Department of Botany and Plant Sciences, University of California, Riverside, California 92521, USA.
Department of Biology, Berry College, Mount Berry, Georgia 30149, USA.
Plant Physiol. 2022 Sep 28;190(2):1440-1456. doi: 10.1093/plphys/kiac328.
Karrikins (KARs) are chemicals in smoke that can enhance germination of many plants. Lettuce (Lactuca sativa) cv. Grand Rapids germinates in response to nanomolar karrikinolide (KAR1). Lettuce is much less responsive to KAR2 or a mixture of synthetic strigolactone analogs, rac-GR24. We investigated the molecular basis of selective and sensitive KAR1 perception in lettuce. The lettuce genome contains two copies of KARRIKIN INSENSITIVE2 (KAI2), which in Arabidopsis (Arabidopsis thaliana) encodes a receptor that is required for KAR responses. LsKAI2b is more highly expressed than LsKAI2a in dry achenes and during early stages of imbibition. Through cross-species complementation assays in Arabidopsis, we found that an LsKAI2b transgene confers robust responses to KAR1, but LsKAI2a does not. Therefore, LsKAI2b likely mediates KAR1 responses in lettuce. We compared homology models of KAI2 proteins from lettuce and a fire-follower, whispering bells (Emmenanthe penduliflora). This identified pocket residues 96, 124, 139, and 161 as candidates that influence the ligand specificity of KAI2. Further support for the importance of these residues was found through a broader comparison of pocket residues among 281 KAI2 proteins from 184 asterid species. Almost all KAI2 proteins had either Tyr or Phe identity at position 124. Genes encoding Y124-type KAI2 are more broadly distributed in asterids than in F124-type KAI2. Substitutions at residues 96, 124, 139, and 161 in Arabidopsis KAI2 produced a broad array of responses to KAR1, KAR2, and rac-GR24. This suggests that the diverse ligand preferences observed among KAI2 proteins in plants could have evolved through relatively few mutations.
卡尔基里因（KARs）是烟雾中的化学物质，可以促进许多植物的发芽。生菜（Lactuca sativa） cv. 大激流对纳米摩尔的卡瑞卡丁内酯（KAR1）有反应。生菜对 KAR2 或合成的 Strigolactone 类似物混合物 rac-GR24 的反应要差得多。我们研究了生菜中对 KAR1 选择性和敏感性感知的分子基础。生菜基因组包含两个 KARRIKIN INSENSITIVE2（KAI2）副本，在拟南芥（Arabidopsis thaliana）中，该基因编码 KAR 反应所必需的受体。在干燥的瘦果和吸胀的早期阶段，LsKAI2b 的表达水平高于 LsKAI2a。通过在拟南芥中的种间互补测定，我们发现 LsKAI2b 转基因赋予了对 KAR1 的强烈反应，但 LsKAI2a 没有。因此，LsKAI2b 可能介导生菜中的 KAR1 反应。我们比较了生菜和火跟随者风铃草（Emmenanthe penduliflora）的 KAI2 蛋白的同源模型。这确定了口袋残基 96、124、139 和 161 作为影响 KAI2 配体特异性的候选物。通过在 184 个 Asterid 物种的 281 个 KAI2 蛋白中更广泛地比较口袋残基，进一步支持了这些残基的重要性。几乎所有的 KAI2 蛋白在位置 124 都具有 Tyr 或 Phe 身份。编码 Y124 型 KAI2 的基因在 Asterid 中的分布比 F124 型 KAI2 更为广泛。在拟南芥 KAI2 中的残基 96、124、139 和 161 处的取代产生了对 KAR1、KAR2 和 rac-GR24 的广泛反应。这表明植物中 KAI2 蛋白之间观察到的多样化配体偏好可能是通过相对较少的突变进化而来的。
