Graduate School of Life Sciences, Tohoku University, Sendai 980-8577, Japan.
Graduate School of Biological Sciences, Nara Institute of Science and Technology, Ikoma 630-0192, Japan.
Nat Plants. 2017 Jun 26;3:17096. doi: 10.1038/nplants.2017.96.
In plants, cell-cell recognition is a crucial step in the selection of optimal pairs of gametes to achieve successful propagation of progeny. Flowering plants have evolved various genetic mechanisms, mediated by cell-cell recognition, to enable their pistils to reject self-pollen, thus preventing inbreeding and the consequent reduced fitness of progeny (self-incompatibility, SI), and to reject foreign pollen from other species, thus maintaining species identity (interspecific incompatibility). In the genus Brassica, the SI system is regulated by an S-haplotype-specific interaction between a stigma-expressed female receptor (S receptor kinase, SRK) and a tapetum cell-expressed male ligand (S locus protein 11, SP11), encoded by their respective polymorphic genes at the S locus. However, the molecular mechanism for recognition of foreign pollen, leading to reproductive incompatibility, has not yet been identified. Here, we show that recognition between a novel pair of proteins, a pistil receptor SUI1 (STIGMATIC UNILATERAL INCOMPATIBILITY 1) and a pollen ligand PUI1 (POLLEN UNILATERAL INCOMPATIBILITY 1), triggers unilateral reproductive incompatibility between plants of two geographically distant self-incompatible Brassica rapa lines, even though crosses would be predicted to be compatible based on the S haplotypes of pollen and stigma. Interestingly, SUI1 and PUI1 are similar to the SI genes, SRK and SP11, respectively, and are maintained as cryptic incompatibility genes in these two populations. The duplication of the SRK and SP11 followed by reciprocal loss in different populations would provide a molecular mechanism of the emergence of a reproductive barrier in allopatry.
在植物中，细胞间的识别是选择最佳配子对以成功繁殖后代的关键步骤。开花植物进化出了各种遗传机制，通过细胞间的识别来实现雌蕊拒绝自花花粉，从而防止近亲繁殖和后代适应度降低（自交不亲和性，SI），以及拒绝来自其他物种的异花花粉，从而维持物种身份（种间不亲和性）。在芸薹属植物中，SI 系统由柱头表达的雌性受体（S 受体激酶，SRK）和绒毡层细胞表达的雄性配体（S 位点蛋白 11，SP11）之间的 S 单倍型特异性相互作用调节，由其在 S 位点的各自多态基因编码。然而，识别导致生殖不相容的异花花粉的分子机制尚未确定。在这里，我们表明，一对新的蛋白质，柱头受体 SUI1（STIGMATIC UNILATERAL INCOMPATIBILITY 1）和花粉配体 PUI1（POLLEN UNILATERAL INCOMPATIBILITY 1）之间的识别，触发了来自两个地理位置遥远的自交不亲和甘蓝型油菜品系的植物之间的单侧生殖不相容性，尽管根据花粉和柱头的 S 单倍型预测，这些杂交是相容的。有趣的是，SUI1 和 PUI1 分别与 SI 基因 SRK 和 SP11 相似，并且在这两个种群中作为隐性不相容基因被保留。SRK 和 SP11 的重复，随后在不同种群中相互丢失，为异地生殖障碍的出现提供了一种分子机制。
