Cytoskeleton-Associated Large RNP Complexes in Tobacco Male Gametophyte (EPPs) Are Associated with Ribosomes and Are Involved in Protein Synthesis, Processing, and Localization

Abstract

The progamic phase of male gametophyte development involves activation of synthetic and catabolic processes required for the rapid growth of the pollen tube. It is well-established that both transcription and translation play an important role in global and specific gene expression patterns during pollen maturation. On the contrary, germination of many pollen species has been shown to be largely independent of transcription but vitally dependent on translation of stored mRNAs. Here, we report the first structural and proteomic data about large ribonucleoprotein particles (EPPs) in tobacco male gametophyte. These complexes are formed in immature pollen where they contain translationally silent mRNAs. Although massively activated at the early progamic phase, they also serve as a long-term storage of mRNA transported along with the translational machinery to the tip region. Moreover, EPPs were shown to contain ribosomal subunits, rRNAs and a set of mRNAs. Presented results extend our view of EPP complexes from mere RNA storage and transport compartment in particular stages of pollen development to the complex and well-organized machinery devoted to mRNA storage, transport and subsequent controlled activation resulting in protein synthesis, processing and precise localization. Such an organization is extremely useful in fast tip-growing pollen tube. There, massive and orchestrated protein synthesis, processing, and transport must take place in accurately localized regions. Moreover, presented complex role of EPPs in tobacco cytoplasmic mRNA and protein metabolism makes them likely to be active in another plant species too. Expression of vast majority of the closest orthologues of EPP proteins also in Arabidopsis male gametophyte further extends this concept from tobacco to Arabidopsis, the model species with advanced tricellular pollen.

Dynamics of protein expression during pollen germination in canola (Brassica napus)

“…Together, these studies conWrm that, as for pollen transcripts (Becker et al 2003;Honys and Twell 2003;Pina et al 2005), mature pollen store a large number of pre-synthesized proteins which could provide the immediate resource for rapid germination and early tube growth. Recent study on tobacco pollen has shown the presence of ribonucleoprotein particles (EPP) complexes (which contain rRNAs and mRNAs) in mature pollen and are highly activated during pollen germination indicating their role in protein synthesis, processing and transport in tube growth (Honys et al 2009). …”

“…These results are reXective of the importance of rapid protein turnover (Sheng et al 2006) and proper folding in pollen germination and early tube growth. In tobacco mature and germinating pollen, a number of proteins related to protein synthesis, processing and localization were identiWed (Honys et al 2009). In contrast, relatively few proteins involved in protein synthesis, degradation and folding were identiWed during pollen germination in rice (Dai et al 2007b).…”

Intercellular communication in Arabidopsis thaliana pollen discovered via AHG3 transcript movement from the vegetative cell to sperm

“…The mechanisms responsible for translational repression are currently unknown, as are the intercellular routes promoting transport within the male germ unit in pollen. However, AHG3 is most likely not an exceptional case, because it has been shown that ribonucleoprotein particles in pollen contain translationally silent mRNAs (34). These ribonucleoprotein complexes have been interpreted to serve as long-term storage of mRNAs that are transported to the pollen tube tip, along with the translational machinery, during pollen tube growth.…”

De novo post-pollen mitosis II tobacco pollen tube transcriptome

“…Among this set of genes there are potential receptor proteins that may respond to female cues for guidance of the pollen-tube tip growth. In the light of recent identification and biochemical characterization of long-term RNA-storage and transport of EPP particles, 3,9 it is likely that these newly synthesized and accumulated transcripts are stored within these particles.…”