Abstract
Rapid Alkalinization Factors (RALFs) are plant peptides that rapidly increase the pH of plant suspension cell culture medium and inhibit root growth. A pollen-specific tomato (Solanum lycopersicum) RALF (SlPRALF) has been identified. The SlPRALF gene encodes a preproprotein that appears to be processed and released from the pollen tube as an active peptide. A synthetic SlPRALF peptide based on the putative active peptide did not affect pollen hydration or viability but inhibited the elongation of normal pollen tubes in an in vitro growth system. Inhibitory effects of SlPRALF were detectable at concentrations as low as 10 nm, and complete inhibition was observed at 1 mum peptide. At least 10-fold higher levels of alkSlPRALF, which lacks disulfide bonds, were required to see similar effects. A greater effect of peptide was observed in low-pH-buffered medium. Inhibition of pollen tube elongation was reversible if peptide was removed within 15 min of exposure. Addition of 100 nm SlPRALF to actively growing pollen tubes inhibited further elongation until tubes were 40 to 60 mum in length, after which pollen tubes became resistant to the peptide. The onset of resistance correlated with the timing of the exit of the male germ unit from the pollen grain into the tube. Thus, exogenous SlPRALF acts as a negative regulator of pollen tube elongation within a specific developmental window.
Highlights
Rapid Alkalinization Factors (RALFs) are plant peptides that rapidly increase the pH of plant suspension cell culture medium and inhibit root growth
While additional experiments to confirm and characterize pollen Leu-rich repeat extensin chimera (LRX)-RALF interactions are in progress, the effects of the pollenexpressed RALF on pollen tube germination and growth became the focus of this study
Like the previously identified vegetative tissue-expressed SlRALF (Pearce et al, 2001), the Pol2 gene encodes a prepropeptide predicted to be targeted to the endomembrane system and proteolytically processed near a conserved dibasic site that, in Arabidopsis, is required for propeptide processing of both AtRALF1 and AtRALF23 (Matos et al, 2008; Srivastava et al, 2009)
Summary
Rapid Alkalinization Factors (RALFs) are plant peptides that rapidly increase the pH of plant suspension cell culture medium and inhibit root growth. The SlPRALF gene encodes a preproprotein that appears to be processed and released from the pollen tube as an active peptide. Addition of 100 nM SlPRALF to actively growing pollen tubes inhibited further elongation until tubes were 40 to 60 mm in length, after which pollen tubes became resistant to the peptide. RALF precursors possess a conserved dibasic site upstream to the active peptide, suggesting that the processing mechanism may be similar. Recent studies show that both AtRALF23 and AtRALF1 are processed at the dibasic site by Golgi-located plant subtilisin-like Ser proteases and that this processing step is required for the activation of the peptide (Matos et al, 2008; Srivastava et al, 2009)
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