Lessons on signalling in plant self-incompatibility systems - Intercellular Communication in Plants - page 247

the extracellular matrix of the stylar transmitting tissue, it may somehow mediate
the uptake of S-RNase into pollen tubes.
Group 3 factors are required for pollen rejection and also are essential for pollina-
tion and/or fertilisation. These characteristics make their identification problematic,
as null mutations are unlikely to be heritable. Perhaps as a result of this, Group 3
factors have not been demonstrated genetically. There is, however, some biochem-
ical evidence for the existence of such factors. A number of stylar proteins have
been identified that bind to S-RNase columns (McClure et al. , 2000). Most signif-
icantly, the protein NaTTS, the tobacco homologue of TTS (Wu et al. , 1995), was
identified (for discussion of all proteins identified, see Cruz-Garcia et al. , 2003).
There is evidence that TTS stimulates pollen tube growth in vitro , and in vivo TTS
is associated with the surface of pollen tubes and appears to be deglycosylated by
growing tubes (Wu et al. , 1995). TTS appears to fuel pollen tube growth and the
demonstration that it binds S-RNase suggests that it could conceivably fall into the
Group 3 category of SI modifiers.
10.2.5 Model for the operation of S-RNase-based SI
Currently two separate gene products have been established as being essential to
the process of signalling the pistil S -genotype to the pollen tubes. S-RNases encode
allelic specificity and possess RNase activity that is required for pollen rejection.
Self-pollen tubes are most likely inhibited through degradation of RNA (Huang
et al. , 1994), but which form(s) of RNA is the primary target is less clear. S-RNases
do not demonstrate detectable substrate specificity in vitro (Singh et al. , 1991),
though whether this is the case in vivo is not certain. Ribosomal RNA (rRNA) is a
prime potential target as rRNA genes are apparently not transcribed in pollen tubes
(McClure et al. , 1990; Mascarenhas, 1993). As a result, degradation of rRNA would
rapidly lead to an irreversible cessation of protein synthesis. Degradation of rRNA
in pollen tubes specific to SI pollinations has been reported (McClure et al. , 1990)
supporting this hypothesis, but as the tips of incompatible pollen tubes frequently
burst, spilling out their contents into an extracellular matrix with high concentrations
of S-RNase, it is not clear whether this is a cause or effect of SI. In addition, data has
been reported that suggests that SI may be reversible, at least in some pollen tubes.
By grafting the upper segment of a self-incompatibly pollinated pistil onto the lower
segment of a compatible pistil, it has been demonstrated that at least some of the
pollen tubes can recover from the effects of S-RNase (Lush & Clarke, 1997). This
suggests that either another class of RNA is the target of S-RNases, or if rRNA is
indeed the target, the general belief that rRNA genes cannot be transcribed in pollen
tubes is false. The second pistil component that is required for SI is the HT protein
(McClure et al. , 1999); the precise function of HT is not clear but it is envisaged
to be involved in a complex with S-RNases and to be required for their entry into
pollen tubes.
Recent confirmation that SLF is the gene that encodes pollen S -specificity, and
that transgenics expressing this gene behave in accordance with the phenomenon of