An Auxin Transport Inhibitor Interferes With Unicellular Gravitropism in Protonemata of the Moss Ceratodon purpureus

Abstract: Gravitropism of the protonemata of the moss Ceratodon purpureus (Hedw.) Brid. was studied after treatment with auxin transport inhibitors and auxin-related substances. The phytotropins NPA (naphthylphthalamic acid) and PBA (pyrenoylbenzoic acid), known to block auxin efflux in higher plants, strongly inhibited gravitropic curvature of the apical protonemal cell. At 3 μM NPA or PBA, approximately 60 % inhibition of curvature was observed; growth rates were less affected. Tyrphostin A47, a known antagonist of NPA effects in higher plants, released the inhibition of moss protonemal gravitropism and restored the full curvature response. Exogenous IAA, even at high concentrations (40 μM), did not interfere with protonemal gravitropism. To account for the results, modified hypotheses for auxin transport and action are discussed.