A species-specific functional module controls formation of pollen apertures

Abstract

Pollen apertures are an interesting model for the formation of specialized plasma-membrane domains. The plant-specific protein INP1 serves as a key aperture factor in such distantly related species as Arabidopsis, rice, and maize. Although INP1 orthologs likely play similar roles throughout flowering plants, they show significant sequence divergence and often cannot substitute for each other, suggesting that INP1 might require species-specific partners. Here, we present a new aperture factor, INP2, which satisfies the criteria for being a species-specific partner for INP1. Both INP proteins display similar structural features, including the plant-specific DOG1 domain, similar patterns of expression and mutant phenotypes, as well as signs of co-evolution. These proteins interact with each other in a species-specific manner and can restore apertures in a heterologous system when both are expressed, but not when expressed individually. Our findings suggest that the INP proteins form a species-specific functional module that underlies formation of pollen apertures.

Members of the ELMOD protein family specify formation of distinct aperture domains on the Arabidopsis pollen surface

“…Thus, mcr mutations reduce the number of apertures, but do not affect their furrow morphology, longitudinal orientation, and equidistant placement. Like all the other previously characterized pollen aperture mutants in Arabidopsis , including inp1 and inp2, which completely lack apertures ( Dobritsa and Coerper, 2012 ; Dobritsa et al, 2011 ; Lee et al, 2021 ), mcr mutants exhibited no obvious fertility defects.…”

“…Pollen of the mcr d6pkl3 double mutants developed single ring-shaped apertures that were partially covered by exine, indicating that the two genes have an additive effect on aperture phenotype ( Figure 2B ). In contrast, pollen grains of mcr inp1 and mcr inp2 completely lacked apertures, phenocopying single inp1 and inp2 mutants ( Dobritsa and Coerper, 2012 ; Lee et al, 2021 ; Figure 2C and D ). This indicates that INP1 and INP2 are epistatic to MCR , consistent with their roles as factors absolutely essential for aperture formation.…”

“…We also examined the genetic interactions between MCR and other aperture factors, including the recently discovered INP2 ( Lee et al, 2021 ), by combining their mutations. d6pkl3 single mutants develop three apertures partially covered by exine ( Lee et al, 2018 ).…”

“…Our genetic analysis places MCR and ELMOD_A at the beginning of the aperture formation pathway, upstream of the previously discovered aperture factors D6PKL3, INP1, and, likely, INP2, the recently identified partner of INP1. Previous studies showed that INP1 and INP2 act as the executors of the aperture formation program, absolutely essential for aperture development but not able on their own to influence the number and positions of aperture domains ( Dobritsa et al, 2018 ; Lee et al, 2021 ; Li et al, 2018 ; Reeder et al, 2016 ). D6PKL3 was proposed to act upstream of INP1, defining the features of aperture domains, yet it also largely lacks the ability to initiate completely new domains ( Lee et al, 2018 ; Zhou and Dobritsa, 2019 ).…”

“…Pollen from wild-type Col- and L er has indistinguishable aperture phenotypes. The following genotypes were also used: mcr-1 , mcr-2 , mcr-3 , mcr-4 , mcr-5 (CS853233), mcr-6 (SALK_205528C), mcr-7 (SALK_203827C), elmod_c (SALK_076565), elmod_d (SALK_031512), elmod_e (SALK_082496), elmod_f (SALK_010379), inp1-1 ( Dobritsa and Coerper, 2012 ), inp2-1 ( Lee et al, 2021 ), d6pkl3-2 ( Lee et al, 2018 ), inp1-1 DMC1pr:INP1-YFP ( Dobritsa et al, 2018 ), d6pkl3-2 D6PKL3pr:D6PKL3-YFP ( Lee et al, 2018 ), tes (SALK _113909), MiMe ( d’Erfurth et al, 2009 ), and cenh3-1 GFP-tailswap (CS66982). mcr-1 through mcr-4 mutants were discovered in a forward genetic screen performed on an EMS-mutagenized L er population ( Plourde et al, 2019 ).…”