Inhibition of Pollen Tube Elongation by Microinjected Anti-Rop1Ps Antibodies Suggests a Crucial Role for Rho-Type GTPases in the Control of Tip Growth

Abstract

Microinjection of anti-Rop1Ps antibodies was used to assess the function of a tip-localized Rho-type GTPase, Rop, in controlling pollen tube growth. Injected antibodies induced sustained growth arrest within 1 to 2 min after injection but did not affect cytoplasmic streaming. Coinjection with Rop rescued antibody-induced growth inhibition, indicating that injected antibodies specifically block the activity of Rop GTPases. Antibody-induced inhibition was significantly enhanced in the presence of a lower threshold of extracellular [Ca2+] or a subinhibitory dosage of caffeine. In contrast, injection of the C3 toxin, which inactivates a different Rho-type GTPase, arrested tube elongation 10 to 20 min after injection. C3-induced growth arrest was accompanied by the cessation of cytoplasmic streaming. These data suggest that Rho-type GTPases play a pivotal role in the control of pollen tube elongation. We propose that Rop may regulate a Ca2+-dependent pathway involved in vesicle docking/fusion, whereas a C3-sensitive Rho GTPase may mediate cytoplasmic streaming.

GTP-binding proteins in plants

“…Rop was also shown to be a substrate for geranylgeranylation, but not farnesylation [248]. Microinjected anti-Rop antibodies inhibited pollen tube elongation, confirming a role for Rop in tip growth [249]. The inhibitory effects of microinjected anti-Rop antibodies were enhanced in low Ca 2 + or subinhibitory concentrations of caffeine, which is known to dissipate Ca 2 + gradients in plants.…”

“…Microinjection of the Rho GTPase inhibitor C3 ADPribosyltransferase inhibited cytoplasmic streaming, resulting in cessation of pollen tube growth. Since microinjected antibody did not inhibit cytoplasmic streaming, it was proposed that Rop is not sensitive to this enzyme and that the effects of C3 exoenzyme are caused by the inhibition of an unidentified C3 exoenzyme-sensitive Rho GTPase [249]. Because no clear evidence exists for cortical actin at the pollen tube tip, it was proposed that Rop might regulate a Ca 2 + -dependent pathway involved in vesicle docking and fusion.…”

ROPs in the spotlight of plant signal transduction

“…Polar growth applies to tip-growing cells represented by pollen tubes and root hairs, but also to regions in diffuse growing epidermal cells and probably trichomes [27][28][29][30]. ROPs were localized at the PM in the apical region of pollen tubes where growth takes place [31], and negative manipulations of the pollen-specific ROPs AtROP1 and AtROP5 inhibited tube growth [32][33][34]. In contrast, OX wt ROPs or CA mutants were ectopically localized at the PM and caused growth depolarization, implying that the establishment of polarity depends on the localized activation of ROP in the tip region [33,34].…”

A distinct mechanism regulating a pollen-specific guanine nucleotide exchange factor for the small GTPase Rop in Arabidopsis thaliana

“…The role of Rops in pollen tube growth has been extensively studied. Overexpression of a CA-Rop1 resulted in isotropic growth (11,12), whereas reduced Rop levels inhibited pollen tube growth (13).…”