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

Summary 1. 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.     

Metabolism of cationized lipoproteins by human fibroblasts: biochemical and morphologic correlations

Human plasma low density lipoprotein (LDL) that had been rendered polycationic by coupling with N, N-dimethyl-1, 3-propanediamine (DMPA) was shown by electron microscopy to bind in clusters to the surface of human fibroblasts. The clusters resembled those formed by polycationic ferritin (DMPA-feritin), a visual probe that binds to anionic site on the plasma membrane. Biochemical studies with (125)I-labeled DMPA-LDL showed that the membrane-bound lipoprotein was internalized and hydrolyzed in lysosomes. The turnover time for cell bound (125)I-DMPA-LDL, i.e., the time in which the amount of (125)I-DMPA-LDL degraded was equal to the steady-state cellular content of the lipoprotein, was about 50 h. Because the DMPA-LDL gained access to fibroblasts by binding nonspecifically to anionic sites on the cell surface rather than by binding to the physiologic LDL receptor, its uptake failed to be regulated under conditions in which the uptake of native LDL was reduced by feedback suppression of the LDL receptor. As a result, unlike the case with native LDL, the DMPA-LDL accumulated progressively within the cell, and this led to a massive increase in the cellular content of both free and esterified cholesterol. Studies with (14)C-oleate showed that at least 20 percent of the accumulated cholesteryl esters represented cholesterol that had been esterified within the cell. After 4 days of incubation with 10 μg/ml of DMPA-LDL, fibroblasts had accumulated so much cholesteryl ester that neutral lipid droplets were visible at the light microscope level with Oil Red O staining. By electron microscopy, these intracellular lipid droplets were observed to lack a tripartite limiting membrane. The ability to cause the overaccumulation of cholesteryl esters within cells by using DMPA-LDL provides a model system for study of the pathologic consequences at the cellular level of massive deposition of cholesteryl ester.     

Mechanism of the inhibition of phloem loading by sodium sulfite: Effect of the pollutant on the transmembrane potential difference

Sodium sulfite (Na₂SO₃) decreased uptake from 1 m M sucrose by the parenchyma and by the veins of leaves of broadbean ( Vicia faba L. cv. Aguadulce). The decrease depended on the concentration of the pollutant and the duration of pretreatment. The inhibition was non-competitive. Sulfite affected the transmembrane potential difference (PD) of the leaf tissues. The short-term response obeyed an 'all or nothing' law. At 0.1 m M and above, sulfite led to a quick depolarization of one-third of the initial potential after a lag phase of about 5 min; for concentrations lower than 0.1 m M , sulfite did not affect the potential. By contrast, the long-term effect of Na₂SO₃ on the transmembrane PD strongly depended on its concentration. After 2–12 h of pretreatmemt there was no effect at 10 μ M , a weak effect at 0.1 m M , and then increasing depolarization as the pollutant concentration increased. The inhibitory effect of Na₂SO₃ on sucrose uptake is thus, at least partly, due to its effect on a component of the proton-motive force. ΔΨ. However, the lack of correlation noticed with 0.1 m M Na₂SO₃ between the effect on sucrose uptake and the long-term effect on transmembrane PD suggests numerous sites of sulfite action.     

Inhibition of Loading of C Assimilates by p-Chloromercuribenzenesulfonic Acid : Localization of the Apoplastic Pathway in Vicia faba

The apoplast of mature leaves excised from broadbean (Vicia faba L.) plants was infiltrated with 2 millimolar p-chloromercuribenzenesulfonic acid (PCMBS) via the transpiration stream, and the ability of the tissues to take up sugars was tested. An infiltration time of 75 minutes was sufficient to obtain a maximal (75%) inhibition of exogenous [¹⁴C]sucrose (1 millimolar) uptake. This infiltration affected neither CO₂ assimilation nor the transmembrane potential difference of leaf cells but strongly inhibited phloem loading of endogenous [¹⁴C] assimilates. The study of the symplastic relations between the different cell types of the mature leaf showed that the density of the plasmodesmata is generally very low in comparison with other species investigated so far, particularly when considering the mesophyll/bundle sheath and the bundle sheath/phloem cells connections, as well as the connections of the transfer cell-sieve tube complex with the surrounding cells. These three successive barriers therefore strongly limit the possibilities of symplastic transit of the assimilates to the conducting cells. The comparison of the densities of plasmodesmata in an importing and an exporting leaf suggests that the maturation of the leaf is characterized by a marked symplastic isolation of the phloem, and, within the phloem itself, by the isolation of the conducting complex. As a consequence, these physiological and cytological data demonstrate the apoplastic nature of loading in the mature leaf of Vicia faba, this species undoubtedly presenting a typical model for apoplastic loading.     

Seasonal variations in the polar-transport pathways and retention sites of [3H]indole-3-acetic acid in young branches ofFagus sylvatica L.

Branches were cut from young beeches (Fagus sylvatica L.) at various stages of the annual cycle and [³H]indole-3-acetic acid (0.35 nmol) was applied to the whole surface of the apical section of each branch, just below the apical bud. The labelled pulse (moving auxin) and the following weakly radioactive zone (auxin and metabolites retained by the tissues) were localized by counting: microautoradiographss were made using cross sections from these two regions. During the second fortnight of April, auxin was transported by nearly all the cells of the young primary shoot, but the label was more concentrated in the vascular bundles. Auxin transport became the more localized: the cortical parenchyma appeared to lose its ability to transport the hormone (end of April), followed in turn by the pith parenchyma (May). Polar auxin movement at that time was limited to the outer part of the bundle (cambial zone and phloem) and to the inner part (protoxylem parenchyma). Later protoxylem parenchyma ceased to carry auxin. During the whole period of cambial activity, auxin was transported and retained mainly by the cambial zone and its recent derivatives. In September, before the onset of dormancy, and in February, at the end of the resting period, the transport pathways and retention sites for auxin were mainly in the phloem, where sieve tubes often completely lacked radiolabel. When cambial reactivation occurred in the one-year shoot, auxin was mainly carried and retained again in the cambial zone and differentiating derivatives.Abbreviation IAA indole-3-acetic acid