The ARC1 E3 Ligase Promotes Two Different Self-Pollen Avoidance Traits inArabidopsis   

Abstract

Flowering plants have evolved various strategies for avoiding self-pollen to drive genetic diversity. These strategies include spatially separated sexual organs (herkogamy), timing differences between male pollen release and female pistil receptivity (dichogamy), and self-pollen rejection. Within the Brassicaceae, these outcrossing systems are the evolutionary default state, and many species display these traits, including Arabidopsis lyrata. In contrast to A. lyrata, closely related Arabidopsis thaliana has lost these self-pollen traits and thus represents an excellent system to test genes for reconstructing these evolutionary traits. We previously demonstrated that the ARC1 E3 ligase is required for self-incompatibility in two diverse Brassicaceae species, Brassica napus and A. lyrata, and is frequently deleted in self-compatible species, including A. thaliana. In this study, we examined ARC1's requirement for reconstituting self-incompatibility in A. thaliana and uncovered an important role for ARC1 in promoting a strong and stable pollen rejection response when expressed with two other A. lyrata self-incompatibility factors. Furthermore, we discovered that ARC1 promoted an approach herkogamous phenotype in A. thaliana flowers. Thus, ARC1's expression resulted in two different A. lyrata traits for self-pollen avoidance and highlights the key role that ARC1 plays in the evolution and retention of outcrossing systems.

The ARC1 E3 Ligase Promotes a Strong and Stable Self-Incompatibility Response in Arabidopsis Species: Response to the Nasrallah and Nasrallah Commentary

“…Based on our work and others (see Samuel et al, 2009;Safavian and Goring, 2013;Indriolo et al, 2014 and references therein), our working models for the acceptance of compatible pollen versus the rejection of selfincompatible pollen by the stigmatic papillae are shown in Figures 1C and 1D. Compatible pollen is accepted through a basal signaling pathway that leads to vesicle secretion at the pollen contact site in the stigmatic papilla, which in turn promotes pollen hydration and pollen tube entry into the stigma ( Figure 1C).…”

“…Transgene expression levels were quantified by quantitative RT-PCR (qRT-PCR) using RNA samples extracted from anther or stigma tissue (harvested from T2 flowers) at the correct developmental stage for all samples (flower buds just starting to open; before anther dehiscence). The transgene qRT-PCR expression analyses included an A. lyrata control for comparison, and the overall conclusion was that ARC1's impact on the SI response was not due to variable transgene expression levels (see Indriolo et al [2014] for a detailed discussion).…”

“…The effect of ARC1 was not due to changes in A. lyrata SRKb expression levels as shown by qRT-PCR analyses . In addition, the Indriolo et al (2014) study was conducted in a true wild-type Col-0 background and not in the RNA-dependent RNA polymerase6 mutant background described as a "modifier" by Nasrallah and Nasrallah (2014) (2014) identified A. thaliana Sha:AlSCRb-SRKb transformants that met the definition of self-incompatibility with statistically significant reductions in seed set in the absence of ARC1 (16.6 seeds/silique in the strongest line). Therefore, both studies show that Sha:AlSCRbSRKb transformants have self-incompatibility responses preventing self-fertilization.…”

“…Pseudocoloring has been added to distinguish the pollen (brown) from the stigmatic papilla (green). Autophagy is detected with the autophagic vacuole in the vacuole (see Rose et al [2006] and Indriolo et al [2014] for details). (Figures 1C to 1E adapted from Indriolo et al [2014], Figures 9 and 10.…”

“…In addition, secretory vesicles are degraded in the vacuole through autophagy. An unknown signaling protein (yellow) also has activity in the Sha ecotype in blocking exocytosis (see Samuel et al [2009], Safavian and Goring [2013], and Indriolo et al [2014] for further details and references therein). (E) Transmission electron microscopy image of a self-incompatible pollen-stigmatic papillar interaction at 10 min postpollination from the transgenic SCRb-SRKb1ARC1 Sha ecotype.…”