitability, particularly in California, the major producer of processing tomatoes in the world.

    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2021)
    The increased susceptibility of ripe fruit to fungal pathogens poses a substantial threat to crop production and marketability. Here, we coupled transcriptomic analyses with mutant studies to uncover critical processes associated with defense and susceptibility in tomato (Solanum lycopersicum) fruit. Using unripe and ripe fruit inoculated with three fungal pathogens, we identified common pathogen responses reliant on chitinases, WRKY transcription factors, and reactive oxygen species detoxification. We established that the magnitude and diversity of defense responses do not significantly impact the interaction outcome, as susceptible ripe fruit mounted a strong immune response to pathogen infection. Then, to distinguish features of ripening that may be responsible for susceptibility, we utilized non-ripening tomato mutants that displayed different susceptibility patterns to fungal infection. Based on transcriptional and hormone profiling, susceptible tomato genotypes had losses in the maintenance of cellular redox homeostasis, while jasmonic acid accumulation and signaling coincided with defense activation in resistant fruit. We identified and validated a susceptibility factor, pectate lyase (PL). CRISPR-based knockouts of PL, but not polygalacturonase (PG2a), reduced susceptibility of ripe fruit by >50%. This study suggests that targeting specific genes that promote susceptibility is a viable strategy to improve the resistance of tomato fruit against fungal disease.
      Cover page: Host susceptibility factors render ripe tomato fruit vulnerable to fungal disease despite active immune responsesCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2023)
      Postharvest fungal pathogens benefit from the increased host susceptibility that occurs during fruit ripening. In unripe fruit, pathogens often remain quiescent and unable to cause disease until ripening begins, emerging at this point into destructive necrotrophic lifestyles that quickly result in fruit decay. Here, we demonstrate that one such pathogen, Botrytis cinerea, actively induces ripening processes to facilitate infections and promote disease in tomato (Solanum lycopersicum). Assessments of ripening progression revealed that B. cinerea accelerated external coloration, ethylene production, and softening in unripe fruit, while mRNA sequencing of inoculated unripe fruit confirmed the corresponding upregulation of host genes involved in ripening processes, such as ethylene biosynthesis and cell wall degradation. Furthermore, an enzyme-linked immunosorbent assay (ELISA)-based glycomics technique used to assess fruit cell wall polysaccharides revealed remarkable similarities in the cell wall polysaccharide changes caused by both infections of unripe fruit and ripening of healthy fruit, particularly in the increased accessibility of pectic polysaccharides. Virulence and additional ripening assessment experiments with B. cinerea knockout mutants showed that induction of ripening depends on the ability to infect the host and break down pectin. The B. cinerea double knockout Δbc polygalacturonase1 Δbc polygalacturonase2 lacking two critical pectin degrading enzymes was incapable of emerging from quiescence even long after the fruit had ripened at its own pace, suggesting that the failure to accelerate ripening severely inhibits fungal survival on unripe fruit. These findings demonstrate that active induction of ripening in unripe tomato fruit is an important infection strategy for B. cinerea.
        Cover page: Botrytis cinerea infection accelerates ripening and cell wall disassembly to promote disease in tomato fruitCreative Commons 'BY' version 4.0 license
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