Photosynthate Metabolism in the Source Leaves of N₂-Fixing Soybean Plants

Abstract

adaptation of these enzyme activities in the leaves of N2-fixing plants, and this contributed to an increase in starch accumulation. Another major causal factor associated with increased starch accumulation was the elevation in foliar levels of fructose-6-phosphate, glucose-6-phosphate, and glucose-i-phosphate (G 1 P), which had risen to chloroplast concentrations considerably in excess of the Km values for their respective target enzymes associated with starch synthesis, e.g. elevated GI P with respect to adenosine diphosphate glucose pyrophosphorylase (ADPG-PPiase) binding sites. The cofactor glucose-1,6-bisphosphate (G1,6BP) was found to be obligate for maximal PGM activity in soybean leaf extracts of N2-fixing as well as N-supplemented plants, and G1,6BP levels in N2-fixing plant leaves was twice that of levels in N-supplied treatments. However the concentration of chloroplastic G1,6BP in illuminated leaves was computed to be saturating with respect to PGM in both N2-fixing and N-supplemented plants. This Soybean plants coexist symbiotically with Bradyrhizobium japonicum, the bacteria from which those plants can derive all of their reduced nitrogen through N2-fixation processes. Symbiotic soybeans often display very large accumulations of starch and/or sucrose in their source leaves, while concurrently, these same leaves exhibit much reduced soluble protein levels (3, 4). When N2-fixing plants are supplemented with N03-and NH4', or they are grown bacteria free from emergence with a sufficient supply of N,2 their source leaves exhibit a reallocation of carbon photosynthate skeletons to support amino acid and soluble leaf protein accompanied by a diminution of sucrose and starch (3, 4). Further, N2-fixing plants often possess foliar net CO2 assimilatory rates equivalent to those observed in leaves of plants supplemented with growth sufficient levels of N (4, 24). This has been interpreted to mean that approximately the same amount of newly synthesized carbon photosynthate (e.g. TP) that normally was channeled into the syntheses of amino acids and soluble leaf protein in N-supplemented plants, instead was directed to support the syntheses of starch and sucrose within N2-fixing soybean plant leaves (4). Excessive foliar starch accumulation in N2-fixing plants was similar to that observed in N-deficient plants, although N2-fixing soybeans were relatively healthy (compare refs. 3, 4, and 16 with 23 and 25). 2Abbreviations: N, inorganic nitrogen, i.e., N03-and/or NH4+; ADPG-PPiase, ADP-glucose pyrophosphorylase; FBPase, fructose-1,6-bisP phosphatase; PGM, phosphoglucomutase; PHI, phosphohexoisomerase; HK, hexokinase; ALD, aldolase; PFK, phosphofructokinase; SP, starch phosphorylase; AMY, amylase; TP, triose phosphate (glyceraldehyde-3-P and dihydroxyacetone-P); PVM, paraveinal mesophyll; TEM, transmission electron microscopy. NS plants, soybean plants propagated in vermiculite from seedling emergence through experiment termination (4 weeks), and supplied daily with nutrient solution containing 6 mm NH4NO3. Pri...

Leaflet photosynthesis rate and carbon metabolite accumulation patterns in nitrogen-limited, vegetative soybean plants

“…Metabolites were extracted from 10 or 20 mg lyophilized leaf tissue samples, and they were enzymatically quantified in the neutralized, 5% perchloric acid extracts described in the preceding section on Pi quantification. PGA, F6P, G6P, and G1P were enzymatically quantified as described previously (Robinson 1984;de Veau et al 1992). FBP and GAP+DHAP were quantified employing the enzyme analyses described by Michal and Beutler (1974) except that the measuring reaction mixture buffer was 50 mM HEPES, pH 7.6.…”

“…When N level is limiting to plant growth (Table 1), then demand is diminished for carbon assimilates, triose phosphates and hexose phosphates, which normally would be transported to intra-and intercellular sites to support NO3-uptake and reductive N assimilation. The result is that starch and sucrose synthesis and accumulation increases, while anaplerotic carbon flow, amino acid syntheses and leaflet soluble protein decreases (Table 4, Figure 1; Paul et al 1978;Bassham et al 1981;Rufty et al 1984;Robinson and Baysdorfer 1985;Oparka et al 1987;Rufty et al 1988;Khamis and Lamaze 1990;de Veau et al 1990;de Veau et al 1992;Champigny and Foyer 1992;Huppe and Turpin 1994).…”

“…SPS, in the leaves of N-Lim plants were sufficiently activated and leaflet mesophyll cell hexose phosphate substrates were present in sufficient concentration to support the elevated synthesis of carbohydrates. Evidence for this was observed in the leaflets of N stressed, Bradyrhyzobiumjaponicum inoculated soybean plants (cv Williams) which were not yet fully symbiotic, and in which starch accumulated to relatively high levels in the source leaflets (de Veau et al 1990;de Veau et al 1992). In N2-fixing, N stressed plants, leaflet enzymes associated with the starch synthesis pathways including FBPasepH 8.0, phosphohexoisomerase, phosphoglucomutase, and ADPG-PPiase displayed, per unit protein, similar or even elevated activities when compared with enzymes from N-sufficient control plant leaves (de Veau et al 1992).…”

“…For example, leaves of many plant species which have been adapted to growth limiting N levels for prolonged periods (e.g. ~30 or more days) display much higher daily levels of sucrose and lower soluble protein than leaves of N-sufficient plants (Robinson and Baysdorfer 1985;Oparka et al 1987; Khamis and Lamaze 1990;de Veau et al 1990de Veau et al , 1992Rufty et al 1984Rufty et al , 1988. Also, based on single time point determinations, leaves of N-limited, non-N2-fixing soybean plants assimilate carbon metabolites in the anaplerotic pathway, although at reduced levels; this was reflected by the lower PYR kinase and PEPCase activities in leaflets of N-limited compared with those of N-sufficient plants (Robinson and Baysdorfer 1985).…”

Influence of different nutrient regimes on the regulation of carbon metabolism in Norway spruce [Picea abies (L.) Karst.] seedlings

“…A surplus of carbohydrate and amino acid metabolism (Paul et nitrogen can divert photosynthates away from the al, 1978), There is experiniental evidence that the formation of storage or transport carbohydrates such content of F26BP is increased when nitrogen supply as starch or sucrose to amino acid or protein synthesis is high (Selenastrum minutum\ Turpin et al, 1990; (Snyder & Tolbert, 1966;Blackwood & Miflin, Rhizinus communis: Geigenberger & Stitt, 1991). 1976Platt, Plaut & Bassham, 1977;Paul, Cornweil This could lead to a better availability of carbon & Bassham, 1978;de Veau et al, 1990de Veau et al, , 1992. skeletons for amino acid synthesis.…”