Secretory Activity Is Rapidly Induced in Stigmatic Papillae by Compatible Pollen,but Inhibited for Self-Incompatible Pollen in the Brassicaceae

[In the Brassicaceae, targeted exocytosis to the stigmatic papillar plasma membrane under the compatible pollen grain is hypothesized to be essential for pollen hydration and pollen tube penetration. In contrast, polarized secretion is proposed to be inhibited in the stigmatic papillae during the rejection of self-incompatible pollen. Using transmission electron microscopy (TEM), we performed a detailed time-course of post-pollination events to view the cytological responses of the stigmatic papillae to compatible and self-incompatible pollinations. For compatible pollinations in Arabidopsis thaliana and Arabidopsis lyrata, vesicle secretion was observed at the stigmatic papillar plasma membrane under the pollen grain while Brassica napus stigmatic papillae appeared to use multivesicular bodies (MVBs) for secretion. Exo70A1, a component of the exocyst complex, has been previously implicated in the compatible pollen responses, and disruption of Exo70A1 in both A. thaliana and B. napus resulted in a loss of secretory vesicles/MVBs at the stigmatic papillar plasma membrane. Similarly, for self-incompatible pollinations, secretory vesicles/MVBs were absent from the stigmatic papillar plasma membrane in A. lyrata and B. napus; and furthermore, autophagy appeared to be induced to direct vesicles/MVBs to the vacuole for degradation. Thus, these findings support a model where the basal pollen recognition pathway in the stigmatic papilla promotes exocytosis to accept compatible pollen, and the basal pollen recognition pathway is overridden by the self-incompatibility pathway to prevent exocytosis and reject self-pollen.     

A QTL on chromosome 3q23 influences processing speed in humans

Processing speed is a psychological construct that refers to the speed with which an individual can perform any cognitive operation. Processing speed correlates strongly with general cognitive ability, declines sharply with age and is impaired across a number of neurological and psychiatric disorders. Thus, identifying genes that influence processing speed will likely improve understanding of the genetics of intelligence, biological aging and the etiologies of numerous disorders. Previous genetics studies of processing speed have relied on simple phenotypes (eg, mean reaction time) derived from single tasks. This strategy assumes, erroneously, that processing speed is a unitary construct. In the present study, we aimed to characterize the genetic architecture of processing speed by using a multidimensional model applied to a battery of cognitive tasks. Linkage and QTL‐specific association analyses were performed on the factors from this model. The randomly ascertained sample comprised 1291 Mexican‐American individuals from extended pedigrees. We found that performance on all three distinct processing‐speed factors (Psychomotor Speed; Sequencing and Shifting and Verbal Fluency) were moderately and significantly heritable. We identified a genome‐wide significant quantitative trait locus (QTL) on chromosome 3q23 for Psychomotor Speed (LOD = 4.83). Within this locus, we identified a plausible and interesting candidate gene for Psychomotor Speed (Z = 2.90, P = 1.86 × 10^?03).     

A comprehensive expression analysis of the Arabidopsis proline-rich extensin-like receptor kinase gene family using bioinformatic and experimental approaches

The Arabidopsis proline-rich extensin-like receptor kinase (PERK) family consists of 15 predicted receptor kinases. A comprehensive expression analysis was undertaken to identify overlapping and unique expression patterns within this family relative to their phylogeny. Three different approaches were used to study AtPERK gene family expression, and included analyses of the EST, MPSS and NASCArrays databases as well as experimental RNA blot analyses. Some of the AtPERK members were identified as tissue-specific genes while others were more broadly expressed. While in some cases there was a good association between these different expression patterns and the position of the AtPERK members in the kinase phylogeny, in other cases divergence of expression patterns was seen. The PERK expression data identified by the bioinformatics and experimental approaches were found generally to show similar trends and supported the use of data from large-scale expression studies for obtaining preliminary expression data. Thus, the bioinformatics survey for ESTs and microarrays is a powerful comprehensive approach for obtaining a genome-wide view of genes in a multigene family.     

ARC1 is an E3 ubiquitin ligase and promotes the ubiquitination of proteins during the rejection of self-incompatible Brassica pollen

ARC1 is a novel U-box protein required in the Brassica pistil for the rejection of self-incompatible pollen; it functions downstream of the S receptor kinase (SRK). Here, we show that ARC1 has E3 ubiquitin ligase activity and contains several motifs that influence its subcellular localization. ARC1 can shuttle between the nucleus, cytosol, and proteasome/COP9 signalosome (CSN) when expressed in tobacco BY-2 suspension-cultured cells. However, ARC1 localization to the proteasome/CSN occurs only in the presence of an active SRK. In the pistil, ubiquitinated protein levels increase specifically with incompatible pollinations, but they do not change in ARC1 antisense-suppressed pistils. In addition, inhibition of the proteasomal proteolytic activity disrupts the self-incompatibility response. We propose that ARC1 promotes the ubiquitination and proteasomal degradation of compatibility factors in the pistil, which in turn leads to pollen rejection.     

Sentinels at the wall: cell wall receptors and sensors

The emerging view of the plant cell wall is of a dynamic and responsive structure that exists as part of a continuum with the plasma membrane and cytoskeleton. This continuum must be responsive and adaptable to normal processes of growth as well as to stresses such as wounding, attack from pathogens and mechanical stimuli. Cell expansion involving wall loosening, deposition of new materials, and subsequent rigidification must be tightly regulated to allow the maintenance of cell wall integrity and co-ordination of development. Similarly, sensing and feedback are necessary for the plant to respond to mechanical stress or pathogen attack. Currently, understanding of the sensing and feedback mechanisms utilized by plants to regulate these processes is limited, although we can learn from yeast, where the signalling pathways have been more clearly defined. Plant cell walls possess a unique and complicated structure, but it is the protein components of the wall that are likely to play a crucial role at the forefront of perception, and these are likely to include a variety of sensor and receptor systems. Recent plant research has yielded a number of interesting candidates for cell wall sensors and receptors, and we are beginning to understand the role that they may play in this crucial aspect of plant biology.     

The ARC1 E3 Ligase Gene Is Frequently Deleted in Self-Compatible Brassicaceae Species and Has a Conserved Role in Arabidopsis lyrata Self-Pollen Rejection

Self-pollen rejection is an important reproductive regulator in flowering plants, and several different intercellular signaling systems have evolved to elicit this response. In the Brassicaceae, the self-incompatibility system is mediated by the pollen S-locus Cys-Rich/S-locus Protein11 (SCR/SP11) ligand and the pistil S Receptor Kinase (SRK). While the SCR/SP11-SRK recognition system has been identified in several species across the Brassicaceae, less is known about the conservation of the SRK-activated cellular responses in the stigma, following self-pollen contact. The ARM Repeat Containing1 (ARC1) E3 ubiquitin ligase functions downstream of SRK for the self-incompatibility response in Brassica, but it has been suggested that ARC1 is not required in Arabidopsis species. Here, we surveyed the presence of ARC1 orthologs in several recently sequenced genomes from Brassicaceae species that had diversified ∼20 to 40 million years ago. Surprisingly, the ARC1 gene was deleted in several species that had lost the self-incompatibility trait, suggesting that ARC1 may lose functionality in the transition to self-mating. To test the requirement of ARC1 in a self-incompatible Arabidopsis species, transgenic ARC1 RNA interference Arabidopsis lyrata plants were generated, and they exhibited reduced self-incompatibility responses resulting in successful fertilization. Thus, this study demonstrates a conserved role for ARC1 in the self-pollen rejection response within the Brassicaceae.     

Interrelationships between Cytoplasmic Ca2+ Peaks, Pollen Hydration and Plasma Membrane Conductances during Compatible and Incompatible Pollinations of Brassica napus Papillae

We have investigated Ca²⁺-involving cell signaling, plasma membranepotentials and conductances and callose formation during earlystages of pollination of papillae of Brassica napus. Using fluorescenceimaging of calcium green-1, we found that application of a rangeof pollen types and controls all rapidly produced small localizedpeaks in papillar cytoplasmic [Ca²⁺]. This response was morefrequent in compatible than incompatible interactions and wascorrelated with subsequent hydration of the applied pollen grains,indicating that it may be a differential prerequisite of thecompatible signaling pathway leading to successful pollinations.In contrast, a slight trend to increased plasma membrane conductance(but with no indications of action potential-like responses)and also callose deposition in papillae adjacent to pollen grainsfollowed pollination in both SC and SI interactions, indicatingthat alterations in plasma membrane permeability and callosedeposition during early phases of pollination are not primarydeterminants of the fate of attempted pollinations. ²On leave of absence from A.N. Bach Institute of Biochemistry,Russian Academy of Sciences, Leninsky Pr., 33, Moscow, Russia,117071     

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

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

Following the identification of the male (S-locus Cysteine Rich/S-locus Protein 11) and female (S Receptor kinase [SRK]) factors controlling self-incompatibility in the Brassicaceae, research in this field has focused on understanding the nature of the cellular responses activated by these regulators. We previously identified the ARM Repeat Containing1 (ARC1) E3 ligase as a component of the SRK signaling pathway and demonstrated ARC1’s requirement in the stigma for self-incompatible pollen rejection in Brassica napus, Arabidopsis lyrata, and Arabidopsis thaliana. Here, we discuss our findings on the role of ARC1 in reconstructing a strong and stable A. thaliana self-incompatibility phenotype, in the context of the putative issues outlined in a commentary by Nasrallah and Nasrallah. Additionally, with their proposed standardized strategy for studying self-incompatibility in A. thaliana, we offer our perspective on what constitutes a strong and stable self-incompatibility phenotype in A. thaliana and how this should be investigated and reported to the greater community.With many angiosperms possessing hermaphroditic flowers, self-incompatibility (SI) systems have evolved to avoid the deleterious effects of inbreeding (Figures 1A and 1B). As defined by Charlesworth et al. (2005), “plant SI systems all prevent self-fertilization through recognition and rejection of pollen by pistils expressing ‘cognate’ allelic specificity.” In Brassicaceae species, the allele specificity is conferred by two well-characterized polymorphic genes encoding the female S Receptor kinase (SRK) and the male S-locus Protein 11/S-locus Cysteine Rich (SP11/SCR; hereby referred to as SCR) (reviewed in Iwano and Takayama, 2012). The major outstanding area in this field is identifying the signaling proteins activated by SRK, determining their function at the cellular level, and investigating whether these signaling proteins have conserved functions across the self-incompatible species in the Brassicaceae. Despite strong interest in finding these potential factors by us and other groups, only the Brassica rapa M Locus Protein Kinase (Murase et al., 2004; Kakita et al., 2007a, 2007b) and the ARM Repeat Containing1 (ARC1) E3 ligase have emerged as direct downstream signaling proteins. We demonstrated a conserved role for ARC1 in self-incompatible Brassica napus (Gu et al., 1998; Stone et al., 1999, 2003), self-incompatible Arabidopsis lyrata (Indriolo et al., 2012), and self-incompatible Arabidopsis thaliana expressing A. lyrata SCRb, SRKb, and ARC1 transgenes (Indriolo et al., 2014). The commentary by Nasrallah and Nasrallah (2014) focuses on our proposed role for ARC1 in reconstituting self-incompatibility in transgenic A. thaliana.Open in a separate windowFigure 1.Pathways for Compatible and Self-Incompatible Pollen Responses in A. thaliana.(A) Compatible (arrow) and self-incompatible (bar) pollen-pistil interactions.(B) Criteria for assessing compatible versus self-incompatible pollinations.(C) Model for the basal compatible pollen response. An unknown basal pollen response pathway is activated in the stigmatic papilla under the compatible pollen grain leading to the activation of vesicle secretion. Our research on Brassica and Arabidopsis Exo70A1 revealed a putative role for the exocyst complex in docking secretory vesicles at the stigmatic papillae plasma membrane (Samuel et al., 2009; Safavian and Goring, 2013; Safavian et al., 2014). Exo70A1 is proposed to assemble with the remaining subunits of the exocyst complex to dock secretory vesicles (reviewed in Zárský et al., 2013). SNARE proteins mediate vesicle fusion to the plasma membrane, and unknown cargo (ACA13 as one candidate; Iwano et al., 2014) are released to enable pollen hydration pollen tube entry through the stigmatic papillar cell wall (compatible pollen is accepted).(D) Model for the reconstituted self-incompatibility signaling pathway in the Sha ecotype. Following self-pollination in transgenic SCR-SRK+ARC1 Sha ecotype flowers, the pollen SCR ligand binds to SRK at the stigmatic papillar plasma membrane, resulting in the activation of the downstream signaling pathway. The ARC1 E3 ligase is recruited by SRK and targets Exo70A1 for ubiquitination. Even though the basal compatible pollen response pathway has been also activated, ubiquitinated Exo70A1 is somehow inhibited so that exocyst-mediated vesicle secretion to the self-incompatible pollen grain is blocked. 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-SRKb+ARC1 Sha ecotype. 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.)