Institute of Plant Stress Adaptation and Genetic Enhancement, Zhejiang Provincial Key Laboratory of Biotechnology on Specialty Economic Plants, College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua, Zhejiang 321004, P.R. China.
Key Laboratory of Mollisols Agroecology, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, No. 4888 Shengbei Street, Changchun 130102, China.
Plant Commun. 2023 Mar 13;4(2):100468. doi: 10.1016/j.xplc.2022.100468. Epub 2022 Oct 28.
Cytokinins (CKs), primarily trans-zeatin (tZ) and isopentenyladenine (iP) types, play critical roles in plant growth, development, and various stress responses. Long-distance transport of tZ-type CKs meidated by Arabidopsis ATP-binding cassette transporter subfamily G14 (AtABCG14) has been well studied; however, less is known about the biochemical properties of AtABCG14 and its transporter activity toward iP-type CKs. Here we reveal the biochemical properties of AtABCG14 and provide evidence that it is also required for long-distance transport of iP-type CKs. AtABCG14 formed homodimers in human (Homo sapiens) HEK293T, tobacco (Nicotiana tabacum), and Arabidopsis cells. Transporter activity assays of AtABCG14 in Arabidopsis, tobacco, and yeast (Saccharomyces cerevisiae) showed that AtABCG14 may directly transport multiple CKs, including iP- and tZ-type species. AtABCG14 expression was induced by iP in a tZ-type CK-deficient double mutant (cypDM) of CYP735A1 and CYP735A2. The atabcg14 cypDM triple mutant exhibited stronger CK-deficiency phenotypes than cypDM. Hormone profiling, reciprocal grafting, and H-iP isotope tracer experiments showed that root-to-shoot and shoot-to-root long-distance transport of iP-type CKs were suppressed in atabcg14 cypDM and atabcg14. These results suggest that AtABCG14 participates in three steps of the circular long-distance transport of iP-type CKs: xylem loading in the root for shootward transport, phloem unloading in the shoot for shoot distribution, and phloem unloading in the root for root distribution. We found that AtABCG14 displays transporter activity toward multiple CK species and revealed its versatile roles in circular long-distance transport of iP-type CKs. These findings provide new insights into the transport mechanisms of CKs and other plant hormones.
细胞分裂素（CKs），主要是反式玉米素（tZ）和异戊烯基腺嘌呤（iP）类型，在植物生长、发育和各种胁迫响应中发挥关键作用。拟南芥 ATP 结合盒转运蛋白亚家族 G14（AtABCG14）介导的 tZ 型 CK 的长距离运输已经得到了很好的研究；然而，对于 AtABCG14 的生化特性及其对 iP 型 CK 的转运活性知之甚少。在这里，我们揭示了 AtABCG14 的生化特性，并提供了证据表明它也是 iP 型 CK 长距离运输所必需的。AtABCG14 在人（Homo sapiens）HEK293T、烟草（Nicotiana tabacum）和拟南芥细胞中形成同源二聚体。在拟南芥、烟草和酵母（Saccharomyces cerevisiae）中进行的 AtABCG14 转运活性测定表明，AtABCG14 可能直接转运多种 CKs，包括 iP 和 tZ 型物质。iP 诱导 AtABCG14 在 CYP735A1 和 CYP735A2 的 tZ 型 CK 缺陷双突变体（cypDM）中表达。与 cypDM 相比，atabcg14 cypDM 三突变体表现出更强的 CK 缺陷表型。激素分析、相互嫁接和 H-iP 同位素示踪实验表明，iP 型 CKs 的根到茎和茎到根的长距离运输在 atabcg14 cypDM 和 atabcg14 中受到抑制。这些结果表明，AtABCG14 参与了 iP 型 CKs 循环长距离运输的三个步骤：根向木质部装载以供向地上部运输、地上部韧皮部卸载以供地上部分配、以及根向韧皮部卸载以供根分配。我们发现 AtABCG14 对多种 CK 物质表现出转运活性，并揭示了其在 iP 型 CK 循环长距离运输中的多功能作用。这些发现为 CKs 和其他植物激素的运输机制提供了新的见解。
