A key gene that controls chloroplast development in plants has been found to also regulate red pigment production in kiwifruit through a previously unknown dual-action mechanism. The study shows that the transcription factor AcGLK2 promotes anthocyanin accumulation by simultaneously activating a positive regulator and suppressing a competing inhibitor. This discovery resolves the long-standing question of how GLK family genes—traditionally associated with photosynthesis—contribute to fruit coloration and provides valuable genetic resources for breeding nutritionally enhanced kiwifruit varieties.
The GOLDEN2-LIKE (GLK) family of transcription factors has been extensively studied for its conserved function in promoting chloroplast development and photosynthesis across the plant kingdom. In recent years, however, emerging evidence has suggested that GLK genes may also influence anthocyanin production—the pigments responsible for red, purple, and blue coloration in plants. Studies in Arabidopsis have shown that GLK homologs can promote anthocyanin accumulation, but whether this function operates through similar or entirely different mechanisms in fruit crops remained unclear. The regulatory network governing red pigmentation in kiwifruit, particularly in commercially valuable red-fleshed varieties, has also been only partially understood. Due to these gaps, a systematic investigation into how GLK genes control anthocyanin biosynthesis in kiwifruit has been urgently needed.
Now, a research team from Anhui Agricultural University and Sichuan University has cracked this puzzle. Their findings were published (DOI: 10.1093/hr/uhag105) in Horticulture Research , a journal from Oxford University Press. The researchers identified AcGLK2 as the key driver behind the deep red coloration in kiwifruit flesh and demonstrated that it operates through a previously unknown dual regulatory mechanism—simultaneously turning on a positive regulator and shutting down a negative one to fine-tune pigment production.
The team began by comparing red-fleshed and green-fleshed kiwifruit cultivars and found that AcGLK2 expression was dramatically higher in tissues that accumulate anthocyanins. When they overexpressed AcGLK2 in Arabidopsis and kiwifruit, the transgenic plants turned visibly redder, with anthocyanin levels soaring. Conversely, silencing the gene via RNA interference stripped the fruit of its red pigment. To understand how AcGLK2 exerts this control, the researchers combined transcriptome profiling (RNA-Seq) with chromatin immunoprecipitation sequencing (ChIP-Seq) to map the gene's direct targets across the entire kiwifruit genome. This genome-wide approach revealed that AcGLK2 binds directly to the promoters of two key MYB transcription factors: AcMYB5 and AcTRY . AcMYB5 , an R2R3-MYB activator, promotes anthocyanin production by partnering with the bHLH protein AcbHLH42 to switch on downstream structural genes such as AcANS and AcF3GT1 . In contrast, AcTRY , an R3-MYB protein, acts as a molecular thief—it competes with positive regulators like AcMYBF110 for binding to AcbHLH42, forming inactive complexes that block pigment synthesis. Remarkably, AcGLK2 does both at once: it activates AcMYB5 expression while repressing AcTRY , effectively flipping a "go" switch on the accelerator and a "stop" switch on the brake.
"The most exciting finding is that AcGLK2 doesn't just flip a single switch—it orchestrates both sides of the regulatory equation," the authors said. "By simultaneously activating a positive MYB regulator and suppressing a competing inhibitor, the gene fine-tunes anthocyanin production with remarkable precision. This dual mechanism explains why red-fleshed kiwifruit varieties express AcGLK2 at such high levels, and it gives us a powerful genetic handle for engineering pigment accumulation in other crops as well."
The implications extend far beyond kiwifruit aesthetics. Anthocyanins are not only responsible for attractive fruit coloration but also serve as potent antioxidants with documented health benefits. The team also demonstrated that silencing SlGLK2 in purple tomato compromised fruit pigmentation, suggesting that this regulatory mechanism is conserved across species. For breeders, AcGLK2 and its downstream targets AcMYB5 and AcTRY represent valuable genetic resources for enhancing the nutritional quality of kiwifruit and other horticultural crops through precision molecular breeding—potentially delivering fruits with higher antioxidant content and improved consumer appeal.
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References
DOI
Original Source URL
https://doi.org/10.1093/hr/uhag105
Funding information
This work was supported by grants from the Anhui Natural Science Foundation (2025AHGXZK30511) and National Natural Science Foundation of China (U23A20204 and 31900257).
About Horticulture Research
Horticulture Research is an open access journal of Nanjing Agricultural University and ranked number one in the Horticulture category of the Journal Citation Reports ™ from Clarivate, 2023. The journal is committed to publishing original research articles, reviews, perspectives, comments, correspondence articles and letters to the editor related to all major horticultural plants and disciplines, including biotechnology, breeding, cellular and molecular biology, evolution, genetics, inter-species interactions, physiology, and the origination and domestication of crops.
Horticulture Research
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AcGLK2 involves anthocyanin biosynthesis via bidirectionally modulating expression of MYB transcription factors in Actinidia chinensis
1-Jul-2026
The authors declare that they have no competing interests.