Root hair formation: F-actin-dependent tip growth is initiated by local assembly of profilin-supported F-actin meshworks accumulated within expansin-enriched bulges

Plant root hair formation is initiated when specialized elongating root epi dermis cells (trichoblasts) assemble distinct domains at the plasma membran e/cell wall cell periphery complexes facing the root surface. These localit ies show accumulation of expansin and progressively transform into tip-grow ing root hair apices. Experimentation showed that trichoblasts made devoid of microtubules (MTs) were unaffected in root hair formation, whereas those depleted of F-actin by the G-actin sequestering agent latrunculin B had th eir root hair formation blocked after the bulge formation stage. In accorda nce with this, MTs are naturally depleted from early outgrowing bulges in w hich dense P-actin meshworks accumulate. These F-actin caps remain associat ed with tips of emerging and growing root hairs. Constitutive expression of the GFP-mouse talin fusion protein in transgenic Arabidopsis, which visual izes all classes of F-actin in a noninvasive mode, allowed in vivo confirma tion of the presence of distinct F-actin meshworks within outgrowing bulges and at tips of young root hairs. Profilin accumulates, at both. the protei n and the mRNA levels, within F-actin-enriched bulges and at tips of emergi ng hairs. ER-based calreticulin and HDEL proteins also accumulate within ou tgrowing bulges and remain enriched at tips of emerging hairs. All this sug gests that installation of the actin-based tip growth machinery takes place only after expansin-associated bulge formation and requires assembly of pr ofilin-supported dynamic F-actin meshworks. (C) 2000 Academic Press.