**6. The pistil modifier variables**

### **6.1. HT-B and glycoprotein**

nents of ubiquitin–ligase processes, which, along with ubiquitin- initiating enzymes and ubiquitin-conjugating enzymes, mediate protein degradation with the 26S proteosome. Experiments are underway, making use of transgenic plants to discover if SLF is definitely the

Predicted amino acid sequences frompollen SFB and SLF from all functional SI S haplotypes conserve the F-box motif at the Nterminal, indicating that they function as F-box proteins. The F-box protein is known to be a component of a class of E3 ubiquitin ligases, the SCF complex, which regulates protein degradation in the ubiquitin/proteasome proteolytic pathway [19]. The F-box protein functions as a receptor to incorporate proteins targeted for polyubiquitina‐ tion into the SCF complex. The polyubiquitinated targeted proteins are degraded by the 26S proteasome. It was, therefore, first proposed that the SCF complex that contains SFB (SCFSFB) and SLF (SCFSLF) might polyubiquitinate all nonself-SRNases for degradation but specifically interact with its cognate SRNase to leave it active, leading to the arrest of self-pollen tubes [56,

Biochemical experimental results supporting this hypothesis have been obtained with Solanaceae and Plantaginaceae. Coimmunoprecipitation and yeast two hybrid analyses showed that *Antirrhinum* SLF (AhSLF) physically interacted with S-RNase in a nonallelic fashion and polyubiquitination of S-RNase was observed after incubating pollen proteins with compatible but not with incompatible stylar proteins [100]. Furthermore, AhSLF was shown to interact with ASK1 and CUL1-like proteins, suggesting that AhSLF makes an SCF complex. Later, AhSSK1 (*A. hispanicum* SLF-interacting SKP1-like1) that may work as an adaptor of the putative SCFSLF was cloned [42]. In contrast to AhSLF, *Petunia* SLF preferentially interacts using its nonself-S-RNase rather than its self-S-RNase [42]. This can be unexpected because the interactions of an allelic product in the pollen S using its self-S-RNase were long viewed as thermodynamically favored over the interactions using its nonself- S-RNase [56]. It would appear that *Petunia* SLF includes a domain that functions of S-RNase presenting (FD2) and two fields that control S-RNase-binding (FD1 and FD3) [41]. The FD1 and FD3 fields are shown to weaken the strong interaction involving FD2 and also S-RNase while in self-interactions,

bringing about preferential connection and polyubiquitination of nonself-S-RNase.

might be a different molecule versus SRNase.

**5. Genes that modulate the real SI reaction**

The existence of different mechanisms of self-recognition in *Prunus* GSI can be suggested by means of molecular analyses of SC PPM S haplotypes as well as the SC/SI behavior in poly‐ ploids in *Prunus*. In contrast to the sign that the substrate of SCFSLF can be nonself-S-RNase in Solanaceae and Plantaginaceae, there are many indications that the substrate of SCFSFB

Although the real S-locus encodes real determinants of S-haplotype specificity, there is evidence for the existence of other unlinked body's genes, termed modifier genes which can be required for SI result (reviewed in [16, 56, 81-84,], which is to be discussed in this article.

pollen S-gene.

174 Plants for the Future

126-128].

HT-B protein, a smaller asparagine-rich protein expressed late in style development, was initially identified by differential cDNA hybridization to screen fashion genes which were expressed in self-incompatible kinds such as *N. alata* however, but not in *N. plumbaginifolia*, any closely related self-compatible kinds [84]. Homologs of HT-B have also been recognized in other genera in the Solanaceae, *Lycopersicon* and *Solanum* [61, 94]. Down-regulation of HT-B appearance by anti-sense shift and RNAi was starving the transformants of the chance to reject self-pollen, suggesting that it is essential for SI [85, 94]. Within a comparative research of self-incompatible and self-compatible taxa of *Lycopersicon*, the appearance of HT-B gene had not been detected in all the self-compatible taxa [61]. Given that no direct interaction of HT-B and S-RNases has been detected, the complete role of HT-B of SI result was not yet determined until a newly released immunolocalization research [25].

This sort of revealed that in self-pollen pipes, HT-B was more likely to help S-RNase move from a great endomembrane compartment for the cytoplasm, where they might exert cyto‐ toxicity, bringing about the arrest of pollen tube growth, when it is in compatible pollen pipes; the HT-B amount was appreciably down-regulated as well as the S-RNases were compart‐ mentalized [25].

The 120 kDa glycoprotein (120K) is an abundant protein inside the stylar ECM and taken up by the real growing pollen pipes [71]. This 120K protein binds to S-RNase in vitro and, like HT-B, reductions of its expression by means of RNAi stopped self-pollen rejection [16, 33]. With recent immunolocalization trials, antibodies for the 120 kDa glycoprotein were found to label the real compartment tissue layer that enters the S-RNases inside the pollen pipes. However, given that S-RNase uptake is usual in 120K protein defective plant life; its specific role in SI continues to evade researchers [25].
