Department of Biochemistry, Interdisciplinary Plant Group, 117 Schweitzer Hall, University of Missouri, Columbia, MO 65211, USA.
Ann Bot. 2011 Sep;108(4):647-58. doi: 10.1093/aob/mcr179. Epub 2011 Jul 28.
S-RNase-based self-incompatibility (SI) occurs in the Solanaceae, Rosaceae and Plantaginaceae. In all three families, compatibility is controlled by a polymorphic S-locus encoding at least two genes. S-RNases determine the specificity of pollen rejection in the pistil, and S-locus F-box proteins fulfill this function in pollen. S-RNases are thought to function as S-specific cytotoxins as well as recognition proteins. Thus, incompatibility results from the cytotoxic activity of S-RNase, while compatible pollen tubes evade S-RNase cytotoxicity.
The S-specificity determinants are known, but many questions remain. In this review, the genetics of SI are introduced and the characteristics of S-RNases and pollen F-box proteins are briefly described. A variety of modifier genes also required for SI are also reviewed. Mutations affecting compatibility in pollen are especially important for defining models of compatibility and incompatibility. In Solanaceae, pollen-side mutations causing breakdown in SI have been attributed to the heteroallelic pollen effect, but a mutation in Solanum chacoense may be an exception. This has been interpreted to mean that pollen incompatibility is the default condition unless the S-locus F-box protein confers resistance to S-RNase. In Prunus, however, S-locus F-box protein gene mutations clearly cause compatibility.
Two alternative mechanisms have been proposed to explain compatibility and incompatibility: compatibility is explained either as a result of either degradation of non-self S-RNase or by its compartmentalization so that it does not have access to the pollen tube cytoplasm. These models are not necessarily mutually exclusive, but each makes different predictions about whether pollen compatibility or incompatibility is the default. As more factors required for SI are identified and characterized, it will be possible to determine the role each process plays in S-RNase-based SI.
基于 S-RNase 的自交不亲和性（SI）发生在茄科、蔷薇科和车前科中。在这三个科中，相容性由一个多态 S 基因座编码的至少两个基因控制。S-RNases 决定柱头中花粉的排斥特异性，而 S 基因座 F-box 蛋白在花粉中发挥此功能。S-RNases 被认为具有 S 特异性细胞毒素以及识别蛋白的功能。因此，不亲和性是由 S-RNase 的细胞毒性活性引起的，而相容的花粉管则逃避 S-RNase 的细胞毒性。
S 特异性决定因素是已知的，但仍有许多问题需要解决。在这篇综述中，介绍了 SI 的遗传学，并简要描述了 S-RNases 和花粉 F-box 蛋白的特征。还回顾了各种修饰基因，这些基因也需要 SI。影响花粉相容性的突变对于定义相容性和不兼容性模型尤为重要。在茄科中，导致 SI 破坏的花粉侧突变归因于异等位花粉效应，但在 Solanum chacoense 中的突变可能是一个例外。这意味着花粉不亲和性是默认条件，除非 S 基因座 F-box 蛋白赋予对 S-RNase 的抗性。然而，在李属中，S 基因座 F-box 蛋白基因突变显然会导致相容性。
已经提出了两种解释相容性和不兼容性的替代机制：相容性要么是由于非自身 S-RNase 的降解，要么是由于其分隔化，使得它无法进入花粉管细胞质。这些模型不一定相互排斥，但每个模型对花粉的相容性或不兼容性是否为默认条件都有不同的预测。随着更多的 SI 所需的因素被鉴定和表征，就有可能确定每个过程在基于 S-RNase 的 SI 中所起的作用。
