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 1 (Gamalei, 1991) 2a (Gamalei, 1991) 2b (Gamalei, 1991) FIGURE 3 I Schematic presentation of the structural types and subtypes of minor vein phloem. Absence of plasmodesmal fields at the companion cell/bundle sheath interface is designated by blue, and the presence—by red color; in the types where companion cells could never be shown to contain chloroplasts, plastids are not depicted. Classification into four types according to Gamalei (1991) is shown for reference. Scrophulariaceae, and Verbenaceae. Sometimes, both subtypes 1-1 and l-II occurred simultaneously in leaves of the same plant. In species classified subtype 1-III, in minor veins with three SE-CCCs the abaxial SE-CCC contains a companion cell of type TC-b (Table 1), i.e., a TC with leucoplasts (Figure 4C). This subtype was found in Acanthaceae, Lamiaceae, Plantaginaceae, and Scrophulariaceae. In subtype 1-IV the SE-CCCs contain CC-b companion cells with plasmodesmal fields and chloroplasts (Table 1; Figure 4D). In this subtype, all companion cells within the minor vein are of similar structure. This subtype was found in several gen­ era of the Apocynaceae family, e.g., Alstonia , Alyxia , Amsonia (Supplemental Table 1; Batashev and Gamalei, 2005). A similar organization of minor veins was found in several representatives of Campanulaceae and Convolvulaceae, with the exception that the companion cells contained no distinct plasmodesmal fields but rather highly abundant single plasmodesmata (Madore et al., 1986; Supplemental Table 1). Whether phloem loading in these families is similar to, or distinct from that in the representatives of the Apocynaceae with minor vein subtype 1-IV, needs further investigation. Type 2 was divided into subtypes I-VI (Figures 3, 5A-F). Subtype 2-1 comprises species with SE-CCCs containing OC-b cells with chloroplasts (in rare cases ОС-a with leucoplasts instead of chloroplasts) and phloem parenchyma cells with­ out cell wall ingrowths (Figure 5A). This subtype was found in some representative of the Gentianaceae, Plantaginaceae, and Polemoniaceae, but was broadly represented in the families Boraginaceae and Solanaceae. Subtype 2-11 is distinct from the previous subtype in that the phloem parenchyma cells in the minor veins contains cell wall ingrowths and thus can be clas­ sified as phloem parenchyma transfer cells (Figure 5B). This subtype occurs very rarely in Asteridae and was found only in Polemoniaceae and Plantaginaceae. Subtype 2-III is widespread and can be found in Asteraceae, Boraginaceae, Plantaginaceae, Rubiaceae, and Solanaceae. It is characterized by the presence of companion cells of TC type with chloroplasts (TC-a, Table 1) while phloem parenchyma cells do not possess cell wall ingrowths (Figure 5C). Subtype 2-IV is very similar to 2-111, the only dif­ ference is that in 2-IV cell wall ingrowths are formed in both companion cells and phloem parenchyma cells (Figure 5D). This subtype is common in Asteraceae but rather rare in Solanaceae. Subtype 2-V has been found only in some genera of the Gentianaceae (Supplemental Table 1; Batashev and Gamalei, 2000). The minor veins in these species contain SE-CCCs with rather peculiar transfer cells: their cell wall ingrowths have a flange morphology (Offler et al., 2003), and the cells contain leucoplasts and not chloroplasts (Figure 5E). FIGURE 4 I Structure of minor vein phloem, type 1, and type 0. (A) Scutellaria pallida, subtype 1-1; (B) Mimulus guttatus, subtype 1-11; (C) Dracocephalumperegrinum, subtype 1-111; (D) Amsonia tabernaemontana, subtype 1-IV; (E) Asarina barclaiana] (F) Tracheiospermum luikinense, type 0. CC, companion cell; MIC-a, modified intermediary cell subtype a; SE, sieve element; TC, transfer cell subtype b. Black arrows point on plasmodesmal fields, white arrowheads point on cell wall ingrowths. Magnification: (A-D), (F), x1750; (E), хЗООО. Frontiers in Plant Science | Plant Physiology August 2013 | Volume 4 | Article 312 | 8