Implication des frutanes et du loliose dans la croissance de Lolium perenne L. après défoliation et dans la tolérance à la sécheresse (Identification des formes de transport du carbone)

CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF

Phloem Loading Strategies in Three Plant Species That Transport Sugar Alcohols1[C][OA]

Many plants translocate sugar alcohols in the phloem. However, the mechanism(s) of sugar alcohol loading in the minor veins of leaves are debated. We characterized the loading strategies of two species that transport sorbitol (Plantago major and apple [Malus domestica]), and one that transports mannitol (Asarina scandens). Plasmodesmata are abundant at all interfaces in the minor vein phloem of apple, and in one of two types of phloem in the minor veins of A. scandens. Few plasmodesmata are present in the minor veins of P. major. Apple differs from the other two species in that sugar alcohol and sucrose (Suc) are present in much higher concentrations in leaves. Apple leaf tissue exposed to exogenous [14C]sorbitol, [14C]Suc, or 14CO2 did not accumulate radiolabel in the minor veins, as determined by macroautoradiography. P. major minor veins accumulated radiolabel from [14C]Suc, [14C]sorbitol, and 14CO2. A. scandens minor veins accumulated 14C from [14C]Suc and 14CO2, but not from [14C]mannitol. We conclude that the movement of sugar alcohol from the mesophyll into the phloem in apple and A. scandens is symplastic and passive, but in P. major it involves an apoplastic step and is energized. We also suggest that apple leaves transport sorbitol in high concentrations to avoid the feedback limitation of photosynthesis that would result from driving passive movement of solute into the phloem with high levels of Suc alone. The loading pathways and the mechanisms by which hydrostatic pressure is maintained in the minor vein phloem of these species are discussed

Anatomical and photosynthetic acclimation to the light environment in species with differing mechanisms of phloem loading.

Plants load sugars from photosynthesizing leaves into the phloem of exporting veins either apoplastically (by using H+/sucrose symporters) or symplastically (through plasmodesmata). The ability to regulate photosynthesis in response to the light environment was compared among apoplastic loaders (pea and spinach) and symplastic loaders (pumpkin and Verbascum phoeniceum). Plants were grown under low light (LL) or high light (HL) or transferred from LL to HL. Upon transfer, pea and spinach up-regulated photosynthesis to the level found in HL-acclimated plants, whereas up-regulation in pumpkin and V. phoeniceum was limited. The vein density of pea and spinach was the same in HL and LL. Although spinach did not exhibit anatomical or ultrastructural acclimation to the light environment, in pea, wall invaginations in minor vein companion (transfer) cells were more extensive in HL. Furthermore, upon transfer from LL to HL, these invaginations increased in mature pea leaves. Foliar starch levels in mature leaves of plants transferred from LL to HL were not greater than in HL-acclimated leaves of either apoplastically loading species. In the symplastic loaders, plasmodesmatal frequency per loading cell did not vary with treatment, but vein density and thus total plasmodesmatal frequency were higher in HL. Upon transfer of symplastic loaders, however, vein density remained low, and starch levels were higher than in HL; the incomplete acclimation of photosynthesis upon transfer is thus consistent with a carbon export capacity physically limited by an inability to increase vein and plasmodesmatal density in a mature leaf

Carbon metabolism in grass leaf meristems

International audienceIn Lolium perenne, fructan polymers represent the main storage carbohydrates. Even if leaf meristems, located in elongating leaf bases, act as strong sink for imported assimilates, they also synthesize fructans in substantial amounts. Fructans are generally not evenly distributed. Their highest content is found in the growth zone (0-30 mm from the leaf base) and decreased strongly in the differentiation zone (30-60 mm). Lp1-SST, Lp6G-FFT/1-FFT and Lp6-SFT, encoding the three main fructan synthesizing activities in Lolium perenne, were also predominantly expressed in the growth zone. Their expression declined along the leaf axis, in parallel with the spatial occurrence of fructans, sucrose and enzyme activities. As a response to defoliation, the decline in fructan content occurred not only in the differentiation zone, but also in the growth zone. Before defoliation, the activity of fructan exohydrolase (FEH) was maximal in the differentiation zone. After defoliation, it increased in all segments, but peaked in the growth zone. These data strongly indicate that fructans stored in the leaf growth zone were hydrolyzed and recycled in that zone to sustain refoliation immediately after defoliation. Leaf sheaths represent the other source of fructans. When the product of fructan degradation, fructose, was supplied as 13C-fructose to leaf sheaths at the time of defoliation, its fate showed that the relative supply of C to roots was transiently reduced for the benefit of the growth zone where 13C was allocated first to the proximal part (0-10 mm). This preferential allocation of C could be at least partly explained by a strong and specific increase of the SuSy activity in that zone after defoliation.The results will be discussed in relation to plant development and defoliation tolerance

Carbohydrate and amino acid composition in phloem sap of Lolium perenne L. before and after defoliation

International audienceThis study investigated leaf exudates collected in a 5 mmol litre-1 EDTA solution as predictors of sugar content of phloem sap obtained through excised aphid (Rhopalosiphum padi) stylets, and analysed the effect of defoliation on the amino acid and the carbohydrate composition in the phloem sap of Lolium perenne (cv. Bravo). Results indicate that leaf exudates obtained from leaves devoid of petiole might not be relevant predictors of carbohydrate content of pure phloem sap. Sucrose was the dominating carbohydrate, accounting for 93% of the total soluble sugars in the phloem sap. Myo-inositol, glucose, and fructose were present in trace amounts, while fructans, raffinose, and loliose have never been detected. Predominant amino acid in the phloem sap was glutamine followed by glutamate, aspartate, and serine. Phloem sap component concentration declined during the first hours following defoliation. Sucrose was the main sugar transported in the phloem sap of L. perenne, despite the fact that the product of fructan degradation was fructose and not sucrose. The results are discussed in relation with the physiological mechanisms that contribute to plant recovery after defoliation

Highly efficient radiosensitization of human glioblastoma and lung cancer cells by a G-quadruplex DNA binding compound

International audienceTelomeres are nucleoprotein structures at the end of chromosomes which stabilize and protect them from nucleotidic degradation and end-to-end fusions. The G-rich telomeric single-stranded DNA overhang can adopt a four-stranded G-quadruplex DNA structure (G4). Stabilization of the G4 structure by binding of small molecule ligands enhances radiosensitivity of tumor cells, and this combined treatment represents a novel anticancer approach. We studied the effect of the platinum-derived G4-ligand, Pt-ctpy, in association with radiation on human glioblastoma (SF763 and SF767) and non-small cell lung cancer (A549 and H1299) cells in vitro and in vivo. Treatments with submicromolar concentrations of Pt-ctpy inhibited tumor proliferation in vitro with cell cycle alterations and induction of apoptosis. Non-toxic concentrations of the ligand were then combined with ionizing radiation. Pt-ctpy radiosensitized all cell lines with dose-enhancement factors between 1.32 and 1.77. The combined treatment led to increased DNA breaks. Furthermore, a significant radiosensitizing effect of Pt-ctpy in mice xenografted with glioblastoma SF763 cells was shown by delayed tumor growth and improved survival. Pt-ctpy can act in synergy with radiation for efficient killing of cancer cells at concentrations at which it has no obvious toxicity per se, opening perspectives for future therapeutic applications