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to anthesis (immature pistil are self-compatible) but rapidly accumulate to 1-10%
of total protein at pollen release. These proteins were initially termed S-proteins and
have been estimated to reach concentrations of 10-50 mg/mL in the extracellular
matrix of the stylar transmitting tract of solanaceous species (Jahnen et al. , 1989).
10.2.1 S-RNases encode S-specificity in the pistil
The first gene encoding an S-protein was cloned from Nicotiana alata (Anderson
et al. , 1986) and currently the sequences of more than 70 alleles from a variety of
species belonging to the Solanaceae, Rosaceae and Scrophulariaceae have been re-
ported. S-proteins are highly polymorphic with amino acid identity ranging from 38
to 98%. Nonetheless, sequence comparisons have identified five regions of conser-
vation, named C1 to C5 (Ioerger et al. , 1991; Tsai et al. , 1992). Of these, two (C2 and
C3) share significant sequence similarity with the corresponding regions of fungal
ribonucleases (RNases), RNase T2 (Kawata et al. , 1988) and RNase Rh (Horiuchi
et al. , 1988), a similarity that rapidly led to the discovery that S-proteins are them-
selves RNases (McClure et al. , 1989) and resulted in their renaming as S-RNases.
Direct confirmation of the involvement of S-RNases in SI has been achieved
through the application of transgenic methodologies in Petunia , Nicotiana and
Solanum (Lee et al. , 1994; Murfett et al. , 1994; Matton et al. , 1997). Expression
of a novel S-RNase or repression of a native S-RNase in transgenic plants causes
again or loss of S -specificity, respectively (Lee et al. , 1994; Murfett et al. , 1994).
These studies also demonstrate that the extremely high levels of S-RNase found
in wild-type pistils are necessary for pollen rejection to be complete. These data
are sometimes interpreted as demonstrating the S-RNases that are necessary and
sufficient for SI in the pistil. However, genes not residing at the S -locus have been
shown to affect the SI response (McClure et al. , 1999) and the presence or absence
of active copies of these genes can vary between genetic backgrounds. As a result it
can be concluded that S-RNases are necessary for SI and encode pistil S -specificity
but are not always sufficient for SI.
10.2.2
S-RNase structure/function
The functional involvement of RNase activity in the action of S-RNases in SI was
initially inferred from the study of a self-compatible S -allele in Lycoperiscon peru-
vianum .Inthis allele, one of two histidine residues essential for catalytic activity
was found to be mutated, causing a loss of RNase activity (Royo et al. , 1994).
In the absence of a functional allele of the same specificity and knowledge of the
functionality of the pollen component of the interaction, conclusive evidence that
loss of SI was caused by the loss of RNase activity was lacking. This evidence has
been obtained using site-directed mutagenesis to change a single codon for one of
the two catalytically essential histidine residues to encode for asparagine, gener-
ating a mutant S-RNase lacking RNase activity, and transforming this gene into
plants (Huang et al. , 1994). This experiment showed that production of this mutant
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