Rubisco and Photosynthesis in Cereal Crops(Recent Topics of the Agricultunal Biological Science in Tohoku University)

Genetic modification to enhance Rubisco efficiency in cereal crops would have great agronomic importance as the prospects for enhancement of photosynthesis. There are substantial variations in the kinetic properties of Rubisco exist among higher plants. Since the difference in Rubisco specific activity reflects difference in the photosynthetic rate at atmospheric CO_2 levels, lower specific activity from rice provides the possibility to improve leaf photosynthesis by the introduction of a more efficient Rubisco into rice. An attempt to increase Rubisco content by overexpressing the rbcS gene shows increases in Rubisco content for a given leaf N content in rice. However, it leads to an unbalance between the in-vivo capacities of Rubisco and other photosynthetic limiting factors. To maintain this balance, the enhancement of cytochrome b6/f complex content may be required as a determinant for RuBP regeneration. Another approach to improving photosynthesis is to introduce C_4 characteristics into rice. An optimal protein allocation with a lower amount of Rubisco of a high Vmax/Km(CO_2) type as well as Kranz leaf anatomy is important for an improvement of photosynthesis

Differences Between Rice and Wheat in Temperature Responses of Photosynthesis and Plant Growth

The temperature responses of photosynthesis (A) and growth were examined in rice and wheat grown hydroponically under day/night temperature regimes of 13/10, 19/16, 25/19, 30/24 and 37/31°C. Irrespective of growth temperature, the maximal rates of A were found to be at 30–35°C in rice and at 25–30°C in wheat. Below 25°C the rates were higher in wheat, while above 30°C they were higher in rice. However, in both species, A measured at the growth temperature remained almost constant irrespective of temperature. Biomass production and relative growth rate (RGR) were greatest in rice grown at 30/24°C and in wheat grown at 25/19°C. Although there was no difference between the species in the optimal temperature of the leaf area ratios (LARs), the net assimilation rate (NAR) in rice decreased at low temperature (19/16°C) while the NAR in wheat decreased at high temperature (37/31°C). For both species, the N-use efficiency (NUE) for growth rate (GR), estimated by dividing the NAR by leaf-N content, correlated with GR and with biomass production. Similarly, when NUE for A at growth temperature was estimated, the temperature response of NUE for A was similar to that of NUE for GR in both species. The results suggest that the difference between rice and wheat in the temperature response of biomass production depends on the difference in temperature dependence of NUE for A

RBCS1A and RBCS3B, two major members within the Arabidopsis RBCS multigene family, function to yield sufficient Rubisco content for leaf photosynthetic capacity

Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit (RBCS) is encoded by a nuclear RBCS multigene family in many plant species. The contribution of the RBCS multigenes to accumulation of Rubisco holoenzyme and photosynthetic characteristics remains unclear. T-DNA insertion mutants of RBCS1A (rbcs1a-1) and RBCS3B (rbcs3b-1) were isolated among the four Arabidopsis RBCS genes, and a double mutant (rbcs1a3b-1) was generated. RBCS1A mRNA was not detected in rbcs1a-1 and rbcs1a3b-1, while the RBCS3B mRNA level was suppressed to ∼20% of the wild-type level in rbcs3b-1 and rbcs1a3b-1 leaves. As a result, total RBCS mRNA levels declined to 52, 79, and 23% of the wild-type level in rbcs1a-1, rbcs3b-1, and rbcs1a3b-1, respectively. Rubisco contents showed declines similar to total RBCS mRNA levels, and the ratio of Rubisco-nitrogen to total nitrogen was 62, 78, and 40% of the wild-type level in rbcs1a-1, rbcs3b-1, and rbcs1a3b-1, respectively. The effects of RBCS1A and RBCS3B mutations in rbcs1a3b-1 were clearly additive. The rates of CO2 assimilation at ambient CO2 of 40 Pa were reduced with decreased Rubisco contents in the respective mutant leaves. Although the RBCS composition in the Rubisco holoenzyme changed, the CO2 assimilation rates per unit of Rubisco content were the same irrespective of the genotype. These results clearly indicate that RBCS1A and RBCS3B contribute to accumulation of Rubisco in Arabidopsis leaves and that these genes work additively to yield sufficient Rubisco for photosynthetic capacity. It is also suggested that the RBCS composition in the Rubisco holoenzyme does not affect photosynthesis under the present ambient [CO2] conditions