Reviews pubs.acs.org/jpr
Proteomics Approaches Advance Our Understanding of Plant SelfIncompatibility Response Subramanian Sankaranarayanan, Muhammad Jamshed, and Marcus A. Samuel* Department of Biological Sciences, University of Calgary, BI 392, 2500 University Drive Northwest, Calgary, Alberta T2N 1N4, Canada S Supporting Information *
ABSTRACT: Self-incompatibility (SI) in plants is a genetic mechanism that prevents self-fertilization and promotes out-crossing needed to maintain genetic diversity. SI has been classified into two broad categories: the gametophytic selfincompatibility (GSI) and the sporophytic self-incompatibility (SSI) based on the genetic mechanisms involved in ‘self’ pollen rejection. Recent proteomic approaches to identify potential candidates involved in SI have shed light onto a number of previously unidentified mechanisms required for SI response. SI proteome research has progressed from the use of isoelectric focusing in early days to the latest third-generation technique of comparative isobaric tag for relative and absolute quantitation (iTRAQ) used in recent times. We will focus on the proteome-based approaches used to study self-incompatibility (GSI and SSI), recent developments in the field of incompatibility research with emphasis on SSI and future prospects of using proteomic approaches to study selfincompatibility. KEYWORDS: self-incompatibility, proteomics, mass spectrometry, iTRAQ, 2-D-DIGE
1. INTRODUCTION Flowering plants have evolved complex mechanisms to avoid inbreeding to maintain genetic diversity. Self-incompatibility (SI) is a genetic mechanism that enables the pistil of a flower to distinguish self-pollen (incompatible) from cross-pollen (compatible) to reject the self-pollen. From genetic studies carried out in several plant species it has been concluded that SI in most species is controlled by a single multiallelic locus called the S-locus or S-haplotype, which has a minimum of two tightly linked, polymorphic genes that control the pollen and pistil identities. If the pollen and pistil share the same “S-allele”, then the outcome of interaction will be rejection of the respective self-pollen.1 SI and self-sterility occur widely among all lineages of angiosperms and is seen in 60% of all angiosperm species.2 The phenomenon of SI is of great importance to agriculture because it is used in the large-scale production of hybrid seeds that give rise to plants with hybrid vigor.3,4 Cross-pollinating two plants of different genetic backgrounds is required to produce hybrid seeds, and this laborious process involves manual removal of anthers from the female parent to prevent self-fertilization. Introduction of SI in compatible plants by introducing S-genes can facilitate hybrid seed production without the need for emasculation.5 Failure to set fruits in several fruits and vegetables has been associated with SI. To overcome this problem, farmers have to plant two different varieties (one of them is a pollen donor) in adjacent rows and this is wastage of land area.5 If SI could be broken in such crops it would greatly improve the efficiency of agricultural practices. Thus a better © 2013 American Chemical Society
understanding of SI mechanism can lead to improvements in crop production. Proteomic techniques have been widely used in studying SI, and they have expanded our understanding of this mechanism to a great extent. The use of new and powerful proteomic techniques can further improve the current understanding of SI mechanisms in plants. We will focus on how proteomic approaches have contributed to SI research. SI in plants is classified into two types: the gametophytic selfincompatibility (GSI) and the sporophytic self-incompatibility (SSI). In GSI, the pollen’s own haploid genome determines the outcome of interaction between the pollen and pistil, while in SSI, it is determined by the diploid genome of the parental plant.2 In the GSI system, partial compatibility can occur when the parents share one S-allele. However, in SSI, a similar situation would lead to an incompatible interaction, and for a compatible interaction to happen, both the S-alleles in the parents have to be different.1 Another major difference between them is that the inhibition of pollen tubes usually occurs on the stigmatic surface in SSI, whereas in GSI the pollen tube growth is arrested in the style.2 Special Issue: Agricultural and Environmental Proteomics Received: July 10, 2013 Published: September 18, 2013 4717
dx.doi.org/10.1021/pr400716r | J. Proteome Res. 2013, 12, 4717−4726
Journal of Proteome Research
Reviews
Figure 1. Solanaceae-type self-incompatibility (adapted from Takayama and Isogai, 2005, Goldraij et al., 2006).14,11
compartment.11−13 In compatible pollinations, the pollen avoids activity of S-RNases by degrading HT-B through a hypothetical pollen protein (PP), thus keeping S-RNases compartmentalized in the vacuole (Figure 1, top right).11 In incompatible pollinations, the self-interaction between S-RNase and SLF stabilizes HT-B by inhibiting the PP.11 The presence of stable HT-B leads to an unstable compartment and release of S-RNases that degrade the pollen RNA (Figure 1, bottom right).11
2. PRESENT UNDERSTANDING OF THE MOLECULAR MECHANISMS THAT REGULATE GAMETOPHYTIC AND SPOROPHYTIC SELF-INCOMPATIBILITY 2.1. Gametophytic Self-Incompatibility
The GSI operates in two distinct ways in plants, and they are broadly categorized as Solanaceae-type SI and Papaveraceaetype SI.6 In Solanaceae-type SI (Figure 1) (Solanaceae, Rosaceae, and Scrophulariaceae), SI is the result of the destruction of rRNA in the tube of self-pollen by S-RNAase, and this inhibits protein synthesis needed for the pollen tube to grow through the style.7,8 Whereas in Papaveraceae (poppy family), SI is brought about by a calcium-dependent signaling network, which results in a programmed cell death (PCD) of the growing pollen tube.9,10 Proteomic approaches have been widely employed to study Solanaceae-type incompatibility in the case of GSI, and hence we will focus our attention on Solanaceae-type incompatibility. Mechanism of Solanaceae-Type Self-Incompatibility. In Solanaceae type of incompatibility, SI is determined by the interaction between the S-RNase (female determinant) and the S-locus F-box gene (SLF or SFB) (male determinant),8 which reside in the S-locus. S-RNase encodes S-RNase, a glycoprotein with one or more glycan chains and is secreted in large amounts in the extracellular matrix (ECM) of the style after pollination and its role is of a cytotoxin that degrades pollen RNA (Figure 1, bottom left).8 The male determinant SLF/SFB is a member of F-box family of proteins that functions in ubiquitin-mediated proteasomal degradation of nonself RNases (Figure 1, top left). Once the “self” pollen tube grows trough the style, the SRNases degrade the RNA in the pollen tubes, preventing its growth, while in the case of “nonself” pollen, this process is prevented by the action of SLF, which targets S-RNases for proteasomal degradation (Figure 1, left).8 Goldraij et al. 2005 proposed an alternate model for GSI called the compartmentalization model (Figure 1, right) wherein the S-RNase is taken up from the ECM by endocytosis and sequestered to a vacuolar compartment in the pollen tubes.11 HT-B, a small asparaginerich protein expressed in a later stage of style development, is necessary for the release of S-RNases from this vacuolar
2.2. Sporophytic Self-Incompatibility in Brassicaceae
The Spororophytic incompatibility in Brassicaceae is controlled by two, linked, multiallelic loci, encoding SP11/SCR as the male determinant and S-locus receptor kinase (SRK) as the female determinant, respectively.15 SRK is linked to the S locus, and the SRK protein consists of a S-locus glycoprotein (SLG)like extracellular domain, a transmembrane domain, and an intracellular serine/threonine kinase domain.16 SRK is expressed exclusively in the papillary cells of the stigma, and its expression coincides with flower opening when the stigma acquires SI. SRK also exhibits allelic sequence diversity among all of the S-haplotypes, and lack of SRK resulted in breakdown of SI, which led to the hypothesis that SRK is the female determinant of SI.17,18 The male determinant was later identified as a small cysteine rich protein (
