A Pollen-Specific RALF from Tomato That Regulates Pollen Tube Elongation      

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 μ m 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 μm 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.

RALFs

“…22,23 In the case of tomato pollen SlRALF, mature peptide was only detected in the medium in an in vitro pollen germination system. 18 These results are also consistent with localization of a Nicotiana benthamiana RALF fused to GFP, which localized first to the ER and later to the cell wall in N. benthamiana leaf cells. 24 These results together indicate an extracellular localization for processed mature RALF peptides.…”

“…[30][31][32][33] When a tomato pollen-specific LRX recognition domain was used as a bait in a Yeast 2 Hybrid screen, a pollen specific RALF was identified. 18 While it is intriguing to note that several plant peptide receptors bind to their ligands via Leucine-rich Repeats (LRRs), the relationship of the RALFbinding proteins identified by Scheer et al 29 and the cell wall localized LRX proteins is not yet known.…”

“…36 Synthetic tomato pollen SlPRALF inhibits in vitro pollen tube growth but the peptide is effective only during a specific developmental window. 18 When SlPRALF was added to pollen grains in germination medium, pollen tube germination was initiated, resulting in a very small (<10 micron) structure called a "glebula," but normal tip growth was inhibited. When added to actively growing pollen tubes that were less than 50 microns in length, SlPRALF inhibited further growth.…”

“…2,15,16 In some cases, a subset of RALF genes is expressed in a highly tissue-specific manner, for example ovulespecific expression of a subset of RALF genes has been observed in Solanum chacoense and Arabidopsis, 15,16 and pollen-specific RALFs have been identified in broccoli and tomato. 17,18 …”

RLKs orchestrate the signaling in plant male-female interaction

“…RALF family has been linked to cell wall alkalization and inhibits cell expansion in many plant species (Matsubayashi, 2014). Interestingly, a pollen-specific RALF has been reported to inhibit pollen tube elongation in tomato (Solanum lycopersicum) (Covey et al, 2010). Arabidopsis genome encodes 34 RALF members and some of them are expressed in pollen and/or synergids or induced by pollination, suggesting a role in gametophyte interaction or function although none has been functionally characterized Boavida et al, 2011).…”