This study presents the effects of methyl jasmonate (MeJA) on growth, N uptake, N partitioning, and N storage in taproots of non-nodulated alfalfa (cv. Lodi). When compared to untreated plants, addition of 100 micro M MeJA to the nutrient solution for 14 days reduced total growth and modified biomass partitioning between shoots and roots in favour of taproots and lateral roots. MeJA decreased N uptake (after 7 days) and increased N partitioning towards roots after 14 days. This preferential N partitioning to roots was accompanied by increased N storage in taproots as soluble proteins. Compared to total soluble proteins, VSP accumulation occurred earlier (7 days), and was greater (2-fold increase) in plants treated with 100 micro M MeJA. Steady-state transcript levels for two VSPs (32 and 57 kDa) also increased markedly (about 4-fold) in roots of plants treated with 100 micro M MeJA. This suggests that MeJA could act directly (transcriptional regulation) or indirectly (via the changes of N partitioning among alfalfa organs) on N storage as soluble proteins and in particular, VSPs. Because the deduced amino acid sequence of the 32 kDa VSP clone reveals high homology with Class III chitinases, we propose that the 32 kDa VSP may have a role in pathogen defense, in addition to its function as a storage protein.

In perennial forage legumes such as alfalfa (Medicago sativa L.) and white clover (Trifolium repens L.), vegetative storage proteins are extensively mobilized to meet the nitrogen requirements of new shoot growth in spring or after cutting in summer. The 32-kDa alfalfa storage protein possesses high homology with class III chitinases, belonging to a group of pathogenesis-related proteins that possess antifreeze protein properties in some species and exhibit chitinolytic activity in vitro. This protein and the corresponding mRNA accumulate in taproots of cold-hardy cultivars during acclimation for winter, and in response to short-day conditions in controlled environments. The 17.3-kDa storage protein of white clover possesses high homology with pathogenesis-related proteins and abscisicacid-responsive proteins from several legume species and has characteristics common to stress-responsive proteins. Low temperature enhances accumulation of this 17.3-kDa protein and its corresponding transcript. Exogenous abscisic acid stimulates the accumulation of vegetative storage proteins and their transcripts in both legume species. These observations suggest that vegetative storage proteins do not exclusively serve as nitrogen reserves during specific phases of legume development, but may play important adaptive roles in plant protection against abiotic (low temperature) and biotic (pathogen attack) stresses. Résumé :Chez les légumineuses fourragères telles que la luzerne (Medicago sativa L.) et le trèfle blanc (Trifolium repens L.), les protéines de réserve des organes végétatifs sont fortement mobilisées pour couvrir les besoins en azote né-cessaires à la croissance des nouvelles parties aériennes au printemps ou après une coupe estivale. La protéine de réserve de 32 kDa de la luzerne possède une forte homologie avec des chitinases de classe III appartenant à un groupe de protéines induites par des pathogènes, présentant des propriétés de protéines antigel chez d'autres espèces, et qui montre une activité chitinolytique in vitro. Cette protéine et les ARNm correspondants sont accumulés dans les pivots de cultivars endurcis au froid, pendant le processus d'acclimatation hivernale, et en réponse à l'application de jours courts, en conditions contrôlées. La protéine de réserve de 17,3 kDa du trèfle possède une forte homologie de séquence avec des protéines induites par les pathogènes et par l'acide abscissique chez plusieurs légumineuses, et partage des caractéristiques communes aux protéines réagissant aux stress. Les basses températures induisent l'accumulation de la protéine de 17,3 kDa et du transcrit correspondant. L'application exogène d'acide abscissique stimule l'accumulation des protéines de stockage et de leurs transcrits chez les deux espèces. Ces observations suggèrent que les protéines de réserves des organes végétatifs ne serviraient pas exclusivement de réserve azotée pendant des phases spécifiques de développement des légumineuses, mais qu'elles pourraient également jouer des rôles adaptatifs importants dans la...

Our objectives were to study the regulation of N partitioning within tissues of non-nodulated alfalfa (Medicago sativa L.) and N storage in taproots as vegetative storage proteins (VSP) of 15, 19, and 32 kDa and β-amylase (57 kDa) by environmental (photoperiod, temperature, N availability) and endogenous factors (methyl jasmonate). When compared to long-day conditions (LD, 16 h day/8 h night), short-day (SD, 8 h day/16 h night), exposure to low temperature (5˚C) or application of methyl jasmonate (MeJA, 100 M ) for 35 d reduced the biomass shoot/ root ratio and modified the source–sink relationships for N. SD and MeJA treatments resulted in partitioning of N to taproots and a concomitant accumulation of VSPs. In comparison with LD, SD treatment also stimulated β-amylase gene expression 2.5-fold. Although low temperature increased the N partitioning to root tissues and the accumulation of soluble proteins in taproot, VSP concentration and β-amylase mRNA levels remained low. Increasing N concentration from 1 to 5 mM KNO3 doubled the total dry matter but did not affect the N partitioning within the plant, VSP accumulation, or ‚ β-amylase expression. These results suggested that short photoperiod can result in preferential N allocation toward taproots with a concomitant induction of VSP accumulation.

No abstract

Our objective was to study the effect of short-day photoperiod for 28, 42 and 56 d on growth, N uptake and N partitioning, particularly vegetative storage protein (VSP) accumulation in taproots of two alfalfa (Medicago�sativa L.) cultivars (Lodi and Europe). For both varieties, the reduction of daylength from 16 h (long day,�LD) to 8 h (short day, SD) for 28 d reduced total plant growth by decreasing shoot growth. Nitrogen uptake and N distribution within the plant was determined by 15N labeling. N uptake decreased with SD treatment duration, and was 2- and 3-fold lower for Europe and Lodi, respectively, for 56 d in SD conditions when compared with LD plants. The SD treatment resulted in preferential partitioning of N to taproots in comparison with LD conditions (19�vs 9% for Lodi and 12 vs 5% for Europe after 28 d). For both cultivars, the SD-induced changes in N allocation to taproots did not significantly affect taproot soluble protein concentrations during 42 d of daylength treatment. In contrast, VSP accumulation occurred after only 28 d for plants grown in SD conditions (6.2 vs 4.8 mg g–1 DW for Lodi and 5.1 vs 1.4 mg g–1 DW for Europe). SD exposure also increased vsp 57 and vsp 32 mRNA transcript levels in Lodi and Europe (up to 2-fold higher) taproots in SD for 28 d compared with LD conditions. Overall results indicate that photoperiod modulates taproot N accumulation in alfalfa by enhancing both β-amylase (vsp 57) and vsp 32 gene expression and accumulation. The enhanced VSP accumulation by short-day photoperiod may result from altered VSP gene expression / transcript stability or occur indirectly through altered N source–sink relationships. Additionally, when SD treatment included a night break with 15 min illumination with sodium high pressure light or red light, our results suggest that the induction of vsp 57 and vsp 32 gene expressions by SD signal is mediated by the phytochrome system.