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.

Batashev et al. Minor vein phloem in Asteridae Table 1 I Classification of companion cells in Asteridae according to their structural characteristics. CC type CC subtype PF Branching of PD in PF CWI Morphology of CWI Chloroplasts (instead of leucoplasts) Stachyose synthesis Ordinary cells ОС-a (ОС with leucoplasts)1 - - - - - - OC-b (ОС with chloroplasts)2 - - - - + - Transfer cells TC-a3 - - + + + reticulate + - TC-b4 (present in minor veins together with IC/M IC) - - + + reticulate - - TC-c (in some Gentianaceae species; this study) - - + + + flange - - Intermediary cells and IC-like cells IC6 + + + asymmetric - - (starch in leucoplasts never observed) + + + ICL (IC-like cells with starch in leucoplasts; this study) + + + asymmetric - (starch in leucoplasts normally found) ? CC with PF and/or many PD CC-a (CC in minor veins of Cornaceae, Eucommiaceae, Griseliniaceae and Hydrangeaceae; this study) + + symmetric CC-b (CC in minor veins of Amsonia tabernaemontana and some other Apocynaceae; this study) + + Asymmetric (no branching in case of multiple single PD) + MIC-a6 + asymmetric + reticulate - + MIC-b (CC in minor veins of some hemiparasitic Orobanchaceae; this study) + + + asymmetric + + + reticulate -(?) CC, companion cell; PF, plasmodesmal fields; PD, plasmodesmata; CWI, cell wall ingrowths. Number of + indicates the relative degree of the manifestation of a feature. Several examples illustrating the trait combinations can be found in 1Turgeon et al. (19931 for Digitalis grandiflora, Fleicie et al. (2009) for Plantago major, ' Gunning and Pate (1969) for Impatiens balsamina, 4Turgeon etal. (1993) for Nemesia strumosa, 5Fisher (1986) for Coleus blumei; °Turgeon etal. (1993) forAsarina scandens; —, feature absent; ?, not clear. in combination with ICs or MICs (e.g., Turgeon et al., 1993. for Nemesia strumosa), or, as the only type of phloem com­ panion cells, in minor veins of some representatives of the Gentianaceae family (Batashev and Gamalei, 2000). Figure ID illustrates a TC found in minor vein phloem together with ICs; these TCs possess leucoplasts and small cell-wall protuber­ ances and were named TC-b. Figure IE shows TCs which were found only in some representatives of the Gentianaceae family; these TCs possess cell wall ingrowths with flange morphology rather than reticulate morphology as typically observed in com­ panion cells (Offler et al., 2003). This feature was consistently observed in four out of 15 studied Gentianaceae species and was found to be independent of the angle of sectioning. Companion cells of this structure, to our knowledge, have not been reported before in any other species. We named these cells TC-c (Table 1). Companion cells with abundant plasmodesmal fields in the cell walls facing the bundle sheath and with leucoplasts are widespread in the Asteridae. First, there are ICs, a type of com­ panion cells with very distinct features. The main structural characteristic of IC is the presence of highly developed plasmod­ esmal fields, asymmetrically branched, with more branches on the IC side (Gamalei, 1974; Turgeon et al., 1975; Volk et al., 1996). To date, a perfect correlation exists between the presence of the ICs and the synthesis of RFOs as phloem transport sugars. Another well-known structural trait of ICs is the presence of many small vacuoles instead of one large vacuole; these unusual vacuoles were found in all ICs of stachyose-transporting plants leading to the speculation that they might represent the compartment for RFO synthesis (Gamalei, 1990; Voitsekhovskaja, 2002). However, we did not include this feature in the present classification because, in contrast to other features, the presence and the number of these Frontiers in Plant Science | Plant Physiology August 2013 | Volume 4 | Article 312 | 4