LOCALIZATION OF ACTIN MESSENGER-RNA DURING THE ESTABLISHMENT OF CELL POLARITY AND EARLY CELL DIVISIONS IN FUCUS EMBRYOS

Localization of mRNA is a well-described mechanism to account for the asymmetric distribution of proteins in polarized somatic cells and emb ryos of animals. In zygotes of the brown alga Fucus, F-actin is locali zed at the site of polar growth and accumulates at the cell plates of the first two divisions of the embryo, We used a nonradioactive, whole -mount in situ hybridization protocol to show the pattern of actin mRN A localization. Until the first cell division, the pattern of actin mR NA localization is identical to that of total poly(A)(+) RNA, that is, a symmetrical distribution in the zygote followed by an actin-depende nt accumulation at the thallus pole at the time of polar axis fixation , At the end of the first division, actin mRNA specifically is redistr ibuted from the thallus pole to the cell plates of the first two divis ions in the rhizoid. This specific pattern of localization in the zygo te and embryo involves the redistribution of previously synthesized ac tin mRNA. The initial asymmetry of actin mRNA at the thallus pole of t he zygote requires polar axis fixation and microfilaments but not micr otubules, cell division, or polar growth. However, redistribution of a ctin mRNA from the thallus pole to the first cell plate is insensitive to cytoskeletal inhibitors but is dependent on cell plate formation. The F-actin that accumulates at the rhizoid tip is not accompanied by the localization of actin mRNA. However, maintenance of an accumulatio n of actin protein at the cell plates of the rhizoid could be explaine d, at least partially, by a mechanism involving localization of actin mRNA at these sites. The pattern and requirements for actin mRNA local ization in the Fucus embryo may be relevant to polarization of the emb ryo and asymmetric cell divisions in higher plants as well as in other tip-growing plant cells.