Ploidy Effects in Isogenic Populations of Alfalfa

125

The in vitro ribulose-1,5-bisphosphate (RuBP) carboxylase activity per unit of leaf nitrogen was found to be 30% greater in Triticum aestiaum than in T. monococcum. This was due to a higher specific activity of the enzyme from T. aestirxm, as the amount of RuBP carboxylase protein per unit of total leaf nitrogen did not differ between the genotypes. The occurrence of higher specific activity of RuBP carboxylase is shown to correlate with possession of the lrge subunit derived from the B genome of wheat.Despite the greater RuBP carboxylase activity per unit of leaf nitrogen in T. aestivum, the initial slopes ofcurves relating rate ofC02 assimilation to intercellular p(CO2) are similar in T. aestivuam and T. mowococcum for the same nitrogen content per unit leaf area. The similarity of the initial slopes is the result of a greater resistance to CO2 transfer between the intercellular spaces and the site of carboxylation in T. astivum than in T. monococcum.Studies ofthe photosynthetic performance ofplants have often been made using a wide variety of species representing diverse evolutionary backgrounds. One advantage of studying the physiology of wheat, apart from the fact that much is already known, is that comparisons can be made between species from different stages in the evolution of the crop. The derivation of modem hexaploid wheat has been well established from a combination of classical genetic and taxonomic work (25) with more recent DNA and protein techniques (5, 30).Several studies have used a broad series of wheats to examine the changes in photosynthetic capacity that have occurred with domestication and the increase in ploidy. Evans and Dunstone ( 11) showed that there has been a parallel increase in the size of both seeds and leaves which is associated with a decline in the maximum rate of photosynthesis per unit of leaf area. This decline was thought to be associated with increased mesophyll resistance due to the lower surface to volume ratio of the larger cells in the hexaploid. However, Dunstone et al. (8) This study examines differences in biochemical and anatomical limitations to photosynthesis between primitive and modem lines of wheat. We have concentrated on in vivo and in vitro analysis of the performance of RuBP3 carboxylase in relation to photosynthetic capacity of the intact leaf. Differences in photosynthesis between wheat genotypes could result from differential nitrogen allocation to RuBP carboxylase protein, alterations in the kinetic properties of the enzyme, or anatomical differences which affect the CO2 pressure at the site of carboxylation. We have examined these possibilities through a comparison of a modem hexaploid wheat, T. aestivum, and a diploid ancestor, T. monococcum. We have also extended this study to include a more thorough examination of the relationship between kinetic properties of RuBP carboxylase and wheat genome.MATERIALS AND METHODS Plant Material. Seed of the wheat accessions (Table I)

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Differences between Wheat Genotypes in Specific Activity of Ribulose-1,5-bisphosphate Carboxylase and the Relationship to Photosynthesis

Evans¹,

Seemann²

1984

125

“…The production and purification of maize PPDK-and alfalfa RuBPCase-specific immunoglobulin G were described earlier (4,16). Rocket immunoelectrophoresis was used to quantify both proteins as previously described (4 …”

mentioning

confidence: 99%

Relationships between CO₂ Exchange Rates and Activities of Pyruvate,Pi Dikinase and Ribulose Bisphosphate Carboxylase, Chlorophyll Concentration, and Cell Volume in Maize Leaves

Baer¹,

Schrader²

1985

Self Cite

The relationships between CO2-exchange rate (CER), DNA and chlorophyll (Chl) concentrations, pyruvate,Pi dikinase (PPDK) and ribulose bisphosphate carboxylase (RuBPCase) activities in ten maize (Zea mays L.) genotypes were investigated. The in vivo degrees of activation of PPDK and RuBPCase were estimated to make meaningful comparisons with CER. In leaves at a photosynthetic photon flux density (PPFD) of 720 micromoles per square meter per second, in vivo PPDK degree of activation was 80% of that of PPDK fully activated in vitro, whereas RuBPCase could not be further activated in vitro, suggesting that RuBPCase was fully activated in vivo. CER varied about 50% among the genotypes tested. Significant genetic differences were observed for the average weight of a cell (estimated by gram fresh weight per milligram DNA), but this character was not correlated with CER expressed on a fresh weight basis. CER was correlated with Chl concentration, and with estimates of the in vivo degree of activation of PPDK and RuBPCase. We concluded that in maize, CER is controlled by the metabolic components of photosynthesis rather than by membrane resistances to CO2. If the latter factor were controlling CER, then smaller cells with higher amounts of exposed cell surface area per unit cell volume would have lower resistance to CO2 diffusion, and therefore higher CER. When data were expressed on a DNA basis (proportional to a per cell basis), results indicated that larger cells (i.e. those with higher fresh weight per milligram DNA) have a higher content of Chl, and higher PPDK and RuBPCase activities, resulting in higher CER than in smaller cells.Recent advances in molecular biology should, in the future, permit modification and/or transfer and expression of specific genes in plants. The genes that should be modified or transferred must be identified in order to use this new technology efficiently.

“…However, concentrations of RuBPCase were not significantly different between ploidy levels of isogenic 2X-4X ryegrass (Lolium perenne L.) (21) and isogenic 2X-4X sets of alfalfa (Medicago sativa L.) (17). Cellular content of RuBPCase was found to increase proportionately with the amount of DNA per cell in these alfalfa polyploids (18).…”

mentioning

confidence: 87%

Ploidy Effects in Isogenic Populations of Alfalfa (Medicago sativa L.)

Meyers

Brinegar²,

Schrader³

et al. 1983

Self Cite

Ribulose-l,5-blsphosphate carboxylase/oxygenase (RuBPCase) was isolated from isonic diplold-tetrapiold and tetraploid-octoploid sets of alfalfa (Medicago sativa L.) leaves. Molecular weights of RuBPCase subunits were silr across ploldy levels ofboth Isogenic sets with subunits of 52,000 and 14,000. Apparent K,(CO2) values and substrate specificity factors (V.K./VK,,K) of RuBPCase were simflar, across ploldy levels of both Isogenc sets. These results indicate that ploldy had no effect on the kinetic properties of RuBPCase in alfalfa.Increases in nuclear ploidy have been associated with changes in both the quantitative and qualitative expression of enzymes and isozymes (1, 4-6, 12, 22). It has been suggested that increased genetic expression ofRuBPCase3 (12) and a selective enhancement of carboxylase activity relative to oxygenase activity of RuBPCase (10, 11) may result in higher net photosynthetic rates and increased photosynthetic efficiency, respectively.Joseph et al (12) reported that the concentration of RuBPCase in leaf extracts and net photosynthesis increased significantly with ploidy in an allopolyploid series (4X, 6X, 8X, and lOX) of tall fescue (Festuca arundinacea Schreb.). However, concentrations of RuBPCase were not significantly different between ploidy levels of isogenic 2X-4X ryegrass (Lolium perenne L.) (21) and isogenic 2X-4X sets of alfalfa (Medicago sativa L.) (17). Cellular content of RuBPCase was found to increase proportionately with the amount of DNA per cell in these alfalfa polyploids (18).Garrett (4) reported that purified RuBPCase from tetraploid ryegrass had a lower apparent Km(CO2) value and a higher isoelectric point than RuBPCase from comparable isogenic diploids. Rathnam and Chollet (20) reported that tetraploid ryegrass '