MLPK function is not required for self-incompatibility in the S29 haplotype of Brassica rapa L.

Abstract

Key message S29 haplotype does not require the MLPK function for self-incompatibility in Brassica rapa. Abstract Self-incompatibility (SI) in Brassicaceae is regulated by the self-recognition mechanism, which is based on the S-haplotype-specific direct interaction of the pollen-derived ligand, SP11/SCR, and the stigma-side receptor, SRK. M locus protein kinase (MLPK) is known to be one of the positive effectors of the SI response. MLPK directly interacts with SRK, and is phosphorylated by SRK in Brassica rapa. In Brassicaceae, MLPK was demonstrated to be essential for SI in B. rapa and Brassica napus, whereas it is not essential for SI in Arabidopsis thaliana (with introduced SRK and SP11/SCR from related SI species). Little is known about what determines the need for MLPK in SI of Brassicaceae. In this study, we investigated the relationship between S-haplotype diversity and MLPK function by analyzing the SI phenotypes of different S haplotypes in a mlpk/mlpk mutant background. The results have clarified that in B. rapa, all the S haplotypes except the S29 we tested need the MLPK function, but the S29 haplotype does not require MLPK for the SI. Comparative analysis of MLPK-dependent and MLPK-independent S haplotype might provide new insight into the evolution of S-haplotype diversity and the molecular mechanism of SI in Brassicaceae.

Stop and go signals at the stigma–pollen interface of the Brassicaceae

“…In a separate study, analysis of a self-fertile B. rapa cultivar identified the membrane-localized protein kinase MLPK as a potential effector of SRK ( Kakita et al 2007 ). However, MLPK function is not required for the operation of at least one S haplotype in B. rapa ( Ohata et al 2023 ). Moreover, neither the SRK-ARC1-Exo70A1 pathway nor MLPK are required for the SI response in transgenic self-incompatible A. thaliana plants ( Kitashiba et al 2011 ; Nasrallah and Nasrallah 2014 ).…”

“…relates to the induction of autophagy in self-pollinated papillae ( Abhinandan et al 2022 ): autophagosomes, which are proposed to function as an additional process for clearing compatibility factors, have been observed in the papillae of self-incompatible Arabidopsis plants but not in those of Brassica ( Table 2 ). Setting aside the curious intraspecific difference regarding the requirement of MLPK in SI ( Ohata et al 2023 ), two possible explanations for the discrepancies observed between Brassica and Arabidopsis may be advanced. One explanation is that the role of MLPK, ARC1, Exo70A1, and autophagy in SI is not universal among crucifers and that distinct SRK-mediated pollen rejection pathways may operate in different species.…”

Development of self-compatible Chinese cabbage lines of Chiifu through marker-assisted selection

“…Two distinct forms of MLPK , MLPK form 1 ( MLPKf1 ) and MLPK form 2 ( MLPKf2 ), localize to the papilla cell membrane and directly interact with SRK to transduce SI signaling ( Kakita et al., 2007 ). The Chiifu accession carries the S 60 haplotype on chromosome A07 ( Yamamoto et al., 2023 ), characterized by MLPK -dependent self-incompatibility ( Ohata et al., 2023 ). However, the practical challenge of breeding self-compatible Chinese cabbage lines based on MLPK remains unresolved.…”