Agriculture Reference
In-Depth Information
tube growth even in the absence of S-RNases. This could be assessed by generating
plants in which
SLF
is 'knocked out' by transformation with an antisense transgene.
These plants could then be used to pollinate both wild-type plants and plants in
which both alleles of S-RNase are 'knocked out' by antisense or co-suppression,
and determining whether the antisense SLF transgene is present in either of the re-
sulting progenies. Such an experiment is currently in progress (T.-h. Kao, personal
communication, 2004).
10.2.4
Non-S-linked components of S-RNase-based SI
Although S-RNase and SLF have been established as being the specificity-encoding
components of this system, various studies, particularly of the breakdown of SI, pro-
vide evidence that additional factors are involved. As these are not directly involved
in specificity there is no requirement for them to be located at the
S
-locus and,
indeed, there is considerable genetic evidence that otherwise functional
S
-loci lose
their ability to cause pollen rejection in certain genetic backgrounds (Martin, 1968;
Ai
et al.
, 1991; Bernatzky
et al.
, 1995). The loci responsible for this breakdown
have been termed 'modifier' loci and three separate classes have been designated
(McClure
et al.
, 2000).
Group 1 factors directly affect the expression of
S
-locus genes at the level of tran-
scription, translation or post-translational modification. For example, in a particular
background, self-fertility has been shown to result from the lack of transcription
of the otherwise functional
S
13
allele (and only this allele) of
Petunia axillaris
,
though the precise mechanism and the gene(s) responsible have not been identified
(Tsukamoto
et al.
, 1999, 2003).
Group 2 factors are required for pollen rejection, but do not directly affect
the
S
-locus products and are not essential to pollination and/or fertilisation. Self-
compatible cultivars of
Petunia hybrida
(Ai
et al.
, 1991) and interspecies hybrids of
Lycopersicon hirsutum
esculentum
(Bernatzky
et al.
, 1995) possess and express
apparently functional S-RNases and apparently possess genes or alleles that rep-
resent Group 2 modifiers (Cruz-Garcia
et al.
, 2003). One gene encoding a Group
2 modifier, designated
HT
, has been identified by differentially screening pistil
cDNAs expressed in
N
. (SI) and
N. plumbaginifolia
(SC) (McClure
et al.
, 1999).
Confirmation that this gene is essential for the SI response was achieved in an an-
tisense experiment, where down-regulation of
HT
expression was shown to cause
self-compatibility without affecting S-RNase expression (McClure
et al.
, 1999).
The role of
HT
is unclear, as it does not have significant homology to any protein
of known function. Homologues of this gene have also been identified and found to
modify SI in
Lycopersicon
and
Solanum
(Kondo
et al.
, 2002). A putative N-terminal
secretion signal and an ND domain comprising a stretch of asparagine and aspartate
residues near the C-terminus are conserved between the three homologues, but the
overall amino acid identity excluding the secretion signal is only
×
32% (McClure
et al.
, 1999; Kondo
et al.
, 2002). Although the mode of action of
HT
is unknown,
as the HT protein must be present for SI to act and also appears to be secreted into
∼
