Self-incompatibility: how plants avoid illegitimate offspring

Disulphide bonding in a stylar self-incompatibility ribonuclease of Nicotiana alata

Many flowering plants have developed a self-incompatibility mechanism, which is controlled by a single polyallelic locus (the S-locus), to prevent inbreeding. The products of the S-locus in the styles of solanaceous plants are an allelic series of glycoproteins with RNase activity [McClure, B. A., Haring, V., Ebert, P. R., Anderson, M. A., Simpson, R. J., Sakiyama, F. & Clarke, A. E. (1989) Nature 342, 955-957]. These S-RNases show some amino-acid-sequence similarity with two fungal RNases (T2 and Rh), including the presence of two active-site His residues, which suggests a common three-dimensional structure. Disulphide bonding is important in the maintenance of the three-dimensional structure of the fungal RNases [Kurihara, H., Mitsui, Y., Ohgi, K., Irie, M., Mizuno, H. & Nakamura, T. (1992) FEBS Lett. 306, 189-192] and the S-RNases [Tsai, D. S., Lee, H.-S., Post, L. C., Kreiling, K. M. & Kao, T.-H. (1992) Sex. Plant Reprod. 5, 256-263]. We have used the S2-allele RNase of Nicotiana alata, which has nine Cys residues, to establish the pattern of disulphide bonding. The disulphide bonds Cys16-Cys21, Cys45-Cys94, Cys153-Cys182 and Cys165-Cys176 are consistent with the S2-RNase having a similar three-dimensional structure to RNase Rh. A free Cys residue (Cys95) adjacent to Cys45-Cys94 promotes a rapid specific disulphide migration when the protein is exposed to denaturing conditions.

Does the genome of Corylus avellana L. contain sequences homologous to the self-incompatibility gene of Brassica?

Self-incompatibility is a genetic mechanism enforcing cross-pollination in plants. Hazelnut (Corylus avellana L.) expresses the sporophytic type of self-incompatibility, for which the molecular genetic basis is characterized only in Brassica. The hypothesis that the hazelnut genome contains homologs of Brassica self-incompatibility genes was tested. The S-locus glycoprotein gene (SLG) and the kinase-encoding domain of the S-receptor kinase (SRK) gene of B. oleracea L. were used to probe blots of genomic DNA from six genotypes of hazelnut. Weak hybridization with the SLG probe was detected for all hazelnut genotypes tested; however, no hybridization was detected with PCR-generated probes corresponding to two conserved regions of the SLG gene. One of these PCR probes included the region of SLG encoding the 11 invariant cysteine residues that are an important structural feature of all S-family genes. The present evidence suggests that hazelnut DNA hybridizing to SLG differs significantly from the Brassica gene, and that the S-genes cloned from Brassica will not be useful for exploring self-incompatibility in hazelnut.

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.

Self-incompatibility (S) alleles of the Rosaceae encode members of a distinct class of the T2/S ribonuclease superfamily

Stylar ribonucleases (RNases) are associated with gametophytic self-incompatibility in two plant families, the Solanaceae and the Rosaceae. The self-incompatibility-associated RNases (S-RNases) of both the Solanaceae and the Rosaceae were recently reported to belong to the T2 RNase gene family, based on the presence of two well-conserved sequence motifs. Here, the cloning and characterization of S-RNase genes from two species of Rosaceae, apple (Malus x domestica) and Japanese pear (Pyrus serotina) is described and these sequences are compared with those of other T2-type RNases. The S-RNases of apple specifically accumulated in styles following maturation of the flower bud. Two cDNA clones for S-RNases from apple, and PCR clones encoding a further two apple S-RNases as well as two Japanese pear S-RNases were isolated and sequenced. The deduced amino acid sequences of the rosaceous S-RNases contained two conserved regions characteristic of the T2/S-type RNases. The sequences showed a high degree of diversity, with similarities ranging from 60.4% to 69.2%. Interestingly, some interspecific sequence similarities were higher than those within a species, possibly indicating that diversification of S-RNase alleles predated speciation in the Rosaceae. A phylogenetic tree of members of the T2/S-RNase superfamily in plants was obtained. The rosaceous S-RNases formed a new lineage in the tree that was distinct from those of the solanaceous S-RNases and the S-like RNases. The findings suggested that self-incompatibility mechanisms in Rosaceae and Solanaceae are similar but arose independently in the course of evolution.

Antisense suppression of S-RNase expression in Nicotiana using RNA polymerase II- and III-transcribed gene constructs

In the Solanaceae, self-incompatibility is controlled by a single, multi-allelic ('S') locus. One product of this locus is a ribonuclease, the S-RNase, which is expressed predominantly in mature pistils and has recently been shown to cause allele-specific pollen rejection in transgenic plants. Hybrid Nicotiana plumbaginifolia x N. alata plants were used to test the effects of antisense suppression of the SA2-RNase from N. alata using three different gene constructs: two driven by RNA polymerase II-transcribed promoters, and the third, containing a truncated soybean tRNA (met-i) gene, transcribed by RNA polymerase III. All three constructs caused suppression of S-RNase activity in the transgenic plants. Unexpectedly, the CaMV 35S promoter was more effective for antisense suppression than the tissue specific tomato ChiP promoter. Antisense suppression of S-RNase correlated with low sense SA2 transcript levels and high antisense SA2 transcript levels. Untransformed hybrids that contained the N. alata SA2 allele were incompatible with N. alata SA2 pollen, while transgenic plants with suppressed SA2 gene expression accepted the pollen. The utility of this hybrid plant system for studying some aspects of antisense gene suppression is discussed.

Self-incompatibility in flowering plants

Fertilization in flowering plants begins with a pollen grain bearing the male gametes landing on the female stigma. Several mechanisms enable the stigma to discriminate between the different types of pollen that it may receive, of which the best studied is self-incompatibility. The molecules that regulate self-incompatibility are well characterized in two plant families, the Solanaceae and Brassicaceae. This list has recently been extended to include candidates for self-incompatibility molecules from the Rosaceae, Papaveraceae and Poaceae. The information provided by the sequences of these molecules gives insight into the mechanisms and evolution of self-incompatibility in the different families of flowering plants.

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.

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.