Evidence for functional heterogeneity of sieve element–companion cell complexes in minor vein phloem of Alonsoa meridionalis

Abstract

Two modes of phloem loading have been proposed, apoplastic and symplastic, depending on the structure of sieve element-companion cell complexes (SE-CCCs) in minor vein phloem. Species are usually classified as either apoplastic or symplastic loaders although the cytology of SE-CCCs in minor veins of the majority of plants indicates that both mechanisms can be simultaneously involved in phloem loading. The functions of structurally different SE-CCCs in minor veins of the stachyose-translocating plant Alonsoa meridionalis were examined. A stachyose synthase gene, AmSTS1, was expressed in intermediary cells but not in the ordinary companion cell of the same vein. In contrast, sucrose transporter AmSUT1 protein was present in ordinary companion cells but not in the neighbouring intermediary cells. These data reveal the principles of phloem sap formation in A. meridionalis and, probably, in many other dicots. The two types of SE-CCCs within one and the same minor vein load different carbohydrates, using contrasting mechanisms for their delivery into the phloem. Lateral sieve pores in the minor vein phloem lead to mixing of the carbohydrates soon after loading. While symplastic and apoplastic pathways can function simultaneously during phloem loading, they are separated at the level of different SE-CCCs combined in phloem endings.

Cytology of the minor-vein phloem in 320 species from the subclass Asteridae suggests a high diversity of phloem-loading modes†

“…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.…”

“…In all these cases, there are asymmetrically branched plasmodesmata the exclusion limit of which is probably regulated, analogous to the situation in ICs. The polymer trap model proposing RFO synthesis as a mechanism regulating barrier properties of such plasmodesmata in species with ICs was cast into doubt based on results obtained using the ICs-containing species Alonsoa meridionalis (Voitsekhovskaja et al, 2006, 2009); moreover, in leaves of species containing MIC-b and CC-b, no RFOs could be detected by GC-MS which, considering the sensitivity of the method, leads to the conclusion that RFOs cannot be the main transport form in these species, at least not at all developmental stages.…”

“…At the same time, many plants contain companion cells of more than one structural type in their minor veins, a fact which renders them beyond the scope of a division into “open” and “closed” types and becomes important when analyzing phloem loading mechanisms and their relative contribution to phloem transport in a given species. An example is Alonsoa meridionalis , a type 1 species, which contains ICs in its minor veins and was originally classified as a putative symplasmic loader but was shown to perform also apoplasmic sucrose loading in a second type of companion cells present in its minor veins, the OC (Voitsekhovskaja et al, 2009). Thus, a comprehensive view of a phloem ending as a functional unit can rely only on the cytology of all cells involved in assimilate loading.…”

Phloem Transport of Resources

“…Consistent with this hypothesis is that downregulation of a key RFO biosynthetic gene depressed photoassimilate export (McCaskill & Turgeon 2007). In some RFO-translocating species, SUTs may be present in SE/CC complexes but absent from ICs, indicating concurrent sym-and apoplastic loading within the same vein (e.g., Voitsekhovskaja et al 2009). However, transgenic downregulation of SUT activity does not affect photoassimilate export, suggesting SUTs in symplastic species serve to retrieve sucrose leaked from transport phloem rather than a loading function per se (Zhang & Turgeon 2009).…”

Phloem Loading through Plasmodesmata: A Biophysical Analysis

“…However, available evidence does not support the presence of complementary apoplastic and symplastic mechanisms in intermediary cells: The Suc transporter does not immunolocalize to the intermediary cell plasma membrane (Voitsekhovskaja et al, 2009). It is possible, even likely, that apoplastic loading occurs in other companion cells in RFO plants (those that do not express the RFO pathway), but if so this contribution is apparently minimal since blocking the Suc transporter chemically (Turgeon and Gowan, 1990) or by RNAi (Zhang and Turgeon, 2009) does not noticeably inhibit growth.…”