Considerable excitement surrounds the idea that small G-proteins, namely Rops (Rho-related proteins of plants), regulate pollen tube growth, with an important aspect of that control mechanism involving the modulation of Ca2+ influx. Rops localize to the apex of the pollen tube where they appear to associate with the plasma membrane (Kost et al. 1999; Li et al. 1999; chapter by Hwang and Yang, this volume). Overexpression of these proteins causes the apex of the pollen tube to swell into balloon-shaped structures (Kost et al. 1999), whereas dominant negative forms of the protein or the injection of function-inhibiting antibodies to Rop block pollen tube growth (Li et al. 1999). Because these antibodies were shown to also eliminate the tip-focused Ca2+ gradient, the conclusion has been made that Rop regulates Ca2+ influx (Li et al. 1999). Despite this provocative conclusion, we hasten to note that the connection between Rop and Ca2+ may not be as compelling or direct as suggested. Several experimental conditions that block pollen tube elongation, such as injection of BAPTA buffers, mild thermal shock, culture in caffeine, or treatment with elevated osmoticum, also eliminate the tip-focused Ca2+ gradient (Pierson et al. 1994, 1996). The fact that Rop inactivation blocks growth is indeed interesting, but the data thus far do not establish that it does so by first blocking Ca2+ influx. The results from the growth/Ca2+ relationship in oscillating pollen tubes indicate that growth defines the subsequent Ca2+ influx, and not the reverse (Messerli et al. 2000).
Despite these reservations, recent work establishes a firmer connection between Rop and Ca2+, and also with the control of the actin cytoskeleton (Gu et al. 2005). In Arabidopsis pollen tubes, Rop1 seems to control tube growth through the coordinate activity of two interacting CRIB (Cdc42/Rac-interactive binding) proteins, called RIC3 and RIC4 (Gu et al. 2005). Experimentation suggests that RIC3 promotes Ca2+ influx, which may affect pollen tube growth through the modulation of F-actin dynamics. Support for these conclusions stems from the observation that cells expressing RIC3 germinate and express a tip-focused Ca2+ gradient at a lower extracellular [Ca2+] than the untransformed controls. In addition, the results show that overexpression of RIC3 causes an apparent degradation of the fine actin filaments in the apical domain.
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