While growing in the pistil, pollen tubes are exposed to many extracellular stimuli that could function as guidance cues. Several signals/molecules, including nutrients, have been reported to be involved in tube guidance including lipids, Ca2+ ions and cyclic nucleotides (Hulskamp et al. 1995; Malho, this volume; Zarsky et al., this volume). However, it is not clear if these molecules act as chemotropic signals.
Nitric oxide (NO) was recently suggested to reorient lily pollen tubes in vitro (Prado et al. 2004). In contrast to Chemocyanin and TTS, which behave as positive chemotropic factors, NO acted as a negative chemotropic factor, causing tubes to turn away from a point source. A gradient of NO was established by pipetting the NO donor, s-nitrosoacetylpenicillamine (SNAP), on to the surface of pollen growth medium. As tubes grew into the gradient, their growth rate slowed or stopped before the tip turned an average of 98° away from the point source. Interestingly, a mutant of Arabidopsis with a defect in NO production has reduced fertility, but tube guidance in this mutant has not been characterized (Guo et al. 2003). Rapid production of NO following entry of a tube into the micropyle could redirect subsequent tubes away from an ovule that is already being fertilized. It will thus be interesting to determine the sites of NO production within the pistil and to determine the path of pollen tubes growing within this and other NO synthesis mutants.
NO is known to regulate the production of the critical secondary messenger cGMP in animals. Sildenafil citrate causes accumulation of cGMP in animal cells because it inhibits cGMP-degrading phosphodiesterases. Treatment of lily pollen tubes with sildenafil citrate sensitized the response to NO, suggesting that NO may also regulate cGMP production in pollen tubes. Interestingly, another cyclic nucleotide, cAMP has also been implicated in reorientation of tip growth (Moutinho et al. 2001). Cyclic nucleotide synthesis is just beginning to be understood in plants (Malho, this volume) and understanding the regulation of cyclic nucleotide signaling in pollen tubes will be an exciting area of future research.
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