Razumovskaya A.V. Cytology of the minor-vein phloem in 320 species from the subclass Asteridae suggests a high diversity of phloem-loading modes. Frontiers in Plant Science. 2013, V. 4, Article 312.

PLANT SCIENCE ORIGINAL RESEARCH ARTICLE published: 21 August 2013 doi: 10.3389/fpis.2013.00312 Cytology of the minor-vein phloem in 320 species from the subclass Asteridae suggests a high diversity of phloem-loading modes1^ Denis R. Batashev, Marina V. Pakhomova, Anna V. Razumovskaya, Olga V. Voitsekhovskaja * and Yuri V. Gamalei Laboratory of Plant Ecological Physiology, Komarov Botanical Institute, Russian Academy of Sciences, St. Petersburg, Russia Edited by: Aart Van Bel , Justus-Liebig-University Giessen, Germany R eview ed by: Alexander Schulz, University of Copenhagen, Denmark Ted Botha, University of Fort Hare, South Africa *Correspondence: Olga V. Voitsekhovskaja, Laboratory of Plant Ecological Physiology, Komarov Botanical Institute, Russian Academy of Sciences, иI. Professora Popova, 2, 197376 St. Petersburg, Russia e-mail: ovoitse@yandex.ru The discovery of abundant plasmodesmata at the bundle sheath/phloem interface in Oleaceae (Gamalei, 1974) and Cucurbitaceae (Turgeon et al., 1975) raised the questions as to whether these plasmodesmata are functional in phloem loading and how widespread symplasmic loading would be. Analysis of over 800 dicot species allowed the definition of "open" and "closed" types of the minor vein phloem depending on the abundance of plasmodesmata between companion cells and bundle sheath (Gamalei, 1989, 1990). These types corresponded to potential symplasmic and apoplasmic phloem loaders, respectively; however, this definition covered a spectrum of diverse structures of phloem endings. Here, a review of detailed cytological analyses of minor veins in 320 species from the subclass Asteridae is presented, including data on companion cell types and their combinations which have not been reported previously. The percentage of Asteridae species w ith "open" minor vein cytology which also contain sieve-element-companion cell complexes w ith "closed" cytology, i.e., that show specialization for both symplasmic and apoplasmic phloem loading, was determined. Along with recent data confirming the dissimilar functional specialization of structurally different parts of minor vein phloem in the stachyose-translocating species Alonsoa meridionalis (Voitsekhovskaja et al., 2009), these findings suggest that apoplasmic loading is indispensable in a large group of species previously classified as putative symplasmic loaders. Altogether, this study provides formal classifications of companion cells and of minor veins, respectively, in 24 families of the Asteridae based on their structural features, opening the way to a close investigation of the relationship between structure and function in phloem loading. Keywords: phloem loading, companion cells, minor veins, symplast, apoplast, Asteridae INTRODUCTION Since their discovery in 1879 by Tangl (1879), plasmodesmata have been assumed to serve the intercellular transport of metabo­ lites in plants, and the development of numerous plasmodesmal connections was considered an indicator of intensified symplas­ mic exchange between the connected cells (Tyree, 1970). The highest densities of plasmodesmata per cell surface unit occur in minor veins of leaves at the interface between bundle sheath cells and companion cells belonging to the “intermediary cell” type, as are found in some dicot families, e.g., Cucurbitaceae and Oleaceae (Gamalei, 1974; Turgeon et al., 1975). The walls between intermediary cells (ICs) and bundle sheath cells in Fmximts orntis minor veins contain about 60 plasmodesmata per (im 2 surface facing the bundle sheath, and up to 140 plasmodesmata per (im 2 surface on the IC side (Gamalei, 1990, 1991). These extremely high densities of symplasmic connections were the reason why species with ICs in m inor vein phloem were suggested to load 'I hls article is dedicated to o u r colleague and friend Prof. em . Dr. U lrich H eber (Julius-von-Sachs Institute for Biosciences, W urzburg University, W urzburg, G erm any). assimilates into the phloem preferentially via the symplast. In contrast, species containing in their minor vein phloem either “ordinary” companion cells (OCs) with only few plasmodesmata at the bundle sheath side (about 1 p er(im 2; Gamalei, 1991), or transfer companion cells (TCs) characterized by even lower plas­ modesmal density (about 0.1 p er(im 2; Gamalei, 1991) and cell wall ingrowths, were considered apoplasmic phloem loaders. The cytology of companion cells in leaf minor veins of dicotyledonous plants, often in relation with the mode of phloem loading, was subject of extensive studies covering a large num ­ ber of species (e.g., Pate and Gunning, 1969; Peterson and Yeung, 1975; Turgeon et al., 1975,1993; Madore et al., 1986; Fisher, 1986, 1991; Schmitz et al., 1987; Gamalei, 1989, 1990, 1991; van Bel and Gamalei, 1992; van Bel et al., 1992; Kempers et al., 1998; Haritatos et al., 2000; Goggin et al., 2001; Turgeon et al., 2001; Turgeon and Medville, 2004; Reidel et al., 2009). The most com­ prehensive analysis, however, has been performed by Gamalei encompassing over 800 species from over 140 families (Gamalei, 1990). He described the extremes of minor vein organization, type 1 and 2 , which differed both in the pattern of the first divi­ sions of the phloem initial during vein development, and in the www.frontiersin.org August 2013 | Volume 4 | Article 312 | 1