A sugar switch: The molecule guiding apple tree maturity
image: Proposed models illustrating how sorbitol mediates age-dependent changes in apple plant growth strategy through gibberellin signaling.
Credit: Horticulture Research
As apple trees mature, their growth slows—but the biological cues driving this shift have remained elusive. A new study uncovers the role of sorbitol, a sugar alcohol, in signaling this age-dependent transformation. Scientists discovered that MdSDH1, a gene responsible for breaking down sorbitol, is highly active in young plants and essential for maintaining rapid growth through gibberellin (GA₃) hormone regulation. As trees age, the MdSPL1–MdWRKY24 gene duo suppresses MdSDH1, causing sorbitol to accumulate. This sugar buildup dampens GA₃ responses via MdGASA1, leading the plant to adopt a more energy-conserving growth strategy. The findings present a sugar-driven mechanism that reshapes how apple trees grow through time.
Perennial fruit trees like apple experience a dramatic transformation from vigorous youth to measured maturity, marked by changes in leaf structure, photosynthesis, hormone levels, and growth speed. While sugars like sucrose and glucose are known to guide this vegetative phase change in other plants, sorbitol—the dominant photosynthetic product in apples—has remained understudied. At the same time, transcription factors like SPLs and WRKYs are well known for orchestrating developmental timing and hormone pathways. How these metabolic and genetic elements intertwine during growth transitions in apple has long puzzled researchers. Due to these gaps in understanding, an in-depth investigation into sorbitol’s regulatory role was urgently needed.
In a study (DOI: 10.1093/hr/uhae192) published July 11, 2024, in Horticulture Research, scientists from Northwest A&F University shed light on the signaling role of sorbitol in apple tree development. Using a combination of gene expression analysis, metabolic profiling, and genetic manipulation, they revealed how the MdSPL1–MdWRKY24 transcriptional module suppresses MdSDH1, triggering sorbitol accumulation and reducing growth-promoting gibberellin signaling. This discovery repositions sorbitol from a passive energy store to a key decision-maker in the life cycle of apple trees.
The research team studied juvenile (1y), transitional (3y), and adult (5y) stages of tissue-cultured apple plants to track physiological and molecular changes. They found that as trees aged, levels of sorbitol rose while expression of MdSDH1 declined. Silencing MdSDH1 stunted plant height and internode length—effects that were reversed with GA₃ treatment—suggesting sorbitol accumulation suppresses growth by limiting gibberellin signaling. Further investigation revealed that MdSPL1 and MdWRKY24, two age-activated transcription factors, bind directly to the MdSDH1 promoter and suppress its expression. Intriguingly, sorbitol itself enhances the activity of these repressors, creating a feedback loop. The loop also targets MdGASA1, a gibberellin-responsive gene linked to cell expansion. Silencing MdGASA1 resulted in smaller internodes and shorter stem cells, confirming its role in promoting elongation. The study concludes that a sorbitol-mediated regulatory circuit governs the plant’s transition from a rapid-growth mode to a restrained, adult-phase strategy.
“This work uncovers a remarkable story of how a sugar metabolite becomes a messenger,” said Dr. Fengwang Ma, senior author of the study. “By linking sorbitol dynamics to hormone signaling and transcriptional control, we reveal a system where apple trees sense their age and adjust growth accordingly. It's a fundamental insight with real-world relevance for fruit tree biology.”
These discoveries offer promising tools for horticulturists aiming to fine-tune apple growth and productivity. By targeting the MdSDH1–MdSPL1–MdWRKY24–MdGASA1 pathway, it may be possible to engineer trees with shortened juvenile phases or improved stress resilience. Chemical treatments that modulate sorbitol levels or gene expression could provide new ways to manage orchard growth patterns. Beyond apples, the concept of sugar-mediated feedback loops could be applied to other perennial crops, potentially transforming how we guide plant development across aging stages.
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References
DOI
Original Source URL
https://doi.org/10.1093/hr/uhae192
Funding information
This work was supported by the National Key Research and Development Program of China (2023YFD2301000), the Earmarked Fund for the China Agriculture Research System (CARS-27), the Key S&T Special Projects of Shaanxi Province (2020zdzx03-01-02), and the Chinese Universities Scientific Fund (2452023067).
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.