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.

Glycoprotein E2 of classical swine fever virus: expression in insect cells and identification as a ribonuclease

Two regions of amino acids homologous to the ribonuclease catalysis domain of the fungal RNases T2 of Aspergillus oryzae and Rh of Rhizopus niveus and the plant S-glycoproteins of Nicotiana alata are perfectly conserved in the amino acid sequence of the envelope glycoprotein E2 of classical swine fever virus (CSFV). To analyze the functional significance of these conserved sequences, the gene encoding E2 was inserted into the p10 locus of baculovirus and expressed in insect cells. Recombinant virus BacCE2 generated a protein which was similar in size (42 to 46 kDa) to wild-type E2 synthesized in swine kidney cells infected with CSFV. Recombinant E2 was purified by immunoaffinity chromatography from the lysate of cells infected with BacCE2 and assayed for RNase activity. RNase activity coeluted with the E2 fraction, indicating that ribonuclease activity is an inherent property of E2. The ribonuclease-specific activity of the protein fraction containing pure E2 was comparable to that of the N. alata S-glycoproteins.

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.

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

In the solanaceous plant Nicotiana alata, self-incompatibility is controlled by a single, multiallelic locus (S locus) expressed in both pollen and pistil. Previously, we have shown cosegregation between alleles of the S locus and alleles of a gene that encodes a glycoprotein with ribonuclease activity (S-RNase). Furthermore, expression of the S-RNase gene is apparently confined to the pistil and is correlated with the onset of self-incompatibility. In this paper, we report that the S-RNase gene is also expressed at low levels in developing pollen. A transcript in developing pollen hybridized to a cDNA encoding the S2-RNase allele of the parent plant and did not hybridize to cDNAs encoding other S-RNase alleles. Two cDNAs for the S2-RNase were cloned from a library derived from anthers of a plant homozygous for the S2 allele and both corresponded to the coding sequence of the S2-RNase. The product of the S-RNase gene was detected by immunocytochemistry in the intine of mature, hydrated pollen grains. These results are interpreted in the light of current knowledge of the structure of the S locus.

Molecular aspects of self-incompatibility in flowering plants

It is seven years since the first reports of cDNAs encoding pistil glycoproteins that segregated with particular S-alleles. During this time, the S-glycoproteins of the Solanaceae have been identified as RNases. This enzymatic activity relies on the presence of histidine residues at the putative active site of the RNase, and these are conserved in all S-glycoproteins so far characterized. The proteins also contain "hypervariable" regions that may have some role in allelic specificity. It is particularly interesting that putative S-glycoproteins from Japanese pear, which is from a different family, the Rosaceae, are also RNases. To counter the temptation to extrapolate to other families with gametophytic self-incompatibility, there is evidence that in another family, the Papaveraceae, poppy S-glycoproteins are not RNases. The current evidence is consistent with a process in which the S-RNase moves into the incompatible pollen tube and degrades RNA, including rRNA. As rRNA genes are not transcribed in pollen, the resulting degradation would lead to the death of the cell. But still we are left with some important gaps in our knowledge. How does the S-RNase move across the wall and membrane and into the pollen tube? How is the specificity of the interaction controlled? What is the mechanism of signal transduction? A major bottleneck in unraveling the story is understanding the nature of the S-locus product in pollen. Is it related to the stylar S-locus product or is it the product of a different gene in the same locus? Each question underlines the sketchiness of our knowledge of many plant processes that are not specific to pollination, but that we need to understand if we are to work out the details of self-incompatibility. For example, we have a very incomplete understanding of cell wall synthesis generally and pollen wall synthesis in particular. How do macronutrients move through cell walls to the cytoplasm of cells generally and pollen tubes in particular? What is the nature of the receptor-ligand interaction in plant cells generally and pollen tubes in particular? A similar range of questions and gaps in our knowledge exist in the sporophytic system, exemplified by studies in Brassica spp. In this case, we have no known enzymatic or other function for the stigmatic S-glycoproteins. We do, however, know that the S-locus in Brassica includes at least two genes, one encoding a S-glycoprotein and the other encoding a protein kinase.(ABSTRACT TRUNCATED AT 400 WORDS)