Search

SeveralAstragalusspecies have the ability to hyperaccumulate selenium (Se) when growing in their native habitat. Given that the biochemical properties of Se parallel those of sulfur (S), we examined the activity of key S assimilatory enzymes ATP sulfurylase (ATPS), APS reductase (APR), and serine acetyltransferase (SAT), as well as selenocysteine methyltransferase (SMT), in eightAstragalusspecies with varying abilities to accumulate Se. Se hyperaccumulation was found to positively correlate with shoot accumulation ofS-methylcysteine (MeCys) andSe-methylselenocysteine (MeSeCys), in addition to the level of SMT enzymatic activity. However, no correlation was observed between Se hyperaccumulation and ATPS, APR, and SAT activities in shoot tissue. TransgenicArabidopsis thalianaoverexpressing bothATPSandAPRhad a significant enhancement of selenate reduction as a proportion of total Se, whereasSAToverexpression resulted in only a slight increase in selenate reduction to organic forms. In general, total Se accumulation in shoots was lower in the transgenic plants overexpressingATPS,PaAPR, andSAT. Root growth was adversely affected by selenate treatment in bothATPSandSAToverexpressors and less so in thePaAPRtransgenic plants. Such observations support our conclusions that ATPS and APR are major contributors of selenate reductionin planta. However, Se hyperaccumulation inAstragalusis not driven by an overall increase in the capacity of these enzymes, but rather by either an increased Se flux through the S assimilatory pathway, generated by the biosynthesis of the sink metabolites MeCys or MeSeCys, or through an as yet unidentified Se assimilation pathway. [ABSTRACT FROM AUTHOR]

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 µM 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 µ MMeJA. Steady-state transcript levels for two VSPs (32 and 57 kDa) also increased markedly (about 4-fold) in roots of plants treated with 100 µM 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. [ABSTRACT FROM AUTHOR]