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 Subtype 2-VI has been found in some hemiparasitic representatives of the Orobanchaceae family, Euphrasia, Melampyrum, Odontites, and Rhinanthus, where SE-CCCs contained MIC-b companion cells with both highly developed cell wall ingrowths and plasmodesmal fields (Figure 5F) and leucoplasts. The mature minor veins of these group of species often showed no well-defined spatial organization, and more detailed studies on the development of the m inor veins in these species are required to clarify the relationship of subtype 2-VI with the other types of minor vein phloem. Similarly, the minor vein phloem of the two Asarina species, A. harclaiana and A. scan- dens, was difficult to include in the subtypes described above, primarily due to lack of data on their minor vein development. In mature m inor veins, the abaxial SE-CCCs usually contained two companion cells of TC type per one SE while the adaxial part of the phloem consisted of SE-CCCs with MIC-a, with a MICSE ratio of 1 or 2. The absence of phloem parenchyma and of tiered organization, as well as the presence of two different companion cell types within a vein, places these species in type 1, close to subtype 1-III (Figure4E). At the same time, the companion cells (MICs) in Asarina formally resemble those in hemiparasitic Scrophulariaceae, differing from the companion cells of subtype 2-VI in that their plasmodesmal fields and cell wall ingrowths were much less developed. Obviously, the classification of the minor vein phloem in these species needs further investigation. Type 0 represents a rather heterogeneous group and the rare occurrence of this type in the Asteridae (Figure 6) pre­ vented its comprehensive analysis. The following combinations of structural features could be found within this group: minor veins with SE-CCCs containing OCs with leucoplasts, lack­ ing phloem parenchyma cells, e.g., Swertia in Gentianaceae, Cinchona in Rubiaceae (Batashev and Gamalei, 2000; Gamalei et al., 2008); minor veins with similar SE-CCCs but containing phloem parenchyma cells (in Digitalis species, not shown); sim­ ilar m inor veins where phloem parenchyma cells developed cell wall ingrowths (in several Veronica species, not shown); minor veins with SE-CCCs containing OCs with chloroplasts, lacking phloem parenchyma cells (e.g., Trachelospermum in Apocynaceae; Figure 4F). The variability of the cellular composition, the absence of highly specialized structures like plasmodesmal fields or cell wall ingrowths, and the lack of well-defined spatial orga­ nization distinguish the minor vein phloem of this group from types 1 and 2. DISTRIBUTION OF THE TYPES AND SUBTYPES OF MINOR VEIN PHLOEM WITHIN ASTERIDAE The large number of species analyzed in this study has allowed the determination of the distribution of the defined struc­ tural types of minor vein phloem within the studied families of subclass Asteridae (Figure 6). Of the eleven subtypes, five (1-1, 1-II, 2-1, 2-III, and 2-IV) dominate, i.e., were found in over 80% of all species analyzed. The other six subtypes are less wide-spread. The distribution of the types and subtypes of minor vein phloem among the studied families is shown in Table 3. The most heterogeneous family was found to be the Plantaginaceae s.L, where almost all subtypes could be found (Table 3). 100 J 90 - m 80 -- <D о 70 Ф S-60 ■5 50 J>40 4- | 3 0 - z 2 0 - 10 -I- 0 □ I □ й ■=■ I В I 1-1 1-11 1-ill 1-IV О 2-I 2-II 2-III 2-IV 2-V 2-VI Subtypes of minorvein phloem FIGURE 6 I Distribution of the subtypes of minor vein phloem among 320 species in the Asteridae. For every subtype defined, the number of species assigned to this subtype is shown. FIGURE 5 I Structure of minor vein phloem, type 2. (A) Ehretia cordifolia, subtype 2-I; (B) Veronica chamaedrys, subtype 2-II; (C) Galium kryiovianum, subtype 2-III; (D) Aster alpinus, subtype 2-IV; (E) Gentiana aquatica, subtype 2-V; (F) Rhinanthus minor, subtype 2-VI. CC, companion cell; PR phloem parenchyma; SE, sieve element. Magnification: (A,F) x1700; (B) x2000; (С, E) хЗООО; (D) x4000. www.frontiersin.org August 2013 | Volume 4 | Article 312 | 9