How Two Gene Interactions Affect Tomato Color and Sweetness

Phil Stevens
24th January, 2024

How Two Gene Interactions Affect Tomato Color and Sweetness

Tomato (Solanum lycopersicum)

Photo adapted from: Liam Steele / CC BY (Source)
Fruit development relies heavily on photosynthesis – the process plants use to convert light into energy – and how that energy is stored as carbohydrates. However, the precise mechanisms controlling this link between chlorophyll (the green pigment essential for photosynthesis) and carbohydrate production haven’t been fully understood. Researchers at Chongqing University have now identified two key proteins, SlGRAS9 and SlZHD17, that act as regulators of both chlorophyll and carbohydrate accumulation in tomato fruit[1]. The study began with the observation that disrupting the function of either SlGRAS9 or SlZHD17 led to a surprising outcome: tomatoes with significantly more chlorophyll. This increase wasn’t just a simple boost in pigment; it also triggered changes in the structure and development of chloroplasts – the compartments within plant cells where photosynthesis takes place. Crucially, these changes were accompanied by higher levels of both starch and soluble sugars in the fruit. To understand how these proteins exerted their influence, the researchers employed a combination of techniques. They analyzed the complete set of gene activity (the ‘transcriptome’) in tomatoes with altered SlGRAS9 and SlZHD17 levels. They also conducted experiments to determine where these proteins bind to DNA – specifically, to the regulatory regions of genes (called promoters) that control gene expression. The results revealed a complex regulatory network. SlGRAS9 and SlZHD17 don’t work in isolation; they interact in multiple ways to fine-tune gene expression. Sometimes, they work sequentially, with one protein influencing the activity of the other. In other cases, both proteins bind to the same gene promoter, amplifying the regulatory effect. Alternatively, they act independently on different target genes. For example, the study found that SlZHD17 directly regulates the expression of SlAGPaseS1 and SlSUS1, genes involved in starch and sucrose metabolism respectively. However, SlGRAS9 controls the expression of SlVI and SlGLK1 – genes crucial for chloroplast development – without needing SlZHD17. Both proteins, however, can directly influence the expression of SlPOR-B, a gene involved in chlorophyll biosynthesis. This intricate interplay highlights the sophisticated control mechanisms governing fruit development. The findings also connect to previous research on starch biosynthesis in maize[2]. That study identified ZmTCP7 as a regulator of ZmBt2, a gene controlling a key step in starch production. While the specific proteins and pathways differ between maize and tomato, both studies demonstrate the importance of transcription factors – proteins that control gene expression – in regulating carbohydrate accumulation. The maize study showed that ZmTCP7 repressed starch production, whereas the current tomato study shows SlGRAS9 and SlZHD17 generally promote it, demonstrating that the same metabolic pathways can be regulated in different ways. Furthermore, the tomato research builds upon the understanding of metabolic changes during fruit ripening[3]. That study highlighted the coordinated changes in metabolite levels and the importance of post-translational regulation (modifications to proteins after they are made). The current work adds another layer to this understanding by identifying key transcriptional regulators that initiate these metabolic shifts. The importance of sugars as signaling molecules, as described in another study[4], is also underscored by the findings, as SlGRAS9 and SlZHD17 directly impact sugar accumulation. Interestingly, the study also touches upon fruit texture. Previous work on tomato fruit texture identified SlMBP3 as a key regulator of locular gel – the substance inside tomato fruit that contributes to its firmness[5]. While the current study doesn’t directly address locular gel, it demonstrates how early developmental stages, controlled by regulators like SlGRAS9 and SlZHD17, influence fundamental metabolic processes that ultimately affect fruit quality traits like texture. The identification of SlGRAS9 and SlZHD17 provides potential targets for breeders aiming to improve fruit quality. By manipulating the activity of these genes, it may be possible to enhance sugar content, improve fruit firmness, and optimize other desirable traits in tomatoes and potentially other fleshy fruits.

FruitsBiochemPlant Science

References

Main Study

1) The SlGRAS9-SlZHD17 transcriptional cascade regulates chlorophyll and carbohydrate metabolism contributing to fruit quality traits in tomato.

Published 23rd January, 2024

https://doi.org/10.1111/nph.19530

Related Studies

2) The novel ZmTCP7 transcription factor targets AGPase-encoding gene ZmBt2 to regulate storage starch accumulation in maize.

https://doi.org/10.3389/fpls.2022.943050

3) Integrated analysis of metabolite and transcript levels reveals the metabolic shifts that underlie tomato fruit development and highlight regulatory aspects of metabolic network behavior.

Journal: Plant physiology, Issue: Vol 142, Issue 4, Dec 2006

4) Molecular basis for optimizing sugar metabolism and transport during fruit development.

https://doi.org/10.1007/s42994-021-00061-2

5) Interaction of two MADS-box genes leads to growth phenotype divergence of all-flesh type of tomatoes.

https://doi.org/10.1038/s41467-021-27117-7


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