Department of Botany and Plant Sciences, University of California, Riverside, California 92521.
Department of Genetics, University of Georgia, Athens, Georgia 30602.
Plant Cell. 2020 Aug;32(8):2639-2659. doi: 10.1105/tpc.19.00752. Epub 2020 May 20.
Karrikins (KARs) are butenolides found in smoke that can influence germination and seedling development of many plants. The KAR signaling mechanism is hypothesized to be very similar to that of the plant hormone strigolactone (SL). Both pathways require the F-box protein MORE AXILLARY GROWTH2 (MAX2), and other core signaling components have shared ancestry. Putatively, KAR activates the receptor KARRIKIN INSENSITIVE2 (KAI2), triggering its association with the E3 ubiquitin ligase complex SCF and downstream targets SUPPRESSOR OF MAX2 1 (SMAX1) and SMAX1-LIKE2 (SMXL2). Polyubiquitination and proteolysis of SMAX1 and SMXL2 then enable growth responses to KAR. However, many of the assumptions of this model have not been demonstrated. Therefore, we investigated the posttranslational regulation of SMAX1 from the model plant Arabidopsis (). We find evidence that SMAX1 is degraded by KAI2-SCF but is also subject to MAX2-independent turnover. We identify SMAX1 domains that are responsible for its nuclear localization, KAR-induced degradation, association with KAI2, and ability to interact with other SMXL proteins. KAI2 undergoes MAX2-independent degradation after KAR treatment, which we propose results from its association with SMAX1 and SMXL2. Finally, we discover an SMXL domain that mediates receptor-target interaction preferences in KAR and SL signaling, laying the foundation for understanding how these highly similar pathways evolved to fulfill different roles.
卡里卡丁（KARs）是烟雾中发现的丁烯内酯，可以影响许多植物的萌发和幼苗发育。KAR 信号机制被假设与植物激素独脚金内酯（SL）非常相似。这两种途径都需要 F-box 蛋白 MORE AXILLARY GROWTH2（MAX2），其他核心信号成分具有共同的祖先。据称，KAR 激活受体 KARRIKIN INSENSITIVE2（KAI2），触发其与 E3 泛素连接酶复合物 SCF 及其下游靶标 SUPPRESSOR OF MAX2 1（SMAX1）和 SMAX1-LIKE2（SMXL2）的关联。然后，SMAX1 和 SMXL2 的多泛素化和蛋白水解使生长对 KAR 产生反应。然而，该模型的许多假设尚未得到证实。因此，我们研究了模式植物拟南芥（Arabidopsis）中 SMAX1 的翻译后调控。我们发现证据表明，SMAX1 被 KAI2-SCF 降解，但也受到 MAX2 独立的周转率的影响。我们确定了 SMAX1 负责其核定位、KAR 诱导降解、与 KAI2 关联以及与其他 SMXL 蛋白相互作用的结构域。KAI2 在 KAR 处理后经历 MAX2 独立降解，我们推测这是由于其与 SMAX1 和 SMXL2 的关联所致。最后，我们发现了一个 SMXL 结构域，介导 KAR 和 SL 信号转导中受体-靶标相互作用偏好，为理解这些高度相似的途径如何进化以发挥不同作用奠定了基础。
