In plants, the stigma represents the initial site of pollen-pistil communication and serves as a crucial location for rejection of incompatible pollen. In self-compatible (SC) plants, such as
Arabidopsis thaliana from the Brassicaceae family, pollen coat protein B (PCP-B) peptides were reported to compete with stigmatic RAPID ALKALINIZATION FACTOR 33 (RALF33) peptides for binding to the FERONIA/ANJEA (FER/ANJ) receptors in the stigma to promote pollen hydration via inhibition of reactive oxygen species (ROS) production (Liu et al.
2021). In self-incompatible (SI) members of the Brassicaceae, however, a different protein, the S-locus cysteine-rich protein (SCR), in the pollen coat was reported to bind to the stigmatic S-locus receptor kinase (SRK) at the same S-locus to trigger the SI response (Franklin-Tong
2002). A recent study showed that, in the SI plant
Brassica rapa, signaling mediated by FER, a
Catharanthus roseus receptor-like kinase-like 1 (CrRLKL1) family receptor (Wang et al.
2022), controls self-pollen rejection through increase in the ROS content of the stigma (Zhang et al.
2021). A long-standing unresolved question has been whether the SI system is involved in the control of interspecific isolation in SI plants. A study conducted in
A. thaliana revealed that a stigmatic transmembrane protein, STIGMATIC PRIVACY 1 (SPRI1), facilitates the selection of interspecific pollen in a SI-independent manner (Fujii et al.
2019). Interestingly, hybridization between different plant species typically follows the SC × SI rule, wherein SC species tend to accept the pollen of SI species (termed unilateral compatibility, UC), whereas pollen is typically rejected in the reciprocal cross (termed unilateral incompatibility, UI). However, the mechanism of the SC × SI rule is unclear.
Recently, in a paper published in Nature, Huang et al. (
2023) reported that, in the SI plant
B. rapa, the SRK receptors not only reject SI pollen, but also reject interspecific pollen. To achieve this, the SCR from self-pollen or unidentified signals from interspecific pollen promote SRK to recruit FER and increase the ROS content in the stigma. If BrSRK or BrFER1 is silenced or the ROS content is reduced in the stigma, the acceptance of interspecific pollen and the efficiency of obtaining interspecific hybrid embryos are improved.
This paper demonstrated that, on SI stigmas, SRK controls rejection of UI pollen through FER-ROS signaling. The supporting evidence was as follows. 1. The expression level of SRK increases as the stigma of B. rapa matures, resulting in an increasingly stringent rejection of SI and UI pollen and, in both cases, ROS contents are increased in the stigma. Sequestration of ROS with sodium salicylate resulted in partial disruption of the rejection of UI pollen, allowing the penetration of B. oleracea pollen tubes into the mature stigma and Barbarea vulgaris pollen tubes into B. rapa stigma at the bud stage. 2. Knock down of BrSRK46 or expression of a transmembrane domain-deleted SRK (BrSRKΔ™) resulted in less stringent rejection of UI pollen, consistent with the observed reduction in ROS content. 3. When SI modules of A. halleri were expressed in transgenic A. thaliana plants, acceptance of interspecific pollen by the A. thaliana stigmas was inhibited, whereas the stigmatic ROS content rapidly increased. These findings suggest that increase in SRK/SRK-dependent stigmatic ROS are associated with UI pollen rejection, similar to the SI response.
Based on the reported role of CrRLK1L receptors in the regulation of ROS production, the authors of this paper aimed to determine whether the stigma-expressed BrFER1 and BrANJ1 were also involved in the UI response. They found that silencing BrFER1 or BrANJ1 resulted in inhibition of the ROS increase, and compromised SI and UI pollen rejection, which was consistent with the phenotype observed in plants with BrRBOHF silenced by antisense technology (AS-BrRBOHF). In addition, BrSRK46 interaction with BrFER1 was enhanced by protein extracts from SI and UI pollen as well as BrSCR46. These results suggest that the SI-determinant SCR from SI pollen and unknown signals from UI pollen promote the recruitment of FER by SRK, thereby activating FER-dependent ROS production to reject incompatible pollen. On the basis of this mechanism, reduction of ROS contents or disruption of Br-SRK/BrFER-to-BrRBOH signaling by AS-oligodeoxyribonucleotides in B. rapa stigmas led to the promotion of interspecific hybridization and the generation of hybrid embryos.
In the SC plant
A. thaliana, the study showed that, although pollen from SI species could be accepted by the stigma, the resulting pollen tubes were much shorter than those of self pollen. The authors found that FER was also involved in this process, because
B. rapa pollen hydrated more rapidly in
fer-4 stigmas and the tubes were much longer than those in wild-type
A. thaliana stigmas. In interspecific pollination, reduction of the stigmatic ROS content was slower, indicating that FER-ROS functions in the discrimination against interspecific pollen to maintain species integrity. Given that PCP-Bs were reported to bind to FER and reduce the ROS content to promote pollen hydration, the authors further found that conspecific PCP-Bs had higher binding efficiency with the extracellular domain of FER, which may favor conspecific pollen for fertilization and thus serve as a potential interspecific barrier. Thus, this report shows that the SI system is capable of serving as an interspecific barrier, shedding light on the role of SRK in interspecific isolation and that of FER-ROS signaling in the establishment of reproductive barriers in SI and possibly SC plants (
Fig. 1).