Developmental regulation of photosynthate distribution in leaves of rice

Abstract

AbstractmRNA expression patterns of genes for metabolic key-enzymes (sucrose phosphate synthase [SPS], phosphoenolpyruvate carboxylase [PEPC], pyruvate kinase, ribulose 1,5-bisphosphate carboxylase/oxygenase, glutamine synthetase 1, and glutamine synthetase 2) were investigated in leaves of rice plants grown at two nitrogen (N) concentrations. The relative gene expression patterns were similar in all leaves except for 9 th leaf, in which mRNA levels were generally depressed.Though an increased N concentrations prolonged the expression period of each mRNA, it did not affect the relative expression intensity of any mRNA in a given leaf. SPS V max , SPS limiting activity, PEPC activity, and carbon flow were examined using 14 CO 2 . The ratio between PEPC activity and SPS V max was higher in leaves developed at the vegetative growth stage (vegetative leaves: 5 th and 7 th 1 leaf) than in leaves developed after the ear primordia formation stage (reproductive leaves: 9 th and flag leaf). PEPC activity and SPS V max decreased with declining leaf N concentration. The 14 C photosynthate distribution to the amino acids fraction was higher in vegetative than in reproductive leaves when compared for the same leaf N status. Thus, at high PEPC activity to SPS activity ratio, more 14 C photosynthate was distributed to the amino acid pool, whereas at higher SPS activity, more 14 C was channeled into the carbohydrate fraction. Thus, leaf ontogeny was an important factor controlling photosynthate distribution to the Nor C-pool, respectively, regardless of leaf N status.

Developmental changes of plant affecting primary photosynthate distribution in rice leaves

“…The carbon flow from photosynthesis has been proposed to be directed by SPS into the carbon pool (sugars and starch; Worrel et al 1991, Signora et al 1998, Murchie et al 1999 or, alternatively, by PEPC into the nitrogen pool (amino acids and proteins; Champigny and Foyer 1992, Foyer et al 1994, Huber et al 1994, Stitt 1999. We have corroborated this idea by an analysis of photosynthate distribution in rice leaves at different levels of nitrogen nutrition (Shinano et al 2006). In rice, the physiological function of each leaf changes during the course of plant development; leaves formed early in the phase of vegetative growth of plant support the development of younger leaves and the root system, while leaves formed towards the end of the vegetative phase of plant tend to translocate almost all of their assimilated carbon to the reproductive organs (Osaki et al 1988, Tanaka 1961.…”

“…2 and 7). Though we had not quantified the sugar phosphate fraction in our previous reports (Shinano et al 2006), it seems that the relative distribution of carbon to sugar phosphates is an indicator of the rate of transformation of primary photosynthates into derived compounds. In the 5 th leaf, the ratio of the V max values of SPS and PEPC was lower than in the 6 th and 7 th leaf.…”

“…This would be expected to decrease the carbon flow to sucrose synthesis, but the proportion of 14 C distribution to the sugar fraction did not corroborate the idea. In our previous study (Shinano et al 2006), we had found a positive relationship between the ratio of the V max of SPS and PEPC on one hand and the proportion of 14 C distribution to sugars on the other, but we had not analyzed the sugar phosphate fraction. Sugar phosphates are metabolic intermediates and form substrates for SPS and PEPC (Champigny and Foyer 1992).…”

“…The relatively higher proportion of primary photosynthate distribution to this fraction seems to indicate a retardation of carbon flow to sugar and/or organic acid synthesis through SPS or PEPC, or possibly a re-distribution of PEP into the chloroplasts (Shinano et al 2005). In rice, carbon distribution between chloroplasts and cytoplasm depends on the plant phosphorus status (Shinano et al 2006), and the ratio of nitrogen to phosphorus contents is greatly modified during the transition from vegetative to reproductive growth of plant (Osaki 1995). This suggests that phosphorus metabolism is another important factor that regulates carbon flow in rice, which will be addressed in future studies.…”

“…In our previous paper (Shinano et al 2006), we have analyzed the effects of nitrogen nutrition on the contents of SPS and PEPC. We found that the leaf nitrogen status contributed little to the regulation of the distribution of 14 C to primary photosynthate compounds, while the developmental status of plant on the leaf appeared to be a more decisive determinant in the regulation of the distribution of photoassimilates into the carbon or nitrogen pool.…”