Involvement of the ubiquitin-proteasome proteolytic pathway in pollen tube growth in kiwifruit

De novo protein synthesis is needed for kiwifruit pollen germination: therefore, it is reasonable to assume that regulated protein turnover accompany biosynthetic activities of growing pollen tubes. The 26S proteasome is a multicatalytic complex which acts as primary protease of the ubiquitin-mediated proteolytic pathway in eucaryotes. Here we report data indicating the key role played by the proteasome in kiwifruit pollen tube growth. Immunoblotting experiments revealed the presence of high levels of free ubiquitin and ubiquitin conjugates in germinating kiwifruit pollen. Specific inhibitors of proteasome function, such as MG-132 and clasto-lactacystin p-lactone, markedly decreased pollen tube growth rate or altered tube morphology. By contrast, hardly any effect could be detected on pollen germination when a non-proteasomal protease inhibitor (E-64) was tested. The data indicate the need of tightly regulated turnover of protein population involved in tube organisation and elongation in kiwifruit, implicating the ubiquitin-26S proteasome as a master proteolytic machinery involved.