Thus, Rop controls polar growth in pollen tubes probably by integrating signals for growth with polarity signals ( Li et al., 1999 Zheng and Yang, 2000a). Evidence indicates that both polar localization of Rop and the regulation of its activity at the tip region of the plasma membrane (PM) are critical for defining the site of growth in pollen tubes ( Lin et al., 1996 Kost et al., 1999 Li et al., 1999 Zheng and Yang, 2000a G. These results suggest that Rop also controls the specification of a cortical site for tip growth in pollen tubes ( Li et al., 1999). Expression of constitutively active (CA) mutants for these Rops induced isotropic growth (i.e., formation of a bulbous tube). Overexpression of wild-type Rop1 or At-Rac2/Rop5 caused depolarized growth in pollen tubes ( Kost et al., 1999 Li et al., 1999), as suggested by swelling of pollen tube tips or an increase in the radial expansion of pollen tubes (referred to as pollen tube expansion in this article). Rop is essential for pollen tube elongation, as indicated by the inhibition of pollen tube elongation by the injection of anti-Rop1 antibodies and the expression of the dominant negative (DN) mutants of Arabidopsis Rop1 and At-Rac2/Rop5 ( Lin and Yang, 1997 Kost et al., 1999 Li et al., 1999). The plant-specific family of Rho GTPases, termed Rop, has emerged as a pivotal molecular switch in plant signal transduction ( Arabidopsis Genome Initiative, 2000 Zheng and Yang, 2000b).

On the basis of these observations, we propose that RICs function as Rop GTPase targets that control various Rop-dependent signaling pathways in plants.

Reverse transcriptaseвЂ“mediated polymerase chain reaction analysis showed that six of the nine RICs are expressed in various parts of Arabidopsis plants. Our results suggest that various RICs have evolved to interact with Rops differentially and to perform distinct functions in pollen tubes. Furthermore, these inhibitory RICs also exhibit distinct patterns of localization in pollen tubes. Expression of all other RICs causes various degrees of growth inhibition in pollen tubes. In contrast, RIC10 (group I) promotes pollen tube elongation but does not affect pollen tube growth polarity and shows Rop1-independent localization to the cytoplasm, suggesting that RIC10 may participate in a Rop1-independent pathway probably controlled by a different Rop. RIC3 (group III) and RIC4 (group V) both cause depolarized growth like Rop1 and display Rop1-enhanced localization to the tip of pollen tubes, suggesting that these RICs may be two distinct targets of Rop1. Overexpression in tobacco pollen tubes of the nine Ric genes that are expressed in Arabidopsis pollen causes distinct phenotypes, implying distinct functions for various RICs. RICs are divergent and classified into five groups that share little sequence homology outside of the conserved Rop-interactive domain. In this study, we have identified 11 Arabidopsis genes encoding novel proteins, termed RICs (for Rop-interactive CRIB motifвЂ“containing proteins), that contain a CRIB (for Cdc42/Rac-interactive binding) motif required for their specific interaction with GTP-bound Rop1. To understand how Rop achieves its functional versatility in signaling, we performed a genome-wide identification of putative Rop targets using a combination of the yeast two-hybrid method, bioinformatic tools, and a robust functional assay in pollen. The plant-specific Rop family GTPases are versatile molecular switches in many processes during plant growth, development, and responses to the environment.

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