for photosynthesis [52,54,70,71,76]. Although the signal transduction pathway for regulation of the
sugar-sensing genes may involve phosphorylation of hexoses, derived from sucrose hydrolysis by acid in-
vertase, via hexokinase [73–75,77–84], unknown gaps still exist between hexose metabolism and repres-
sion of gene expression at elevated growth [CO 2 ] [54,83]. However, future molecular genetic studies of
Arabidopsismutants with altered sensitivity to sugars may aid in elucidating steps along this signaling
pathway [75].
Transcription of the Rubisco small subunit (rbcS), and to a lesser extent the large subunit (rbcL), ap-
pears to be strongly repressed by sucrose and glucose [85,86]. Reduced expression of Rubisco genes and
differential response of other photosynthetic genes have been reported for a variety of crops grown at el-
evated [CO 2 ] [61,69,81,83,87–92]. Table 1 shows the influence of long-term growth at elevated [CO 2 ] on
rbcStranscript levels for various crop plants. For many species, the expression level of rbcStranscripts,
however, does not always correlate with the Rubisco protein content at elevated growth [CO 2 ] [69].
In tomato, transcript levels for Rubisco subunits, chlorophyll a/bbinding protein (Cab), and Rubisco
activase (Rca) decline with CO 2 enrichment, whereas those for core proteins in photosystems I and II re-
main unchanged [87,93]. In wheat, transcripts for Rubisco subunits and phosphoglycerate kinase of the
flag leaves are sensitive to elevated CO 2 , whereas those for sedoheptulose-1,7-bishosphatase and phos-
phoribulokinase are insensitive [89]. For tomato, despite a large accumulation of starch occurring in
leaves of elevated CO 2 –grown plants, transcript levels for ADP glucose pyrophosphorylase show little
change [87]. Furthermore, although photorespiration decreases under elevated [CO 2 ] [52], responses of
the enzymes and/or transcripts associated with the photorespiratory pathway have not been well investi-
gated [83]. Elevated CO 2 has little effect on the transcript level of glycolate oxidase in tomato [87] but in-
hibits the accumulation of hydroxypyruvate reductase messenger RNA (mRNA) in cucumber [94].
- C 4 Species
Although C 4 plants represent only 1% of the total plant species [14], their economic and ecological sig-
nificance is substantial [95]. Over 100 genera contain plants that utilize the C 4 pathway, and about 21%
of gross primary productivity (i.e., annual net CO 2 assimilation per unit ground area) is provided by C 4
plants on a global basis [96,97]. In many tropical regions, the food supply is primarily based on C 4 plants,
including grasses providing grains for many tropical diets and pastures and rangelands supplying forage
for livestock [98]. Sugarcane, maize, sorghum, millet, and amaranth are the most widely grown C 4 crops.
On a land area basis, maize, millet, and sorghum account for 70, 55, and 46% of the cereals grown in
Africa, South America, and North America, respectively [98]. C 4 plants dominate all tropical and sub-
RESPONSES TO RISING CO 2 AND CLIMATE CHANGE 39
TABLE 1 Effect of Long-Term Growth of Plants at Elevated
[CO 2 ] on rbcSTranscript Abundance
rbcSmRNA
Species (% of ambient CO 2 )a Reference
Arabidopsis 40 [69]
Bean 85 [69]
Cotton 54 [69]
Maize 152 [69]
Parsley 60 [69]
Pea 45–110 [69, 88]
Plantain 125 [69]
Radish 83 [69]
Rice 83–94 [90, 91]
Soybean 73–87 [69, 92]
Spinach 135 [69]
Sunflower 69 [69]
Tobacco 92 [69]
Tomato 40–81 [69, 86]
Wheat 50–61 [61, 89]
aThe percentage is expressed relative to the corresponding value for plants
grown at ambient [CO 2 ].