Microautoradiographic localization of imported (14)C-photosynthate in induced sink leaves of two dicotyledonous C4 plants in relation to phloem unloading

Complexity untwined: The structure and function of cucumber (Cucumis sativus L.) shoot phloem

Cucurbit phloem is complex, with large sieve tubes on both sides of the xylem (bicollateral phloem), and extrafascicular elements that form an intricate web linking the rest of the vasculature. Little is known of the physical interconnections between these networks or their functional specialization, largely because the extrafascicular phloem strands branch and turn at irregular angles. Here, export in the phloem from specific regions of the lamina of cucumber (Cucumis sativus L.) was mapped using carboxyfluorescein and ¹⁴ C as mobile tracers. We also mapped vascular architecture by conventional microscopy and X-ray computed tomography using optimized whole-tissue staining procedures. Differential gene expression in the internal (IP) and external phloem (EP) was analyzed by laser-capture microdissection followed by RNA-sequencing. The vascular bundles of the lamina form a nexus at the petiole junction, emerging in a predictable pattern, each bundle conducting photoassimilate from a specific region of the blade. The vascular bundles of the stem interconnect at the node, facilitating lateral transport around the stem. Elements of the extrafascicular phloem traverse the stem and petiole obliquely, joining the IP and EP of adjacent bundles. Using pairwise comparisons and weighted gene coexpression network analysis, we found differences in gene expression patterns between the petiole and stem and between IP and EP, and we identified hub genes of tissue-specific modules. Genes related to transport were expressed primarily in the EP while those involved in cell differentiation and development as well as amino acid transport and metabolism were expressed mainly in the IP.

(Questions)n on phloem biology. 2. Mass flow, molecular hopping, distribution patterns and macromolecular signalling

This review speculates on correlations between mass flow in sieve tubes and the distribution of photoassimilates and macromolecular signals. Since micro- (low-molecular compounds) and macromolecules are withdrawn from, and released into, the sieve-tube sap at various rates, distribution patterns of these compounds do not strictly obey mass-flow predictions. Due to serial release and retrieval transport steps executed by sieve tube plasma membranes, micromolecules are proposed to "hop" between sieve element/companion cell complexes and phloem parenchyma cells under source-limiting conditions (apoplasmic hopping). Under sink-limiting conditions, micromolecules escape from sieve tubes via pore-plasmodesma units and are temporarily stored. It is speculated that macromolecules "hop" between sieve elements and companion cells using plasmodesmal trafficking mechanisms (symplasmic hopping). We explore how differential tagging may influence distribution patterns of macromolecules and how their bidirectional movement could arise. Effects of exudation techniques on the macromolecular composition of sieve-tube sap are discussed.

Pathogen-induced resistance and alarm signals in the phloem

SUMMARY Despite a long-standing notion of long-distance signals triggering systemic acquired resistance (SAR), the translocation pathway and the identity of the signals involved have not been determined with any degree of certainty. A critical assessment indicates that, in parallel to signalling via the phloem, alternative modes for SAR induction such as signalling via the xylem or air-borne signalling by volatile substances may occur. This review further evaluates several classes of compounds as being functional in systemic resistance signalling. Evidence in favour of SAR involvement of phloem-mobile substances such as salicylic acid, lipid-derived molecules, reactive oxygen species and components of the antioxidant machinery is contradictory, circumstantial or inconclusive, at best. Nitric oxide bound to proteins or thiols seems a good candidate for signalling, but has not been found in phloem sap thus far. No convincing support of the involvement in SAR of phloem-mobile substances such as calcium, oligosaccharides, peptides or RNA species, which function in other systemic signalling cascades, has yet been produced. Nevertheless, phloem-mobile macromolecules are considered as potential tools for SAR given their pivotal role in remote gene expression under stress conditions. In this framework, the existence of several cascades for signal generation along the phloem pathway is envisaged. Finally, recent methods for detection of molecular signals in phloem sap and their expression in companion cells are presented.

Tansley Review No. 27 The control of carbon partitioning in plants

This review reports on the processes associated with carbon transfer and metabolism in leaves and growing organs and the role of long-distance transport and vascular links in the regulation of carbon partitioning in plants. Partitioning is clearly influenced by both the supply and demand for photosynthate and is moderated by vascular connections and the storage capacity of the leaves and pathway tissues. However there appears to be little more than circumstantial evidence either that short distance transfer of carbon within either the source or the sink, or that long-distance transport in the phloem, are limiting photosynthesis or growth directly. Although individual biochemical and physiological processes relating to photosynthesis and growth may be well understood, the factors primarily responsible for the control of carbon partitioning in plants have not been clearly identified. There is a need for a greater understanding of organ initiation and development (source and sink formation and potential size), the clear identification of whether growth is sink or source limited (including possible sink-controlled photosynthesis) and a detailed assessment of the role of storage in buffering developmental and environmental changes in sink and source activity. Also more information is needed on the role of hormonal and nutritional factors in regulating source and sink activity (organ interactions not directly associated with carbon transfer). CONTENTS Summary 341 I. Introduction 342 II. General source-sink relationships 342 III. Control at the source 345 IV. The utilization of photosynthate: sink characteristics and limitations 353 V. Vascular constraints and temporary storage 360 VI. Concluding comments 366 Acknowledgements 366 References 367.

A morphometric analysis of the phloem-unloading pathway in developing tobacco leaves

A morphometric analysis of developing leaves of Nicotiana tabacum L. was conducted to determine whether imported photoassimilates could be unloaded by symplastic transport and whether interruption of symplastic transport could account for termination of import. Five classes of veins were recognized, based on numbers of cells in transverse section. Photoassimilate is unloaded primarily from Class III veins in tissue nearing the end of the sink phase of development. Smaller veins (Class IV and V) do not transport or unload photoassimilate in sink tissue because the sieve elements of these veins are immature until after the tissue stops importing. In Class III veins the sieve element-companion cell (SE-CC) complexes are surrounded by phloem parenchyma which abuts the bundle sheath. Along the most obvious unloading route, from SE-CC complex to phloem parenchyma to bundle sheath to mesophyll cells, the frequency of plasmodesmata at each interface increases. To determine whether this pattern of plasmodesmatal contact is consistent with symplastic unloading we first demonstrated, by derivation from Fick's law that the rate of diffusion from a compartment is proportional to a number N which is equal to the ratio of surface area to volume of the compartment multiplied by the frequency of pores (plasmodesmata) which connect it to the next compartment. N was calculated for each compartment within the vein which has the SE-CC complex as its center, and was shown to be statistically the same in all cases except one. These observations are consistent with a symplastic unloading route. As the leaf tissue matures and stops importing, plasmodesmatal frequency along the unloading route decreases and contact area between cells also decreases as intercellular spaces enlarge. As a result, the number of plasmodesmata between the SE-CC complex and the first layer of mesophyll cells declines in nonimporting tissue to 34% of the number found in importing tissue, indicating that loss of symplastic continuity between the phloem and surrounding cells plays a role in termination of photoassimilate unloading.

Photoassimilate-transport characteristics of nonchlorophyllous and green tissue in variegated leaves of Coleus blumei Benth

The nonchlorophyllous (albino) tissue of mature C. blumei leaves is a sink for photoassimilate. Transport from the green to the albino region of the same leaf was inhibited by cold and anoxia. When the green tissue of mature leaves was removed, the remaining albino portion imported labeled translocate from other mature leaves in the phloem. Photoassimilate unloading in the albino region of mature leaves was studied by quantitative autoradiography. The unloading was inhibited by cold but not by anoxia. No labeled photoassimilate could be detected in the free space of mature albino tissue by compartmental efflux analysis as phloem unloading proceeded in a N2 atmosphere, indicating that unloading, may occur by a symplastic pathway as it apparently does in sink leaves of other species. The minor veins of mature albino leaf tissue did not accumulate exogenous [(14)C]sucrose. Minor veins of green tissue in the same leaves accumulated [(14)C]sucrose but, in contrast to other species studied to date, this accumulation was insensitive to the inhibitor p-chloromercuribenzensulfonic acid (PCMBS).In its capacity to import and unload photoassimilate, and in the inability, of the minor veins to accumulate exogenous sucrose, the albino region of the mature C. blumei lamina differs from mature albino tobacco leaves and darkened mature leaves of other species. This, together with evidence indicating that phloem loading in C. blumei and other species may occur by different routes and with different sensitivity to PCMBS, indicates that the mechanism of transfer of photoassimilates between veins and surrounding tissues, and the mechanism of the sink-source transition, may not be the same in the leaves of all species. It is speculated that the unusual properties of the C. blumei leaf may be a consequence of the presence, in the minor veins, of "intermediary cells", large companion cells connected to the bundle sheath by abundant plasmodesmata.