Jean-Louis Bonnemain

Source-to-sink transport of sugar and regulation by environmental factors

Source-to-sink transport of sugar is one of the major determinants of plant growth and relies on the efficient and controlled distribution of sucrose (and some other sugars such as raffinose and polyols) across plant organs through the phloem. However, sugar transport through the phloem can be affected by many environmental factors that alter source/sink relationships. In this paper, we summarize current knowledge about the phloem transport mechanisms and review the effects of several abiotic (water and salt stress, mineral deficiency, CO2, light, temperature, air, and soil pollutants) and biotic (mutualistic and pathogenic microbes, viruses, aphids, and parasitic plants) factors. Concerning abiotic constraints, alteration of the distribution of sugar among sinks is often reported, with some sinks as roots favored in case of mineral deficiency. Many of these constraints impair the transport function of the phloem but the exact mechanisms are far from being completely known. Phloem integrity can be disrupted (e.g., by callose deposition) and under certain conditions, phloem transport is affected, earlier than photosynthesis. Photosynthesis inhibition could result from the increase in sugar concentration due to phloem transport decrease. Biotic interactions (aphids, fungi, viruses…) also affect crop plant productivity. Recent breakthroughs have identified some of the sugar transporters involved in these interactions on the host and pathogen sides. The different data are discussed in relation to the phloem transport pathways. When possible, the link with current knowledge on the pathways at the molecular level will be highlighted.

Compatible plant-aphid interactions: How aphids manipulate plant responses

To access phloem sap, aphids have developed a furtive strategy, their stylets progressing towards sieve tubes mainly through the apoplasmic compartment. Aphid feeding requires that they overcome a number of plant responses, ranging from sieve tube occlusion and activation of phytohormone-signalling pathways to expression of anti-insect molecules. In addition to bypassing plant defences, aphids have been shown to affect plant primary metabolism, which could be a strategy to improve phloem sap composition in nutrients required for their growth. During compatible interactions, leading to successful feeding and reproduction, aphids cause alterations in their host plant, including morphological changes, modified resource allocation and various local as well as systemic symptoms. Repeated salivary secretions injected from the first probe in the epidermal tissue up to ingestion of sieve-tube sap may play a crucial role in the compatibility between the aphid and the plant.

Structure and functions of the vascular cambium.

Following a general outline on the functioning of the cambium, the authors review the data acquired over the last 20 years. The interest is focused on: i) the ultrastructural characteristics of the two kinds of initials; ii) the early structural, metabolic and molecular modifications occurring during the first stages of daughter cell differentiation into either xylem or phloem; iii) the complex rhythmic changes of structure, metabolism and activity undergone by cambial cells during the seasonal cycle; iv) the characteristics and control of the cessation of cambial activity in autumn and of its reactivation in spring; v) the main research approaches in cell and molecular biology presently open to the students of the cambial meristem.

Phloem loading in Vicia faba leaves: Effect of N-ethylmaleimide and parachloromercuribenzenesulfonic acid on H+ extrusion, K+ and sucrose uptake

The effects of a penetrating (NEM) and a non-penetrating (PCMBS) sulfhydryl-specific reagent on proton extrusion, 86Rb and [U-14C]sucrose uptake by Vicia faba leaves have been studied. Proton extrusion was strongly or completely inhibited by 0.1 mM NEM. 86Rb and [U-14C]sucrose uptake were markedly reduced by NEM concentrations equal to or higher than 0.5 mM. Under our experimental conditions, PCMBS (1 mM) exerted a strong inhibition on [14C]sucrose uptake but did not inhibit proton extrusion and 86Rb uptake. The sensitivity of phloem loading to PCMBS is thought to be a consequence of sugar-carrier blockage and not of inhibition of the proton pump.

Asymmetric distribution of the plasma-membrane H+-ATPase in embryos of Vicia faba L. with special reference to transfer cells

The ultrastructural localization of the plasma-membrane H+ -ATPase by immunocytochemistry was studied in Vicia faba embryos which absorb nutrients from the maternal organism through the transfer cells of their external epidermis. The samples were embedded in LR White resin and the specificity of immunolabelling was checked by inhibition in the presence of purified H+-ATPase. The following results were obtained: (i) The H+-ATPase density varied according to the cell type, being higher in transfer cells than in other cell types, especially the non-modified cells of the internal epidermis. (ii) There was a marked polarity in transfer cells as proton pumps were more numerous in the area of plasmalemma infoldings where active nutrient uptake is assumed to take place, (iii) No clear immunolabelling occurred on the plasma membrane of plasmodesmata. These results demonstrate that in transfer cells the area of plasmalemma infoldings is highly specialized for active solute transport; they also support the idea of specific structural properties of the plasmalemma in plasmodesmata.

Structural and ultrastructural characteristics of the vascular apparatus of the sensitive plant (Mimosa pudica L.)

Summary1. In motor organs ofMimosa pudica xylem contains living fibriform elements limited by a thick lignified highly pitted wall, whereas in other parts of the plant (stem, petiole, rachis), xylem and protoxylem vessels are closely associated with parenchyma cells which possess wall ingrowths. These ingrowths, at the apex of which the plasmalemma and the tonoplast touch, are localized like those of “transfer cells” of C type described byGunning andPate. Nevertheless, xylem parenchyma cells differ from cells of C type in several characteristics. Moreover, in motor organs, phloem contains cells characterized by wall ingrowths, less abundant on the parts adjacent to the sieve tubes; these cells which are localized near collenchyma cells of primary phloem, look like “transfer cells” of A type defined byGunning andPate; they are absent from internodes, petioles and rachides. 2. In motor organs, three types of vascular cells (companion cells, living xylem fibriform elements and protoxylem parenchyma cells) are characterized by reduced vacuolar volumes and well developed membrane systems, as compared with homologuous cells belonging to other parts of the plant. 3. A symplastic continuity holds from the middle of motor organs to their cortex: it is provided by the presence, in xylem and phloem respectively, of living fibriform elements and collenchyma cells bearing numerous pit fields containing large numbers of plasmodesmata. Several ultrastructural features suggest that the vascular apparatus ofMimosa pudica would be the site of intensive lateral transfer at different levels, specially in motor organs. Possible functions of certain structures observed are discussed in relation to some hypotheses relative to excitatory conduction pathways.

Transport, distribution et métabolisme de l'auxine dans la racine de Vicia faba L. après application de[14C]AIA ou de [3H]AIA sur le bourgeon

SummaryAfter application of [2-14C]IAA or [3H]IAA to the apical bud of intact young broad-beans, the movement of labelled auxin into the roots was followed by liquid scintillation counting and by autoradiographic analyses. Its metabolism was studied by chromatography, and its pathways by autoradiographic analyses coupled with ringing experiments or removal of the stele.The movement of [14C]IAA or [3H]IAA was characterized by a high retention of radioactivity in tissues, particularly in very young plants. The speed, which did not exceed 9 mm·h-1 in old roots, appeared the slower the younger the plants were. However, it seemed possible that small quantities of IAA or its derivates went into sieve tubes in which they moved downwards faster. In the apical part of the root the labelled IAA was more quickly transformed than in the other parts of this organ. 24 h after the application of the IAA, the labelled molecules gathered more densely in the cap itself than in apical meristem.At least 2/3 of the applied auxin moved within the stele, which in a crosssection represents only 1/7 of the whole area. In the older part of the root, the cambial zone located between mature phloem and mature xylem was the preferred pathway of IAA transport, although it is a zone where the hormone is immobilized, used and metabolised. In the younger part of the root, the whole stele was the preferred pathway. Therefore, the auxin is in a privileged situation to take part in the regulation of various processes, especially in the development of secondary vascular tissue, more particularly of xylem.After application of [2-(14)C]IAA or [(3)H]IAA to the apical bud of intact young broad-beans, the movement of labelled auxin into the roots was followed by liquid scintillation counting and by autoradiographic analyses. Its metabolism was studied by chromatography, and its pathways by autoradiographic analyses coupled with ringing experiments or removal of the stele.The movement of [(14)C]IAA or [(3)H]IAA was characterized by a high retention of radioactivity in tissues, particularly in very young plants. The speed, which did not exceed 9 mm·h(-1) in old roots, appeared the slower the younger the plants were. However, it seemed possible that small quantities of IAA or its derivates went into sieve tubes in which they moved downwards faster. In the apical part of the root the labelled IAA was more quickly transformed than in the other parts of this organ. 24 h after the application of the IAA, the labelled molecules gathered more densely in the cap itself than in apical meristem.At least 2/3 of the applied auxin moved within the stele, which in a crosssection represents only 1/7 of the whole area. In the older part of the root, the cambial zone located between mature phloem and mature xylem was the preferred pathway of IAA transport, although it is a zone where the hormone is immobilized, used and metabolised. In the younger part of the root, the whole stele was the preferred pathway. Therefore, the auxin is in a privileged situation to take part in the regulation of various processes, especially in the development of secondary vascular tissue, more particularly of xylem.

Energetics of threonine uptake by pod wall tissues of Vicia faba L.

Kjeldahl assays showed that the pod wall of Vicia faba fruits behaves as a transitory reservoir of nitrogen. We have studied the properties and energetics of amino-acid uptake during the accumulating stage of pod wall development. A comparative analysis using various inhibitors or activators of the proton pump has been carried out i) on threonine uptake, ii) on the acidifying activity of the tissues, and iii) on the transmembrane potential difference of mesocarp cells. Except for the effect of dicyclohexylcarbodiimide which could not be satisfactorily explained, all other results obtained with ATPase inhibitors, uncouplers and fusicoccin were consistent with the view of a transport energized by the proton-motive force. Adding threonine to a medium containing fragments of pericarp or of endocarp induced a pH change (to-wards more alkaline values) of the medium and a membrane depolarization of the storage cells which depended on the amino-acid concentration added. These data indicate H+-threonine cotransport in the pod wall of broad bean. Moreover, because p-chloromercuribenzenesulphonic acid inhibits threonine uptake without affecting the transmembrane potential difference, it is concluded that the threonine carrier possesses a functional SH-group located at the external side of the plasmalemma.

Absence of plasma membrane H+-ATPase in plasmodesmata located in pit-fields of the young reactive pulvinus ofMimosa pudica L.

SummaryImmunocytochemical techniques were employed to study the spatial distribution of the plasma membrane H+-ATPase within various cell types of the young reactive primary pulvinus ofMimosa pudica L. These cells were interconnected by large numbers of plasmodesmata, being concentrated within pit-fields. Although we could routinely detect evidence of the H+-ATPase along the plasma membrane, immunolabelling was rarely, if ever, observed along the plasma membranes of the plasmodesmata. This finding is discussed with respect to the likely specialized supramolecular structure of the plasmodesma.

Transport de l auxine-14C en provenance de jeunes gousses de Vicia faba L.

SummaryAfter the injection of [14C]indole acetic acid (IAA) into very young pods of broad-bean (Vicia faba L.) the movement of the 14C in the peduncle and stem was followed by autoradiography. In samples with only one young pod the basipetal transport was always clearly dominant. Most of the radioactivity was found in the bundles, particularly in the outer region of the bundle and also in the inner region (protoxylem parenchyma). The progression of the tracer was relatively complex. The rate of movement of the radioactive «front» could be as much as 2 cm·h-1 but most of the 14C moved towards the base at rates clearly less than that of the «front». Chromatograms with several solvent systems showed that IAA was the main or the only mobile radioactive substance. During transport, a part of IAA was converted into indole-3-aldehyde (IAld) and indole-3-acetyl-aspartic acid (IAAsp). IAAsp and possibly also IAld, which were found mainly near the donor pod, seemed immobile. This work is part of a study on the interchange of phytohormones between fruit and plant.After the injection of [(14)C]indole acetic acid (IAA) into very young pods of broad-bean (Vicia faba L.) the movement of the (14)C in the peduncle and stem was followed by autoradiography. In samples with only one young pod the basipetal transport was always clearly dominant. Most of the radioactivity was found in the bundles, particularly in the outer region of the bundle and also in the inner region (protoxylem parenchyma). The progression of the tracer was relatively complex. The rate of movement of the radioactive «front» could be as much as 2 cm·h(-1) but most of the (14)C moved towards the base at rates clearly less than that of the «front». Chromatograms with several solvent systems showed that IAA was the main or the only mobile radioactive substance. During transport, a part of IAA was converted into indole-3-aldehyde (IAld) and indole-3-acetyl-aspartic acid (IAAsp). IAAsp and possibly also IAld, which were found mainly near the donor pod, seemed immobile. This work is part of a study on the interchange of phytohormones between fruit and plant.