Identification of a new sucrose transporter in rye-grass (LpSUT2): Effect of defoliation and putative fructose sensing

International audienceRye-grass fast regrowth after defoliation results from an efficient mobilization of C reserves which are transported as sucrose towards regrowing leaves, and which can be supported by one or several sucrose transporters (SUTs) like LpSUT1. Therefore, our objectives were to isolate, identify, characterize and immunolocalize such sucrose transporters.A protein (LpSUT2) showing a twelve spanning trans-membrane domain, extended N terminal and internal cytoplasmic loop, and kinetic properties consistent with well-known sucrose transporters, was isolated and successfully characterized. Along with LpSUT1, it was mainly localized in mesophyll cells of leaf sheaths and elongating leaf bases. These transporters were also found in parenchyma bundle sheath (PBS) cells but they were not detected in the sieve element/companion cell complex of the phloem. Unlike LpSUT1 transcript levels which increased as a response to defoliation in source and sink tissues, LpSUT2 transcript levels were unaffected by defoliation and weakly expressed. Interestingly, sucrose transport by LpSUT2 was inhibited by fructose.LpSUT1 and LpSUT2 appeared to have different functions. LpSUT1 is proposed to play a key role in C storage and mobilization by allowing sucrose transport between PBS and mesophyll cells, depending on the plant C status. LpSUT2 could be involved in sucrose/fructose sensing at sub-cellular level

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

Differential regulation of two sucrose transporters by defoliation and light conditions in perennial ryegrass

International audienceSucrose transport between source and sink tissues is supposed to be a key-step for an efficient regrowth of perennial rye-grass after defoliation and might be altered by light conditions. we assessed the effect of different light regimes (high vs low light applied before or after defoliation) on growth, fructans and sucrose mobilization, as well as on sucrose transporter expression during 14 days of regrowth. Our results reported that defoliation led to a mobilization of C reserves (first sucrose and then fructans), which was parallel to an induction of LpSUT1 sucrose transporter expression in source and sink tissues (i.e. leaf sheaths and elongating leaf bases, respectively) irrespective to light conditions. Light regime (high or low light) had little effects on regrowth and on C reserves mobilization during the first 48 h of regrowth after defoliation. Thereafter, low light conditions, delaying the recovery of photosynthetic capacities, had a negative effect on C reserves re-accumulation (especially sucrose). Surprisingly, high light did not enhance sucrose transporter expression. Indeed, while light conditions had no effect on LpSUT1 expression, LpSUT2 transcripts levels were enhanced for low light grown plants. These results indicate that two sucrose transporter currently identified in Lolium perenne L are differentially regulated by light and sucrose. (C) 2012 Elsevier Masson SAS. All rights reserved

Phylloplane microbiome diversity in fructan- and non-fructan accumulating grassland species and its relationship with carbohydrate metabolism of the host plant

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Short-term effects of defoliation intensity on sugar remobilization and N fluxes in ryegrass

International audienceIn grassland plant communities, the ability of individual plants to regrow after defoliation is of crucial importance since it allows the restoration of active photosynthesis and plant growth. The aim of this study was to evaluate the effects of increasing defoliation intensity (0, 25, 65, 84, and 100% of removed leaf area) on sugar remobilization and N uptake, remobilization, and allocation in roots, adult leaves, and growing leaves of ryegrass over 2 days, using a N-15 tracer technique. Increasing defoliation intensity decreased plant N uptake in a correlative way and increased plant N remobilization, but independently. The relative contribution of N stored before defoliation to leaf growth increased when defoliation intensity was severe. In most conditions, root N reserves also contributed to leaf regrowth, but much less than adult leaves and irrespective of defoliation intensity. A threshold of defoliation intensity (65% leaf area removal) was identified below which C (glucose, fructose, sucrose, fructans), and N (amino acids, soluble proteins) storage compounds were not recruited for regrowth. By contrast, nitrate content increased in elongating leaf bases above this threshold. Wounding associated with defoliation is thus not the predominant signal that triggers storage remobilization and controls the priority of resource allocation to leaf meristems. A framework integrating the sequential events leading to the refoliation of grasses is proposed on the basis of current knowledge and on the findings of the present work

Phylloplane microbiome diversity in fructan- and non-fructan accumulating grassland species and its relationship with carbohydrate metabolism of the host plant

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Microbiota diversity of pastures: from grass to milk

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Microbiota diversity of pastures: from grass to milk

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Characterization of AgMaT2, a Plasma Membrane Mannitol Transporter from Celery, Expressed in Phloem Cells, Including Phloem Parenchyma Cells[OA]

A second mannitol transporter, AgMaT2, was identified in celery (Apium graveolens L. var. dulce), a species that synthesizes and transports mannitol. This transporter was successfully expressed in two different heterologous expression systems: baker's yeast (Saccharomyces cerevisiae) cells and tobacco (Nicotiana tabacum) plants (a non-mannitol-producing species). Data indicated that AgMaT2 works as an H+/mannitol cotransporter with a weak selectivity toward other polyol molecules. When expressed in tobacco, AgMaT2 decreased the sensitivity to the mannitol-secreting pathogenic fungi Alternaria longipes, suggesting a role for polyol transporters in defense mechanisms. In celery, in situ hybridization showed that AgMaT2 was expressed in the phloem of leaflets, petioles from young and mature leaves, floral stems, and roots. In the phloem of petioles and leaflets, AgMaT2, as localized with specific antibodies, was present in the plasma membrane of three ontologically related cell types: sieve elements, companion cells, and phloem parenchyma cells. These new data are discussed in relation to the physiological role of AgMaT2 in regulating mannitol fluxes in celery petioles