Department of Plant Biology, Cornell University, Ithaca, New York 14853.
Plant Physiol. 2013 Nov;163(3):1387-95. doi: 10.1104/pp.113.225672. Epub 2013 Sep 27.
The self-incompatibility (SI) response of the Brassicaceae is mediated by allele-specific interaction between the stigma-localized S-locus receptor kinase (SRK) and its ligand, the pollen coat-localized S-locus cysteine-rich protein (SCR). Based on work in Brassica spp., the thioredoxin h-like proteins THL1 and THL2, which interact with SRK, have been proposed to function as oxidoreductases that negatively regulate SRK catalytic activity. By preventing the spontaneous activation of SRK in the absence of SCR ligand, these thioredoxins are thought to be essential for the success of cross pollinations in self-incompatible plants. However, the in planta role of thioredoxins in the regulation of SI signaling has not been conclusively demonstrated. Here, we addressed this issue using Arabidopsis thaliana plants transformed with the SRKb-SCRb gene pair isolated from self-incompatible Arabidopsis lyrata. These plants express an intense SI response, allowing us to exploit the extensive tools and resources available in A. thaliana for analysis of SI signaling. To test the hypothesis that SRK is redox regulated by thioredoxin h, we expressed a mutant form of SRKb lacking a transmembrane-localized cysteine residue thought to be essential for the SRK-thioredoxin h interaction. We also analyzed transfer DNA insertion mutants in the A. thaliana orthologs of THL1 and THL2. In neither case did we observe an effect on the pollination responses of SRKb-expressing stigmas toward incompatible or compatible pollen. Our results are consistent with the conclusion that, contrary to their proposed role, thioredoxin h proteins are not required to prevent the spontaneous activation of SRK in the A. thaliana stigma.
自交不亲和（SI）反应在十字花科中由柱头定位的 S 基因座受体激酶（SRK）与其配体花粉外壁定位的 S 基因座富含半胱氨酸蛋白（SCR）之间的等位基因特异性相互作用介导。基于 Brassica spp 的研究，已提出与 SRK 相互作用的硫氧还蛋白 h 样蛋白 THL1 和 THL2 作为氧化还原酶发挥作用，它们负调控 SRK 催化活性。通过防止 SRK 在没有 SCR 配体的情况下自发激活，这些硫氧还蛋白被认为对于自交不亲和植物中异花授粉的成功至关重要。然而，硫氧还蛋白在调节 SI 信号转导中的植物内作用尚未得到明确证明。在这里，我们使用从自交不亲和的拟南芥 lyrata 中分离的 SRKb-SCRb 基因对转化的拟南芥植物解决了这个问题。这些植物表现出强烈的 SI 反应，使我们能够利用拟南芥中可用的广泛工具和资源来分析 SI 信号转导。为了检验 SRK 被硫氧还蛋白 h 氧化还原调节的假设，我们表达了一种缺乏跨膜定位半胱氨酸残基的 SRKb 突变体，该残基被认为对于 SRK-硫氧还蛋白 h 相互作用至关重要。我们还分析了拟南芥 THL1 和 THL2 直系同源物的 T-DNA 插入突变体。在这两种情况下，我们都没有观察到对表达 SRKb 的柱头对不亲和或亲和花粉的授粉反应产生影响。我们的结果与结论一致，即与它们预期的作用相反，硫氧还蛋白 h 蛋白不是防止拟南芥柱头中 SRK 自发激活所必需的。
