In phase I, nocturnal CO 2 fixation and accumulation of malic enzyme (ME) occurs in the central vacuole. The CAM type of CO 2 fixation is thought of as integrating into the day/night cycle in four phases. Opuntia can take up relatively large amounts of CO 2 with respect to water loss by transpiration (4–10 mmol CO 2 mol −1 water compared with 1–1.5 mmol in C 3 plants), and the annual aboveground dry mass can be increased by 37% to 40% for Opuntia ficus-indica when the CO 2 level is doubled ( Cui et al., 1993 Nobel et al., 1994). The CAM cycle is expressed under essentially all growing conditions ( Winter et al., 2008). Opuntia species are constitutive Crassulacean acid metabolism (CAM) plants, in which the capacity to induce CAM metabolism is developmentally regulated, with a progression from C 3 to CAM metabolism occurring during cladode development. Our results indicate that the evolution of CAM metabolism could be the result of modified circadian regulation at both the transcriptional and posttranscriptional levels. Expression of OfiCCA1 and OfiPRR9, unlike in Arabidopsis, adapted best to a 12-h rhythm, suggesting that circadian clock gene interactions differ from those of Arabidopsis. Expression of the circadian clock gene OfiTOC1, similar to Arabidopsis, oscillated with a 24-h periodicity, peaking at night. Three kinds of expression profiles were found: transcripts of OfiPPCK oscillated with a 24-h periodicity transcripts of the light-active OfiNADP- ME and OfiPPDK genes adapted to 12-h cycles, while transcript accumulation patterns of OfiPEPC and OfiMDH were arrhythmic. We identified stable reference genes for quantitative polymerase chain reaction and found that OfiSAND, like its counterpart in Arabidopsis ( Arabidopsis thaliana), and OfiTUB are generally appropriate standards for use in the quantification of gene expression in O. We identified genes involved in circadian regulation and CAM metabolism for transcriptomic analysis in plants grown in long days. Sequences were assembled, compared with sequences in the National Center for Biotechnology Information nonredundant database for identification of putative orthologs, and mapped using Kyoto Encyclopedia of Genes and Genomes Orthology and Gene Ontology. As little transcriptomic data are available from obligate CAM plants, we created an expressed sequence tag database derived from different organs and developmental stages.
Current knowledge of CAM metabolism suggests that the enzyme phospho enolpyruvate carboxylase kinase (PPCK) is circadian regulated at the transcriptional level, whereas phospho enolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH), NADP-malic enzyme (NADP-ME), and pyruvate phosphate dikinase (PPDK) are posttranslationally controlled. The cactus Opuntia ficus-indica is a constitutive Crassulacean acid metabolism (CAM) species.
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