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Abstract
Key messagePIN2-like auxin transporters are expressed, preferentially in a polarized manner, in antheridial cells of freshwater green algaChara vulgaris, considered to be the closest relative of the present-day land plants.Chara vulgaris represents a group of advanced multicellular green algae that are considered as the closest relatives of the present-day land plants. A highly specialized structure of its male sex organs (antheridia) includes filaments consisting of generative cells, which after a series of synchronous divisions transform into mature sperm, and non-generative cells comprising outer shield cells, cylindrical manubria, and central complex of capitular cells from which antheridial filaments arise. Immunofluorescence observations indicate that PIN2-like proteins (PIN2-LPs), recognized by antibodies against PIN-FORMED2 (PIN2) auxin transporter in Arabidopsis thaliana, are expressed in both types of antheridial cells and, in most of them, preferentially accumulate in a polarized manner. The appearance of PIN2-LPs in germ-line cells is strictly confined to the proliferative period of spermatogenesis and their quantities increase steadily till antheridial filaments reach the 16-celled stage. An enhanced level of PIN2-LPs observed in the central cell walls separating two asynchronously developing parts of antheridial filaments (characterized by the plugged plasmodesmata) is correlated with an enhanced deposition of callose. Intense PIN2-LPs immunofluorescence maintained in the capitular cells and its altering polarity in manubria suggest a pivotal role of these cells in the regulation of auxin transport directionality during the whole time of antheridial ontogenesis. Immunohistochemical staining of IAA revealed a clear-cut correspondence between localization sites of auxins and PIN2-LPs. It seems probable then that a supplementary developmental mechanism has evolved in Chara, by which all antheridial elements may be integrated at the supra-cellular level via plasma membrane-targeted PIN2-LPs and auxin-mediated processes.
Highlights
The mechanisms responsible for both biosynthesis and translocation of the phytohormone auxin play a fundamental role in a wide range of processes, such as cell proliferation, elongation and differentiation, through which they exert a considerable impact on the ultimate shape and function of tissues and organs in all vascular plants (Benjamins and Scheres 2008; Lau et al 2008; Zhao 2010)
As a rule, the intracellular distribution of these proteins corresponds to that reported for the functional PIN2 in the root meristems of higher plants, we find it more convenient to use the term PIN2-like proteins (PIN2-LPs) because of their unknown structure and possible plurality of the binding sites for antibodies used in our studies
Immunoprecipitation and protein blot assays for PIN2-LPs extracted from apical parts of C. vulgaris carrying whorls with young antheridia and for PIN2 proteins extracted from root tips of A. thaliana (0.5–1 mm in length) were performed according to methods described earlier (Z_abka et al 2015)
Summary
The mechanisms responsible for both biosynthesis and translocation of the phytohormone auxin play a fundamental role in a wide range of processes, such as cell proliferation, elongation and differentiation, through which they exert a considerable impact on the ultimate shape and function of tissues and organs in all vascular plants (Benjamins and Scheres 2008; Lau et al 2008; Zhao 2010). Most of the research concerning intercellular polar auxin transport (PAT) and auxin-mediated signaling pathways is still concentrated mainly on the higher plant models (such as Arabidopsis thaliana), over the past several years there has been a growing interest in the evolution of auxin-dependent cellular polarization, correlative interactions, and long-range control of the developmental patterning in early diverged groups of non-vascular algae, liverworts and mosses (Cooke et al 2002; Fujita et al 2008; Lau et al 2009; Boot et al 2012; Bennett 2015). As judged from an in silico survey, a significant number of genes related to auxin biosynthesis, signaling and transport from land plants have homologs in a relatively small (200 Mb) genome of E
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