Search

Jasmonates are ubiquitous oxylipin-derived phytohormones that are essential in the regulation of many development, growth and defence processes. Across the plant kingdom, jasmonates act as elicitors of the production of bioactive secondary metabolites that serve in defence against attackers (1-3). Knowledge of the conserved jasmonate perception and early signalling machineries is increasing (3-6), but the downstream mechanisms that regulate defence metabolism remain largely unknown. Here we show that, in the legume Medicago truncatula, jasmonate recruits the endoplasmic-reticulum-associated degradation (ERAD) quality control system to manage the production of triterpene saponins, widespread bioactive compounds that share a biogenic origin with sterols (7-9). An ERAD-type RING membrane-anchor E3 ubiquitin ligase is co-expressed with saponin synthesis enzymes to control the activity of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), the rate-limiting enzyme in the supply of the ubiquitous terpene precursor isopentenyl diphosphate. Thus, unrestrained bioactive saponin accumulation is prevented and plant development and integrity secured. This control apparatus is equivalent to the ERAD system that regulates sterol synthesis in yeasts and mammals but that uses distinct E3 ubiquitin ligases, of the HMGR degradation 1 (HRD1) type, to direct destruction of HMGR (10-13). Hence, the general principles for the management of sterol and triterpene saponin biosynthesis are conserved across eukaryotes but can be controlled by divergent regulatory cues., To identify regulators of plant triterpene synthesis, we monitored the transcriptome of suspension-cultured M. truncatula cells, known to increase accumulation of saponins, one of the main metabolite classes in this [...]

There is interest in increasing the protein concentration of soybean [Glycine max (L.) Merr.] seed. Potentially, this could change its protein composition and perhaps its value as a livestock feed. Our objective was to examine the change in Protein composition of soybean seed with change in protein concentration as influenced by nitrogen source supplied. `Harper' soybean Was grown in hydroponic culture MA supplied with different nitrogen sources ([KNO.sub.3], [NH.sub.4][NO.sub.3], [N.sub.2], urea) during pod filling. Protein quality was monitored as a function of soybean seed storage subunit composition. With [N.sub.2] fixation as sole N source, the concentration of total seed protein decreased about 6%, storage protein 5%. Each seed storage protein subunit decreased, but the concentration of Β-conglycinin, especially the sulfur-poor Β-sub-unit, decreased more than glycinin. Seed storage protein concentration was increased up to 4% by substituting reduced-N for [NO.sub.3]. The increased protein concentration resulted from a disproportionate increase of the Β-subunit of Β-conglycinin. The relative abundance of [Alpha]- and [Alpha']-subunits was not changed by N form. Thus, N form seems to control, at least in part, the relative portion of the Β-subunit of Β-conglycinin, and consequently affects the 11S/7S ratio of seed storage protein. This change in composition with increased concentration diminishes the protein quality because the Βsubunit lacks sulfur-amino acids. It is noteworthy that the concentration of glycinin remained constant while the total seed protein concentration increased. We think these variations in seed storage protein composition result from the relative abundance of S- and N-metabolites available to developing soybean seed., Protein and oil are the most valuable components of soybean seed. Glycinin (11S) and Β-conglycinin (7S), the two main classes of multi-subunit seed storage proteins, account for approximately 70% of [...]