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Zhang Kewei's team has made important progress 

Cytokinin (CK) is an important plant hormone that plays an important role in plant growth, development, and stress resistance. CK mainly exists in the form of IP and tZ (trans-zeatin) in plants. CK of tZ type is mainly synthesized in the root tissue and transported to the above-ground tissue through the catheter under the action of transpiration pulling force. The ABC transporter AtABCG14 is responsible for the assembly of root to-type CK from the central cylindric sheath to the catheter, but it is not clear how to-type CK is unloaded and distributed after being transported to the ground tissue through the catheter.

Recently, the team of Professor Zhang Kewei from the School of Biochemistry of Zhejiang Normal University published an online research paper entitled "Phloem unloading via the apoplastic pathway is essential for shoot distribution of root-synthesized cytokinins" in the top international botany journal Plant Physiology, revealing the synthesis of roots. The unloading and distribution mechanism of tZ type CK on the ground.

Zhang Kewei's team has been engaged in research on plant hormones and aging for a long time. Early studies found that the atabcg14 mutant is a scion, and when the wild type is a rootstock, root CK can be transported to the above-ground tissues, but the above-ground tissues cannot be completely restored to the wild-type phenotype (Figure 1), suggesting that AtABCG14 is involved in the unloading of root CK from the above-ground tissues And distribution.

 

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Figure 1. Grafting experiment of wild type and mutant atabcg14

To reveal the molecular and physiological mechanism of CK unloading and distribution on the ground, the team established a CK unloading research system after more than 5 years of hard work. First, semi-thin sections were used to prove the expression of AtABCG14 in the phloem companion cell and xylem tracheid of the veins of the above-ground tissue, and the phloem expression-specific promoter was used to drive AtABCG14 to complement the atabcg14 mutant and the grafting experiment proved that the expression of AtABCG14 in the phloem of the above-ground tissue has a long-distance effect on root CK. Transportation is crucial. Further using atabcg14 and wild-type grafting materials, using isotope tracer, xylem sap, phloem sap, and intercellular sap hormone determination methods to prove that the-type CK is first switched to the sieve for transportation during the upward transportation from the catheter. AtABCG14 located in the sieve tube companion cell is unloaded to the intercellular space, and then transported to the target cell through the apoplast transport pathway, and the excess CK flows back to the root through the sieve tube (Figure 2).

It is worth mentioning that this study found that the same transporter AtABCG14 is expressed in different tissues, and then performs assembly and unloading functions during the long-distance transport of CK from the root to the top, revealing the importance of AtABCG14 in the process of CK transport. At the same time, the previous conclusion that AtABCG14 has no biological function in the above-ground tissue is revised. This study is of great significance for systematically elucidating the molecular mechanism of the long-distance transport of root CK to the top, as well as for the study of the long-distance transport mechanism of other plant hormones and nutrient elements.

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Figure 2. AtABCG14-mediated long-distance transport model of root cytokinin

For this research result, Professor Zhang Kewei is the corresponding author, and Dr. Zhao Jiangzhe from Zhang Kewei’s team and Ding Bingle, who has graduated with a master’s degree, are the first authors of the paper. Professor Liu Changjun from Brookhaven National Laboratory, Professor Zhang Cankui from Purdue University, and Professor Xiao Shi from Sun Yat-sen University participated in this research and provided important support for the progress of the project. This research was supported by the National Natural Science Foundation of China and the National Key Research and Development Project.

In recent years, the team has made a series of research progress in the field of cytokinins. The CK oxidase OsCKX11 (Plant Biotechnol J. 2021,19(2):335-350) that controls leaf senescence has been identified; the CK transporter OsABCG18 (J Exp Bot. 2019, 18;70(21):6277- 6291); established genetic methods (Plant Methods. 2020, 16: 134) and biochemical methods (Front Plant Sci. 2021, 12: 660966) for CK transporter screening. Laid a foundation for in-depth study of the transport mechanism and biological functions of cytokinins.

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