A molecular brake and accelerator: how wild citrus fine-tunes its cold defense
image: A proposed working model for regulation of CiHSP26.5 by ERFs under cold stress in Ichang papeda. In the early stage of cold stress, CiERF023 serves as a transcription repressor, inhibiting the transcription of CiERF041 and CiHSP26.5. With prolonged low-temperature stress, the expression of CiERF023 decreases, relieving its repression on the two genes. In addition, CiERF041 acts as a transcription activator, triggering an active response to drive substantial expression of CiHSP26.5, thereby working to promote cold stress adaptation and tolerance.
Credit: Horticulture Research
When cold strikes, plants face a dilemma: mount a full-scale defense that consumes precious energy, or risk fatal damage. Now, scientists have uncovered how a wild citrus species masterfully navigates this trade-off. They identified a small heat shock protein, CiHSP26.5, that is crucial for cold tolerance. More importantly, they discovered it is controlled by an antagonistic duo of transcription factors—one that acts as a "brake" and another as a "gas pedal"—working in a time-delayed cascade to ensure a measured, rather than a frantic, response to dropping temperatures.
Low temperatures are a major threat to agriculture, particularly for tropical and subtropical fruit trees like citrus, causing billions in losses annually. In response to such stress, plants produce molecular chaperones like heat shock proteins (HSPs) to protect cellular proteins from damage. While the role of HSPs is well-known, the intricate transcriptional "software" that controls their expression during cold stress, especially in fruit crops, remains largely a black box. These HSPs, however, are not always protective on demand; their expression can be a costly, high-stakes maneuver. Based on these challenges, in-depth research is urgently needed to understand how plants precisely time and tune the expression of these critical defense factors, particularly to find ways to boost cold tolerance in elite but sensitive cultivars like lemons.
Researchers from the National Key Laboratory for Germplasm Innovation & Utilization of Horticultural Crops at Huazhong Agricultural University and Hubei Hongshan Laboratory, in collaboration with the Citrus Research Institute, published (DOI: 10.1093/hr/uhag131) their findings in the Horticulture Research. The study reveals that in Citrus ichangensis, a frost-hardy wild relative, the expression of the protective protein CiHSP26.5 is not just turned on or off, but is fine-tuned over time by two transcription factors, CiERF023 and CiERF041, which serve as its repressor and activator, respectively.
The study’s elegance lies in its temporal choreography. Under cold stress, the repressor CiERF023 is the first to respond, rapidly accumulating to bind directly to the CiHSP26.5 promoter and shut down its activity. Simultaneously, CiERF023 also suppresses the expression of its counterpart, CiERF041, acting as a "master brake" on the entire defense pathway. This early suppression appears to prevent a costly, premature stress response. However, as the cold persists for days, CiERF023 levels decline, releasing CiERF041 from its inhibitory grip. CiERF041 then steps in, binding to the same promoter to powerfully activate CiHSP26.5 expression. This delayed activation ensures that the plant’s protective machinery is deployed only when absolutely necessary, akin to a "molecular brake and accelerator." Functional tests confirmed this model: silencing the repressor (CiERF023) made plants hardier, while silencing the activator (CiERF041) made them more susceptible to freezing. Overexpressing CiERF023 in tobacco turned the heat-sensitive plant into a cold-sensitive one, further underscoring its role as a primary negative regulator.
"Cold tolerance is not just about having the right protective genes, but about knowing exactly when and how much to deploy them," the authors said. They added that the plant’s strategy of using a quick-responding 'brake' to initially stall a costly defense program, only to release it for a powerful, late-stage activation, reveals a sophisticated economic model for stress survival. "It’s this regulatory precision—finding the perfect timing—that we believe is key for developing elite crops that can withstand cold without sacrificing growth," they further explained.
This discovery offers a fresh, strategic blueprint for engineering cold tolerance. Instead of simply overexpressing a protective gene, breeders could use gene-editing techniques to fine-tune the balance between the repressor CiERF023 and the activator CiERF041. For example, selectively weakening the repressor's binding or strengthening the activator's activity could lower the threshold for cold defense, priming the plant for faster protection in a volatile climate. This approach has the potential to enhance the resilience of cold-sensitive commercial citrus varieties, such as lemons and pummelos, safeguarding yields against increasingly unpredictable frost events and providing a model for other vital fruit crops.
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References
DOI
Original Source URL
https://doi.org/10.1093/hr/uhag131
Funding information
This work was supported by National Key Research and Development Program of China (2022YFD1200503), the National Natural Science Foundation of China (32330095), and the Key Research and Development Program of Jiangsu Province (BE2023328).
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.