Modification of Carbon Partitioning, Photosynthetic Capacity, and O2 Sensitivity in Arabidopsis Plants with Low ADP-Glucose Pyrophosphorylase Activity1

Abstract

Wild-type Arabidopsis plants, the starch-deficient mutant TL46, and the near-starchless mutant TL25 were evaluated by noninvasive in situ methods for their capacity for net CO 2 assimilation, true rates of photosynthetic O 2 evolution (determined from chlorophyll fluorescence measurements of photosystem II), partitioning of photosynthate into sucrose and starch, and plant growth. Compared with wild-type plants, the starch mutants showed reduced photosynthetic capacity, with the largest reduction occurring in mutant TL25 subjected to high light and increased CO 2 partial pressure. The extent of stimulation of CO 2 assimilation by increasing CO 2 or by reducing O 2 partial pressure was significantly less for the starch mutants than for wild-type plants. Under high light and moderate to high levels of CO 2 , the rates of CO 2 assimilation and O 2 evolution and the percentage inhibition of photosynthesis by low O 2 were higher for the wild type than for the mutants. The relative rates of 14 CO 2 incorporation into starch under high light and high CO 2 followed the patterns of photosynthetic capacity, with TL46 showing 31% to 40% of the starch-labeling rates of the wild type and TL25 showing less than 14% incorporation. Overall, there were significant correlations between the rates of starch synthesis and CO 2 assimilation and between the rates of starch synthesis and cumulative leaf area. These results indicate that leaf starch plays an important role as a transient reserve, the synthesis of which can ameliorate any potential reduction in photosynthesis caused by feedback regulation.Because plant productivity is governed by photosynthetic activity and sink activity for utilizing photosynthate (see Zamski and Schaffer, 1996), it is important to understand the environmental and genetic factors affecting these processes. In general, photosynthesis is limited mainly by light harvesting and assimilatory power under low light and by carboxylation and photorespiration under low CO 2 . Under saturating light and CO 2 , however, photosynthesis may be controlled by processes that convert triose-P into starch and Suc (Sage, 1990(Sage, , 1994Stitt, 1996). Thus, the capacity to utilize triose-P for carbohydrate synthesis can establish an upper limit for the maximum rate of photosynthesis under CO 2 -and light-saturated conditions (Sage, 1990;Sharkey et al., 1995). This is clearly demonstrated under certain conditions by the response of photosynthesis of C 3 plants to low O 2 . In many instances, the increase in CO 2 assimilation attributable to the reduction in photorespiration under low O 2 can be predicted accurately based on the known kinetic properties of Rubisco. However, when the extent of stimulation of photosynthesis by C 3 plants under subatmospheric O 2 is less than predicted, or when there is reversed O 2 sensitivity, photosynthesis is considered to be feedback limited as a result of restrictions on triose-P utilization (Sharkey, 1985; Leegood and Furbank, 1986;Sage and Sharkey, 1987; Hanson, 1990;Sun et al., 1997). L...

Phloem Loading Strategies and Water Relations in Trees and Herbaceous Plants

“…Only 8% to 10% of the newly fixed 14 C is partitioned to starch in poplar (Populus spp. ; Dickson and Larson, 1981) and apple (Cheng et al, 2005), whereas this figure is more than 30% in garden bean (Phaseolus vulgaris; Vassey and Sharkey, 1989), potato (Solanum tuberosum; Rung et al, 2004), and Arabidopsis (Sun et al, 1999). Apparently, the synthesis of transport sugars in passive loading plants diverts carbon away from starch synthesis and provides a pool of soluble carbohydrate for use in metabolism and export at night.…”

Feedback limitation of photosynthesis at high CO2 acts by modulating the activity of the chloroplast ATP synthase

“…Feedback is more likely to occur under high light, high leaf conductance for CO 2 from the atmosphere to Rubisco, and relatively low capacity for synthesis of carbohydrates (Sage and Sharkey 1987;Sharkey et al 1995;Leegood and Edwards 1996;Sun et al 1999a). Feedback becomes increasingly evident when starch synthesis is impaired by mutations in AGP pyrophosphorylase (Neuhaus and Stitt 1990;Sun et al 1999b), and plastid phosphoglucoisomerase activity (Hanson and McHale 1988;Hanson 1990Hanson , 1992Peterson and Hanson 1991;Huber and Hanson 1992;Eichelmann and Laisk 1994;Sharkey et al 1995) or sucrose synthesis by mutants of cytosolic fructose-1,6-biphosphatase activity (Sharkey et al 1988).…”

Analysis of Arabidopsis with Highly Reduced Levels of Malate and Fumarate Sheds Light on the Role of These Organic Acids as Storage Carbon Molecules

“…There is variation in the extent to which C 3 plants store photosynthates in the form of sugars and organic acids in leaves during carbon assimilation (Zeeman and Ap Rees, 1999;Chia et al, 2000;Zeeman et al, 2007). In Arabidopsis, approximately half of the photoassimilates are partitioned into starch (Sun et al, 1999;Zeeman and Ap Rees, 1999). Under short days (SD), the partitioning of assimilates to the formation of starch is greater than in long days (LD; Gibon et al, 2004).…”