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A team headed by Professor Han Yingyan and Professor Hao Jinghong from the College of Plant Science and Technology of BUA, recently published a research paper online in the internationally renowned Plant Biotechnology Journal, entitled LsMAPK6 Phosphorylates the LsCO Protein to Enhance Its Stability and Transcriptional Activity, Promoting Floral Transition Upon High Temperatures in Lettuce. The research reveals the core signaling module and molecular mechanism underlying high temperature-induced bolting of lettuce. It not only deepens the understanding of how plants integrate ambient temperature signals with developmental timing, but provides a key theoretical basis and potential target for the molecular design and breeding of lettuce varieties with enhanced heat tolerance and delayed bolting.

Lactuca sativa L., commonly known as lettuce, is an important vegetable widely cultivated worldwide, whose economic value lies entirely in its rosette leaves formed during the vegetative stage. However, lettuce prefers cool conditions. High-temperature in summer will strongly induce its transition from the vegetative stage to the reproductive stage, namely the initiation of bolting and flowering. This will lead to ceased leaf growth, reduced quality and thus substantial economic losses, making year-round production impossible and severely restricting industrial development, which is a major challenge requiring urgent solutions. Therefore, decoding the molecular mechanism of high temperature-induced bolting is a pressing need for breeding heat-tolerant and late-bolting lettuce varieties.

Figure 1 Molecular model of the LsMAPK6-LsCO-LsFT module regulating high temperature-induced bolting in lettuce

Focusing on the key traits of urban fresh horticultural products, BUA's team for vegetable germplasm innovation and efficient utilization has long been dedicated to the research on heat tolerance and bolting resistance in lettuce, achieving certain headway in elite variety breeding, key gene mining and regulation. The team recently systematically decoded the role of the LsMAPK6-LsCO-LsFT signaling module in high temperature-induced bolting, uncovering a new model in which MAPK signaling directly regulates core photoperiodic factors. Furthermore, the research team proposed a working model of the LsMAPK6-LsCO-LsFT signaling pathway. When lettuce is exposed to high-temperature, the MAPK signaling cascade is triggered, and upstream signals activate the kinase activity of LsMAPK6. Activated LsMAPK6 directly binds with LsCO and specifically phosphorylates its Ser-258 site. This key post-translational modification, on one hand, stabilizes the LsCO protein, leading to its substantial accumulation in the nucleus, and on the other hand, greatly enhances the transcriptional activity of LsCO. Abundant highly active phosphorylated LsCO proteins then strongly bind with and activate the promoter of the downstream core florigen gene LsFT, driving its high-level expression. Ultimately, the high concentration of LsFT protein, as a decisive flowering signal, promotes the transition of the shoot apical meristem from vegetative to reproductive growth, and thus completion of the bolting and flowering process induced by high temperature. Explanation of this LsMAPK6-LsCO-LsFT signaling module provides a new molecular perspective for understanding how plants integrate ambient temperature signals with intrinsic developmental programs.

Master's graduates Wang Tingzhen (now assistant researcher at Zhejiang Academy of Agricultural Sciences), Liu Mingjia and Tian Yufeng, and master degree candidate Tan Qingqing from the College of Plant Science and Technology are co-first authors of the paper. Professor Hao Jinghong and Professor Han Yingyan are co-corresponding authors. Master degree candidates Wang Jiaxuan and Mu Huifang, Associate Professor Liu Chaojie and Wang Huiyu from the College of Plant Science and Technology, and Researcher Liu Ning from Beijing Academy of Agriculture and Forestry Sciences made important contributions to this research. Professor Chen Jiongjiong from Huazhong Agricultural University, Associate Professor Shi Yue from Shenyang Agricultural University, Associate Researcher Jia Meiru from the Chinese Academy of Sciences, and Associate Professor Liu Ziyan from Beijing University of Agriculture provided valuable guidance for this project. The research is based on the "National Huang Danian-Style University Teachers' Team" of Modern Horticulture at BUA, and financed by the National Natural Science Foundation of China, the Joint Key Research and Development Program of the Beijing Natural Science Foundation and the Beijing Municipal Education Commission, and the Spark Program of BUA.

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