title: Cytology of the minor-vein phloem in 320 species from the subclass Asteridae suggests a high diversity of phloem-loading modes.
abstract: 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 with "open" minor vein cytology which also contain sieve-element-companion cell complexes with "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.
authors: Laboratory of Plant Ecological Physiology, Komarov Botanical Institute, Russian Academy of Sciences St. Petersburg, Russia.
authors: Denis R
authors: DR
authors: Batashev
authors: Marina V
authors: MV
authors: Pakhomova
authors: Anna V
authors: AV
authors: Razumovskaya
authors: Olga V
authors: OV
authors: Voitsekhovskaja
authors: Yuri V
authors: YV
authors: Gamalei
keywords: Asteridae; apoplast; companion cells; minor veins; phloem loading; symplast
references: Planta. 1987 May;171(1):19-29
references: 24227267
references: Plant Physiol Biochem. 2011 Apr;49(4):441-8
references: 21330144
references: J Theor Biol. 1970 Feb;26(2):181-214
references: 5434342
references: Plant Physiol. 2004 Nov;136(3):3795-803
references: 15516516
references: Plant Physiol. 1986 Oct;82(2):432-42
references: 16665047
references: Plant Physiol. 1997 Sep;115(1):241-248
references: 12223804
references: Planta. 2001 Jan;212(2):231-42
references: 11216844
references: Plant Cell. 1997 Aug;9(8):1381-1396
references: 12237387
references: Plant J. 2011 Apr;66(2):366-74
references: 21241389
references: Planta. 1992 Mar;186(4):518-25
references: 24186781
references: Plant Physiol. 1984 Nov;76(3):782-6
references: 16663924
references: Planta. 1986 Oct;169(2):141-52
references: 24232544
references: Planta. 2010 Apr;231(5):1137-44
references: 20179964
references: Annu Rev Plant Biol. 2003;54:431-54
references: 14502998
references: Proc Natl Acad Sci U S A. 2009 Aug 18;106(33):14162-7
references: 19666555
references: Proc Natl Acad Sci U S A. 2011 Dec 20;108(51):E1451-60
references: 22106293
references: Proc Natl Acad Sci U S A. 2010 Mar 9;107(10):4623-8
references: 20176954
references: J Exp Bot. 2008;59(11):2945-54
references: 18617505
references: Plant Physiol. 2001 Feb;125(2):891-9
references: 11161046
references: Plant Physiol. 2009 Mar;149(3):1601-8
references: 19129415
references: Am J Bot. 2001 Aug;88(8):1331-9
references: 21669666
references: Plant Physiol. 2006 Jan;140(1):383-95
references: 16377750
references: Planta. 1988 Dec;176(3):307-18
references: 24220859
references: J Exp Bot. 2009;60(6):1873-83
references: 19321649
references: Planta. 2000 Jun;211(1):105-11
references: 10923710