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Phloem loading is the initial step in photoassimilate export and the one that creates the driving force for mass flow. It has been proposed that loading occurs symplastically in species that translocate carbohydrate primarily as raffinose family oligosaccharides (RFOs). In these plants, dense fields of plasmodesmata connect bundle sheath cells to specialized companion cells (intermediary cells) in the minor veins. According to the polymer trap model, advanced as a mechanism of symplastic loading, sucrose from the mesophyll diffuses into intermediary cells and is converted there to RFOs. This process keeps the sucrose concentration low and, because of the larger size of the RFOs, prevents back diffusion. To test this model, the RFO pathway was down-regulated in Verbascure phoeniceum L. by suppressing the synthesis of galactinol synthase (GAS), which catalyzes the first committed step in RFO production. Two GAS genes (VpGAS1 and VpGAS2) were cloned and shown to be expressed in intermediary cells. Simultaneous RNAi suppression of both genes resulted in pronounced inhibition of RFO synthesis. Phloem transport was negatively affected, as evidenced by the accumulation of carbohydrate in the lamina and the reduced capacity of leaves to export sugars during a prolonged dark period. In plants with severe down-regulation, additional symptoms of reduced export were obvious, including impaired growth, leaf chlorosis, and necrosis and curling of leaf margins. plasmodesmata | polymer trap | stachyose | galactinol | RNAi

The author reviews the publications on vascular tissue differentiation from meristematic cells. The topics of interest include the roles of mutants in vascular differentiation and pattern formation at the organ level.

Gymnosperm root protophloem is not well understood. There is a question as to whether root protophloem cells mature as phloem parenchyma, or as sieve elements, or if within the protophloem there is an anatomical and evolutionary gradient having characteristics of both cell types. This question is relevant to understanding anatomical and physiological mechanisms that supply nutrients to the root tip. Anatomical data from a broad range of species show that gymnosperms have one to three layers of parenchymatous protophloem cells located at the vascular cylinder periphery between the pericyle and the metaphloem. In some species, these cells are associated with secretory idioblasts. Near the root apex, protophloem cells develop a large central vacuole and, in transverse sections, their radial walls tend to be radially elongated. When mature, these cells are highly longitudinally elongated. Only these cells exhibit surging toward the root apex during chemical fixation. These data indicate that protophloem of gymnosperm roots lacks sieve elements. Because of its distinctive anatomical characteristics and the absence of sieve elements, gymnosperm root protophloem is a vegetative synapomorphy among extant species. The restriction of this tissue type to gymnosperms supports the hypothesis that it originated in a progenitor of that clade. Key words: gymnosperm root, protophloem, vascular plant evolution. Le protophloeme de la racine des gymnospermes est une structure mal comprise. On ignore si les cellules du protophloeme de la racine evoluent, lors de la maturation, comme parenchyme du phloeme ou comme elements des tubes cribles, ou s'il existe a l'interieur du protophloeme un gradient anatomique et evolutionnaire possedant les caracteristiques des deux types cellulaires. Cette question est pertinente pour comprendre les mecanismes anatomiques et physiologiques qui fournissent les nutriments a l'apex des racines. Les donnees anatomiques obtenues d'un large spectre d'especes montrent que les gymnospermes possedent de une a trois couches de cellules parenchymateuses du protophloeme localisees a la peripherie du cylindre vasculaire entre le pericycle et le metaphloeme. Chez certaines especes, ces cellules sont associees aux idioblastes secretoires. Pres de l'apex des racines, les cellules du protophloeme developpent une vacuole centrale de grande taille, et dans les sections transverses, leurs parois radiales tendent a etre allongees radialement. Lorsqu'elles sont matures, ces cellules sont fortement allongees longitudinalement. Seules ces cellules presentent une spumescence vers l'apex des racines lors de la fixation chimique. Ces donnees indiquent que le protophloeme des racines des gymnospermes est depourvu d'elements des tubes cribles. A cause de ses caracteristiques anatomiques distinctives et l'absence d'elements des tubes cribles, le protophloeme de la racine des gymnospermes constitue une synapomorphie vegetative parmi les especes toujours existantes. La restriction de ce type de tissu aux gymnospermes appuie l'hypothese qu'il provenait d'un progeniteur de ce clade. [Traduit par la Redaction] Mots-cles: racine des gymnospermes, protophloeme, evolution des plantes vasculaires., Introduction Root tissues have been extensively investigated with regard to apical organization (e.g., Allen 1947; Wilcox 1954; Pillai 1964; Byrne and Heimsch 1970; Gunning et al. 1978; Peterson and Vermeer [...]

Phylogenetic analysis provides a rational basis for comparative studies of phloem structure and phloem loading. Although several types of minor vein companion cell have been identified, and progress has been made in correlating structural features of these cells with loading mechanisms, little is known about the phylogenetic relationships of the different types. To add to the available data on companion cells, we analyzed the ultrastructure of minor veins in Euonymus fortunei and Celastrus orbiculatis (Celastraceae) leaves and determined that in these species they are specialized as intermediary cells. This cell type has been implicated in symplastic phloem loading. The data were added to published data sets on minor vein phloem characteristics, which were then mapped to a well-supported molecular tree. The analysis indicates that extensive plasmodesmatal continuity between minor vein phloem and surrounding cells is ancestral in the angiosperms. Reduction in plasmodesmatal frequency at this interface is a general evolutionary trend, punctuated by instances of the reverse. This is especially true in the case of intermediary cells that have many plasmodesmata, but other distinguishing characteristics as well, and have arisen independently at least four, and probably six, times in derived lineages. The character of highly reduced plasmodesmatal frequency in minor vein phloem, common in crop plants, has several points of origin in the tree. Thus, caution should be exercised in generalizing results on apoplastic phloem loading obtained from model species. Transfer cells have many independent points of origin, not always from lineages with reduced plasmodesmatal frequency.