Defoliation, wounding, and methyl jasmonate induce expression of the sucrose lateral transporter LpSUT1 in ryegrass (Lolium perenne L.).

Ryegrass (Lolium perenne L.) regrowth after defoliation results from the mobilization of sugar reserves (mainly fructans) and, simultaneously, the efficient lateral transport of sucrose toward growing tissues. However, as for grasses overall, it is not yet known if the induction of this transport is solely linked to the sugar demand of growing tissues via the modification of sugar content at the tissue or cellular level or if it could be triggered by a wounding signal due to the defoliation itself. Ryegrass plants were therefore submitted to total or partial defoliation, pinning of the leaf blades to simulate wounding, or to leaf spraying with 100 µM methyl jasmonate (MeJA), a phytohormone related to wounding. As a response to total or partial defoliation, fructans were mobilized, and the expression of the sucrose lateral transporter LpSUT1 was induced. This highlights an efficient intra-plant compensatory partitioning of sugar resources between defoliated and intact tillers, resulting in the adaptation to regrow after moderate to severe defoliation. The MeJA treatment strongly decreased fructan content. Pinning and especially MeJA largely and quickly increased sucrose content and LpSUT1 transcript levels in leaf sheaths and elongating leaf bases, suggesting a direct effect of wounding on the upregulation of the sucrose lateral transporter. The overall results suggest that sucrose transport capacity and fructan degradation are induced by defoliation through the modification of source-sink relationships for sugars at the plant level and are mediated by phytohormones associated with wounding, such as jasmonates.

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

In 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 15N 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.

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

Rye-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.

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

Sucrose 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.

Activation of sucrose transport in defoliated Lolium perenne L.: an example of apoplastic phloem loading plasticity.

The pathway of carbon phloem loading was examined in leaf tissues of the forage grass Lolium perenne. The effect of defoliation (leaf blade removal) on sucrose transport capacity was assessed in leaf sheaths as the major carbon source for regrowth. The pathway of carbon transport was assessed via a combination of electron microscopy, plasmolysis experiments and plasma membrane vesicles (PMVs) purified by aqueous two-phase partitioning from the microsomal fraction. Results support an apoplastic phloem loading mechanism. Imposition of an artificial proton-motive force to PMVs from leaf sheaths energized an active, transient and saturable uptake of sucrose (Suc). The affinity of Suc carriers for Suc was 580 microM in leaf sheaths of undefoliated plants. Defoliation induced a decrease of K(m) followed by an increase of V(max). A transporter was isolated from stubble (including leaf sheaths) cDNA libraries and functionally expressed in yeast. The level of L.perenne SUcrose Transporter 1 (LpSUT1) expression increased in leaf sheaths in response to defoliation. Taken together, the results indicate that Suc transport capacity increased in leaf sheaths of L. perenne in response to leaf blade removal. This increase might imply de novo synthesis of Suc transporters, including LpSUT1, and may represent one of the mechanisms contributing to rapid refoliation.

Characterization of AgMaT2, a plasma membrane mannitol transporter from celery, expressed in phloem cells, including phloem parenchyma cells.

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.