S-RNase gene of Nicotiana alata is expressed in developing pollen

Ectopic expression of S-RNase of Petunia inflata in pollen results in its sequestration and non-cytotoxic function

The specificity of S-RNase-based self-incompatibility (SI) is controlled by two S-locus genes, the pistil S-RNase gene and the pollen S-locus-F-box gene. S-RNase is synthesized in the transmitting cell; its signal peptide is cleaved off during secretion into the transmitting tract; and the mature "S-RNase", the subject of this study, is taken up by growing pollen tubes via an as-yet unknown mechanism. Upon uptake, S-RNase is sequestered in a vacuolar compartment in both non-self (compatible) and self (incompatible) pollen tubes, and the subsequent disruption of this compartment in incompatible pollen tubes correlates with the onset of the SI response. How the S-RNase-containing compartment is specifically disrupted in incompatible pollen tubes, however, is unknown. Here, we circumvented the uptake step of S-RNase by directly expressing S(2)-RNase, S(3)-RNase and non-glycosylated S(3)-RNase of Petunia inflata, with green fluorescent protein (GFP) fused at the C-terminus of each protein, in self (incompatible) and non-self (compatible) pollen of transgenic plants. We found that none of these ectopically expressed S-RNases affected the viability or the SI behavior of their self or non-self-pollen/pollen tubes. Based on GFP fluorescence of in vitro-germinated pollen tubes, all were sequestered in both self and non-self-pollen tubes. Moreover, the S-RNase-containing compartment was dynamic in living pollen tubes, with movement dependent on the actin-myosin-based molecular motor system. All these results suggest that glycosylation is not required for sequestration of S-RNase expressed in pollen tubes, and that the cytosol of pollen is the site of the cytotoxic action of S-RNase in SI.

Identification of a S-ribonuclease-binding protein in Petunia hybrida

To investigate protein-protein interactions in gametophytic self-incompatibility, we used a yeast two-hybrid assay to identify proteins that could interact with the S-ribonuclease protein. These assays identified a pollen-expressed protein, which we have named PhSBP1, that appears to bind with a high degree of specificity to the Petunia hybrida S-ribonuclease. Although PhSBP1 activates reporter gene expression only when expressed in tandem with a S-RNAse bait protein, binding is not allele-specific. Sequence analysis demonstrated that PhSBP1 contained a C-terminal cysteine-rich region that includes a RING-HC domain. Because many RING-finger domain proteins appear to function as E3 ubiquitin ligases, our results suggest that ubiquitination and protein degradation may play a role in regulating self-incompatibility interactions. Together, these results suggest that PhSBPI may be a candidate for the recently proposed general inhibitor (RI) of self-incompatibility ribonucleases.

Pollen tubes of Nicotiana alata express two genes from different beta-glucan synthase families

The walls deposited by growing pollen tubes contain two types of beta-glucan, the (1,3)-beta-glucan callose and the (1,4)-beta-glucan cellulose, as well as various alpha-linked pectic polysaccharides. Pollen tubes of Nicotiana alata Link et Otto, an ornamental tobacco, were therefore used to identify genes potentially encoding catalytic subunits of the callose synthase and cellulose synthase enzymes. Reverse transcriptase-polymerase chain reactions (RT-PCR) with pollen-tube RNA and primers designed to conserved regions of bacterial and plant cellulose synthase (CesA) genes amplified a fragment that corresponded to an abundantly expressed cellulose-synthase-like gene named NaCslD1. A fragment from a true CesA gene (NaCesA1) was also amplified, but corresponding cDNAs could not be identified in a pollen-tube library, consistent with the very low level of expression of the NaCesA1 gene. RT-PCR with pollen-tube RNA and primers designed to regions conserved between the fungal FKS genes [that encode (1,3)-beta-glucan synthases] and their presumed plant homologs (the Gsl or glucan-synthase-like genes) amplified a fragment that corresponded to an abundantly expressed gene named NaGsl1. A second Gsl gene detected by RT-PCR (NaGsl2) was expressed at low levels in immature floral organs. The structure of full-length cDNAs of NaCslD1, NaCesA1, and NaGsl1 are presented. Both NaCslD1 and NaGsl1 are predominantly expressed in the male gametophyte (developing and mature pollen and growing pollen tubes), and we propose that they encode the catalytic subunits of two beta-glucan synthases involved in pollen-tube wall synthesis. Different beta-glucans deposited in one cell type may therefore be synthesized by enzymes from different gene families.

Cellular and molecular mechanisms of sexual incompatibility in plants and fungi

Plants and fungi show an astonishing diversity of mechanisms to promote outbreeding, the most widespread of which is sexual incompatibility. Sexual incompatibility involves molecular recognition between mating partners. In fungi and algae, highly polymorphic mating-type loci mediate mating through complementary interactions between molecules encoded or regulated by different mating-type haplotypes, whereas in flowering plants polymorphic self-incompatibility loci regulate mate recognition through oppositional interactions between molecules encoded by the same self-incompatibility haplotypes. This subtle mechanistic difference is a consequence of the different life cycles of fungi, algae, and flowering plants. Recent molecular and biochemical studies have provided fascinating insights into the mechanisms of mate recognition and are beginning to shed light on evolution and population genetics of these extraordinarily polymorphic genetic systems of incompatibility.

A molecular description of mutations affecting the pollen component of the Nicotiana alata S locus

Mutations affecting the self-incompatibility response of Nicotiana alata were generated by irradiation. Mutants in the M1 generation were selected on the basis of pollen tube growth through an otherwise incompatible pistil. Twelve of the 18 M1 plants obtained from the mutagenesis screen were self-compatible. Eleven self-compatible plants had mutations affecting only the pollen function of the S locus (pollen-part mutants). The remaining self-compatible plant had a mutation affecting only the style function of the S locus (style-part mutant). Cytological examination of the pollen-part mutant plants revealed that 8 had an extra chromosome (2n + 1) and 3 did not. The pollen-part mutation in 7 M1 plants was followed in a series of crosses. DNA blot analysis using probes for S-RNase genes (encoding the style function of the S locus) indicated that the pollen-part mutation was associated with an extra S allele in 4 M1 plants. In 3 of these plants, the extra S allele was located on the additional chromosome. There was no evidence of an extra S allele in the 3 remaining M1 plants. The breakdown of self-incompatibility in plants with an extra S allele is discussed with reference to current models of the molecular basis of self-incompatibility.

A relic S-RNase is expressed in the styles of self-compatible Nicotiana sylvestris

We surveyed ribonuclease activity in the styles of Nicotiana spp. and found little or no activity in self-compatible species and in a self-compatible accession of a self-incompatible species. All self-incompatible species had high levels of ribonuclease activity in their style. Interestingly, one self-compatible species, N. sylvestris, had a level of stylar ribonuclease activity comparable to that of some self-incompatible Nicotiana species. A ribonuclease with biochemical properties similar to those of the self-incompatibility (S-)RNases of N. alata was purified from N. sylvestris styles. The N-terminal sequence of this protein was used to confirm the identity of a cDNA corresponding to the stylar RNase. The amino acid sequence deduced from the cDNA was related to those of the S-RNases and included the five conserved regions characteristic of these proteins. It appears that the N. sylvestris RNase may have evolved from the S-RNases and is an example of a 'relic S-RNase'. A number of features distinguish the N. sylvestris RNase from the S-RNases, and the role these may have played in the presumed loss of the self-incompatibility response during the evolution of this species are discussed.

Self-incompatibility and other pollen-pistil interactions

Self-incompatibility allows plants to recognize and reject pollen from the same plant, thereby reducing inbreeding. Although in most cases self-incompatibility is controlled by a single genetic locus, recent results show that surprisingly complex signal transduction pathways and many players are involved in pollen recognition and rejection.

How flowering plants discriminate between self and non-self pollen to prevent inbreeding

Flowering plants have evolved various genetic mechanisms to circumvent the tendency for self-fertilization created by the close proximity of male and female reproductive organs in a bisexual flower. One such mechanism is gametophytic self-incompatibility, which allows the female reproductive organ, the pistil, to distinguish between self pollen and non-self pollen; self pollen is rejected, whereas non-self pollen is accepted for fertilization. The Solanaceae family has been used as a model to study the molecular and biochemical basis of self/non-self-recognition and self-rejection. Discrimination of self and non-self pollen by the pistil is controlled by a single polymorphic locus, the S locus. The protein products of S alleles in the pistil, S proteins, were initially identified based on their cosegregation with S alleles. S proteins have recently been shown to indeed control the ability of the pistil to recognize and reject self pollen. S proteins are also RNases, and the RNase activity has been shown to be essential for rejection of self pollen, suggesting that the biochemical mechanism of self-rejection involves the cytotoxic action of the RNase activity. S proteins contain various numbers of N-linked glycans, but the carbohydrate moiety has been shown not to be required for the function of S proteins, suggesting that the S allele specificity determinant of S proteins lies in the amino acid sequence. The male component in self-incompatibility interactions, the pollen S gene, has not yet been identified. The possible nature of the pollen S gene product and the possible mechanism by which allele-specific rejection of pollen is accomplished are discussed.

Molecular characterisation of an S-like RNase of Nicotiana alata that is induced by phosphate starvation

We characterised a cDNA encoding an S-like RNase (RNase NE) from the styles of the self-incompatible plant, Nicotiana alata. RNase NE is 86% identical to an extracellular RNase from tomato cell cultures, RNase LE. DNA hybridisation experiments indicate that there are ca. 5-6 sequences related to RNase NE in the N. alata genome and that RNase NE is not linked to the self-incompatibility (S) locus. RNase NE is expressed in the styles, petals and immature anthers but not in the vegetative tissues of N. alata plants under normal growth conditions. Under phosphate-limited conditions, RNase NE expression is induced in roots but not leaves of N. alata. A transcript hybridising to RNase NE is also induced in N. plumbaginifolia cell cultures in response to phosphate starvation. RNase NE is likely to play a role in the response of N. alata to phosphate limitation, possibly by scavenging phosphate from sources of RNA in the root environment. We also discuss the evolutionary relationships between the S- and S-like RNase genes in plants.

A retrotransposon-like sequence linked to the S-locus of Nicotiana alata is expressed in styles in response to touch

We have identified a family of repetitive sequences in the genome of Nicotiana alata named Tna1 (Transposon of N. alata). The first element we characterised was a genomic clone for the N. alata s6-ribonuclease (S6-RNase), a gene required for self-incompatibility in this species. The DNA sequence of this element resembles the integrase domain of retrotransposons of the gypsy class and is most similar to a retrotransposon from Lilium henryi. A transcript present in N.alata styles (self-incompatibility genotype S6S6) hybridized to Tna1 and accumulated in the style following either pollination or touching. This transcript was cloned from a cDNA library and was encoded by second, partial Tna1 elements. Neither the transcribed sequence nor the original Tna1 element contain an open reading frame or is likely to be able to transpose. The second element was mapped using a population of N.alata plants segregating for alleles of the self-incompatibility locus and is closely linked to the S6-allele. The Tna1 element is present in a number of Nicotiana species and appears to have been active at least twice during the evolution of this genus.

The S-locus of Nicotiana alata: genomic organization and sequence analysis of two S-RNase alleles

Genomic clones encoding the S2- and S6-RNases of Nicotiana alata Link and Otto, which are the allelic stylar products of the self-incompatibility (S) locus, were isolated and sequenced. Analysis of genomic DNA by pulsed-field gel electrophoresis and Southern blotting indicates the presence of only a single S-RNase gene in the N. alata genome. The sequences of the open-reading frames in the genomic and corresponding cDNA clones were identical. The organization of the genes was similar to that of other S-RNase genes from solanaceous plants. No sequence similarity was found between the DNA flanking the S2- and S6-RNase genes, despite extensive similarities between the coding regions. The DNA flanking the S6-RNase gene contained sequences that were moderately abundant in the genome. These repeat sequences are also present in other members of the Nicotianae.

An S-RNase promoter from Nicotiana alata functions in transgenic N. alata plants but not Nicotiana tabacum

Nicotiana tabacum and Nicotiana alata plants were transformed with genomic clones of two S-RNase alleles from N. alata. Neither the S2 clone, with 1.6 kb of 5' sequence, nor the S6 clone, with 2.8 kb of 5' sequence, were expressed at detectable levels in transgenic N. tabacum plants. In N. alata, expression of the S2 clone was not detected, however the S6 clone was expressed (at low levels) in three out of four transgenic plants. An S6-promoter-GUS fusion gene was also expressed in transgenic N. alata but not N. tabacum. Although endogenous S-RNase genes are expressed exclusively in floral pistils, the GUS fusion was expressed in both styles and leaves.

Cloning a putative self-incompatibility gene from the pollen of the grass Phalaris coerulescens

In Phalaris coerulescens, gametophytic self-incompatibility is controlled by two unlinked genes: S and Z. A probable S gene has now been isolated and sequenced. This represents a novel self-incompatibility gene isolated from pollen in the multilocus system of a monocotyledonous plant. The gene is approximately 3 kb long, split by five introns, and exclusively expressed in the mature pollen. The deduced amino acid sequences from the S1, S2, and part of the S4 alleles showed that the protein has a variable N terminus and a conserved C terminus. The sequence of a complete mutant at the S locus indicated that mutations in the conserved C terminus, a thioredoxin-like region, led to loss of function. We propose that the gene has two distinct sections, a variable N terminus determining allele specificity and a conserved C terminus with the catalytic function. The gene structure and its deduced protein sequences strongly suggest that this monocotyledon has developed a self-incompatibility system entirely different from those operating in the dicotyledons. The possible interactions between S and Z genes in both pollen and stigma are discussed.

The S-ribonuclease gene of Petunia hybrida is expressed in nonstylar tissue, including immature anthers

To determine the ability of isolated S-locus promoter sequences to direct organ-specific gene expression, we used microprojectile bombardment to introduce chimeric S-allele/beta-glucuronidase genes into different tissues of Petunia hybrida for transient expression. Histochemical staining showed that S-locus/beta-glucuronidase fusions were expressed in pistil, ovary, and petal tissue. No expression of the chimeric genes was detected in leaves or in mature pollen, either by histochemical staining or by fluorescence assays. RNA blot hybridization confirmed that low levels of S-locus mRNA accumulate in petals and ovaries in vivo. Analysis of the expression pattern of S-locus promoter deletions showed that sequences in the immediate vicinity of the TATA box were sufficient to confer qualitatively correct organ-specific expression of beta-glucuronidase. To further investigate the potential for S-ribonuclease expression in pollen, we used the polymerase chain reaction to amplify RNA accumulated in developing anthers. These assays demonstrated that mRNA for the S-ribonuclease accumulates to low levels in developing anthers several days prior to corolla opening and pollen anthesis. We discuss these results in light of current models of self-incompatibility.

Self-compatibility in aLycopersicon peruvianum variant (LA2157) is associated with a lack of style S-RNase activity

A series of crosses between a naturally-occurring self-compatible accession ofLycopersicon peruvianum and a closely-related self-incompatible accession were used to demonstrate that the mutation to self-compatibility is located at the S-locus. Progeny of the crosses contain abundant style proteins of about 30 kDa that segregate with the S6and S7-alleles from the SI parent and the Sc-allele from the SC parent. The S6and S7-associated proteins have ribonuclease activity whereas the Sc-associated protein is not an active ribonuclease. This finding indicates that S-RNases are determinants of self-incompatibility in the style and that the ribonuclease activity is essential for their function.

Loss of a histidine residue at the active site of S-locus ribonuclease is associated with self-compatibility in Lycopersicon peruvianum

Gametophytic self-incompatibility in the Solanaceae is controlled by a single, multiallelic locus, the S locus. We have recently described an allele of the S locus of Lycopersicon peruvianum that caused this normally self-incompatible plant to become self-compatible. We have now characterized two glycoproteins present in the styles of self-compatible and self-incompatible accessions of L. peruvianum: one is a ribonuclease that cosegregates with a functional self-incompatibility allele (S6 allele); the other cosegregates with the self-compatible allele (Sc allele) but has no ribonuclease activity. The derived amino acid sequences of the cDNAs encoding the S6 and Sc glycoproteins resemble sequences of other ribonucleases encoded by the S locus. The derived sequence for the Sc glycoprotein differs from the others by lacking one of the histidine residues found in all other S-locus ribonucleases. These findings demonstrate the essential role of ribonuclease activity in self-incompatibility and lend further weight to evidence that this histidine residue is involved in the catalytic site of the enzyme.

Plant self-incompatibility. The key to specificity

Experiments with transgenic plants provide strong evidence that the locus controlling the female side of recognition in self-incompatibility has been correctly identified, and supoort the view that a ribonuclease is involved.

Self-incompatibility: how plants avoid illegitimate offspring

In some families of flowering plants, a single self-incompatibility (S) locus prevents the fertilization of flowers by pollen from the same plant. Self-incompatibility of this type involves the interaction of molecules produced by the S locus in pollen with those present in the female tissues (pistil). Until recently, the pistil products of the S locus were known in only two families, the Brassicaceae (which includes the cabbages and mustards) and Solanaceae (potatoes and tomatoes). A paper in this issue of the Proceedings describes the molecules associated with self-incompatibility in a third family, the Papaveraceae (poppies). We review current research on self-incompatibility in these three families and discuss the implications of the latest findings in poppy on the likely evolution of self-incompatibility in flowering plants. We also compare research into self-incompatibility with recent progress in understanding the mechanisms by which plants overcome infection by certain pathogens.