High glycolate oxidase activity is required for survival of maize in normal air.

A mutant in the maize (Zea mays) Glycolate Oxidase1 (GO1) gene was characterized to investigate the role of photorespiration in C4 photosynthesis. An Activator-induced allele of GO1 conditioned a seedling lethal phenotype when homozygous and had 5% to 10% of wild-type GO activity. Growth of seedlings in high CO2 (1%-5%) was sufficient to rescue the mutant phenotype. Upon transfer to normal air, the go1 mutant became necrotic within 7 d and plants died within 15 d. Providing [1-14C]glycolate to leaf tissue of go1 mutants in darkness confirmed that the substrate is inefficiently converted to 14CO2, but both wild-type and GO-deficient mutant seedlings metabolized [1-14C]glycine similarly to produce [14C]serine and 14CO2 in a 1:1 ratio, suggesting that the photorespiratory pathway is otherwise normal in the mutant. The net CO2 assimilation rate in wild-type leaves was only slightly inhibited in 50% O2 in high light but decreased rapidly and linearly with time in leaves with low GO. When go1 mutants were shifted from high CO2 to air in light, they accumulated glycolate linearly for 6 h to levels 7-fold higher than wild type and 11-fold higher after 25 h. These studies show that C4 photosynthesis in maize is dependent on photorespiration throughout seedling development and support the view that the carbon oxidation pathway evolved to prevent accumulation of toxic glycolate.

The oxidation of glycolate to glyoxylate in higher plants is catalyzed by glycolate oxidase (GO; EC 1.1.3.15), an FMN-containing protein (Zelitch and Ochoa, 1953). The enzyme is located in the peroxisome and performs an essential step in the operation of the oxidative photorespiratory cycle accompanying photosynthetic CO 2 assimilation in C 3 plants (Tolbert, 1971(Tolbert, , 1997. Glycolate arises from 2-phosphoglycolate, which is produced together with 3-phosphoglycerate by ribulose bisphosphate reacting with O 2 as catalyzed by Rubisco (Bowes et al., 1971;Ogren, 2006) in the bundle sheath (BS) cell of C 4 plants (Edwards and Walker, 1983). Dephosphorylation of 2-phosphoglycolate yields glycolate that is transported into the peroxisome. In the peroxisome, GO oxidizes glycolate to produce glyoxylate. Transamination of glyoxylate generates Gly that is converted to Ser and CO 2 in the mitochondrion (Tolbert, 1971). The Ser gives rise to glyceric acid, which is ultimately converted to 3-phosphoglyceric acid in the mesophyll (M) cell chloroplast (Usuda and Edwards, 1980). Thus, the photorespiratory pathway (Fig. 1) is a futile process that diverts energy to refixation of photorespired CO 2 , thereby lowering the quantum efficiency of net carbon assimilation (Zelitch, 2001).
In C 3 leaves, production of photorespiratory CO 2 decreases net CO 2 assimilation by about 25% at 25°C, and photorespiration increases greatly relative to CO 2 assimilation at higher temperatures (Peterson, 1983;Hanson and Peterson, 1986). In C 4 plants such as maize (Zea mays), a carbon shuttle increases local CO 2 concentrations in the BS chloroplasts, where Rubisco is located, to levels more than three times higher than those found in M cells of C 3 plants. As a result, photorespiration is greatly reduced in C 4 tissues (Hatch, 1971;Jenkins et al., 1989;Dai et al., 1993;Laisk and Edwards, 1998).
Several studies point to a low, yet finite, rate of photorespiration in maize. To estimate photorespiration in maize leaves, Zelitch (1973) compared the initial rate of glycolate accumulation in leaf tissue in high light in the presence of an inhibitor of GO, a-hydroxysulfonate, and found that glycolate accumulation was 10% as great in maize as in tobacco (Nicotiana tabacum; C 3 ). Because no mechanism for the metabolism of glycolate is known without its first being converted to glyoxylate, this demonstrated that the rate of CO 2 production by photorespiration was no more than 5% of the net CO 2 assimilation rate (A) in C 4 photosynthesis. Additional studies that monitored 18 O 2 and 13 CO 2 exchange and estimates from electron transport suggested a similar photorespiratory rate in maize of 2% to 7% of A (Volk and Jackson, 1972;de Veau and Burris, 1989;Laisk and Edwards, 1998). Experiments with maize leaves in high light showed that with increasing O 2 level, there was 1 This work was supported by the Hatch Fund (grant no. CONH00246 to R.B.P.) and by the National Science Foundation (grant no. DBI-0076892) to T.P.B. * Corresponding author; e-mail israel.zelitch@po.state.ct.us. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Israel Zelitch (israel.zelitch@po.state.ct.us). [OA] Open Access articles can be viewed online without a subscription.
www.plantphysiol.org/cgi/doi/10.1104/pp.108.128439 some decrease in A, and in limiting light a decrease in the maximum quantum yield was observed, further indicating that some photorespiration occurred (Dai et al., 1993). A number of conditional mutants with lesions in different steps of the photorespiratory pathway were first obtained by Ogren and coworkers in Arabidopsis (Arabidopsis thaliana; Ogren, 1984Ogren, , 2006 and later by others in barley (Hordeum vulgare; Blackwell et al., 1988) and tobacco (McHale et al., 1988). In these studies, mutants were identified that could survive in high CO 2 , when photorespiration was blocked, but succumbed in normal air. Indeed, analyses of metabolite accumulation in these mutants provided biochemical evidence for a photorespiratory cycle (for review, see Reumann and Weber, 2006). However, no mutant lacking GO was ever recovered by this method of selection. The presence of five distinct GO loci in the Arabidopsis genome likely accounts for the failure to recover conditional GO mutants in previous screens.
Transgenic tobacco plants were generated that had reduced GO activity through cosuppression of the endogenous GO (Yamaguchi and Nishimura, 2000). These plants contained variable levels of GO activity that decreased during plant development, with some plants ultimately having as little as 20% of wild-type activity. Transgenic plants with greatly reduced levels of GO exhibited dramatically reduced photosynthetic electron transport rates in high light, whereas plants with moderate levels of GO were less affected.
In this report, we describe the isolation and characterization of a maize GO mutant caused by an Activator (Ac) insertion in GO1 (go1-m1::Ac). Using Ac as a molecular tag, the GO gene was cloned and sequenced.
In addition, mutant seedlings were characterized under both high and low CO 2 conditions. Under high CO 2 conditions, GO deficiency had no visibly adverse effects on plant growth. However, A was rapidly and linearly diminished in mutants in high light and high O 2 . When such mutant plants were transferred from growth in high CO 2 to normal air in the light, glycolate accumulated linearly for 6 h to levels 7-fold greater than the initial concentration. Plants greatly depleted in GO activity were nonviable when grown at ambient CO 2 concentrations. We therefore conclude that photorespiration plays a vital role in C 4 photosynthesis.

Identifying a Maize GO Mutant
The GO mutant was first identified in sandbench screens of Ac-mutagenized families (Kolkman et al., 2005). A single family was identified that segregated a seedling lethal phenotype in 21 of the 71 seedlings examined, suggesting that the mutation was due to disruption of a single gene (x 2 = 0.23; P = 0.63). DNAblot analysis was used to identify an Ac-containing, 3.2kb EcoRI fragment that cosegregated with the mutant phenotype (data not shown). Approximately 800 bp of DNA flanking this sequence was recovered using an inverse PCR technique as described in "Materials and Methods." To recover additional sequences flanking the Ac insertion, we exploited the somatic instability of the active Ac insertion to selectively amplify Ac junction fragments that were generated through local insertions. This method, known as Ac casting (Singh et al., 2003), Figure 1. Schematic diagram of C 4 photosynthetic CO 2 assimilation in maize. The incoming atmospheric CO 2 reacts (1) with phosphoenolpyruvate (PEP) to form oxaloacetate (OAA) in the M cell. The OAA is reduced (2) to malate through the NADPmalate dehydrogenase reaction, and the malate is converted to pyruvate and CO 2 by the NADP-malic enzyme (3) in the chloroplasts of BS cells. The released CO 2 combines with ribulose bisphosphate (RuBP) through the Rubisco reaction (4) to form phosphoglycerate (PGA). The resulting elevated CO 2 level inhibits, but does not eliminate, the oxygenase reaction (5) of Rubisco. Photorespiratory products are likely generated from incomplete suppression of Rubisco oxygenase activity in BS chloroplasts. A specific phosphatase in the chloroplast (6) converts the resulting P-glycolate to glycolate. The glycolate is transported to the peroxisome where GO activity (7), when present at high levels, creates glyoxylate. A series of aminotransferase and decarboxylation reactions leads to the production of Gly and Ser with the release of photorespiratory CO 2 . Note that two Gly combine to produce one Ser and one CO 2 . When GO activity is low or absent, the glycolate, which has no other means of being metabolized, will accumulate at rates equal to the rate of P-glycolate formation. For further details, refer to the introduction. enabled us to recover additional sequence information 1 kb upstream and 1 kb downstream of the original Ac insertion site. This confirmed that the Ac element inserted at the locus was active and enabled us to assemble a contig of approximately 2 kb flanking the Ac insertion site.
The Ac Insertion Is Located in the GO1 Locus Genomic sequences flanking the Ac insertion were used to search GenBank and matched genomic DNA (AC187560.5) and mRNA (AY108197). Further BLAST searches indicated the Ac insertion was in exon 7 of a putative GO gene ( Fig. 2A). We reasoned that the mutant phenotype (necrosis and seedling death) was likely caused by accumulation of glycolate and that it may be rescued under high CO 2 conditions where photorespiration is minimal. In an attempt to rescue the seedling lethality of homozygous go1 mutants, seedlings were grown under high CO 2 conditions (1% to 5% CO 2 in air). Under these conditions, the growth and appearance of the mutants could not be distinguished visually from either the heterozygous or homozygous wild-type individuals (Fig. 3). Seedlings were routinely grown this way for 30 to 60 d. Soon after germination in high CO 2 , the phenotype of an individual plant was established using a sensitive GO assay carried out on extracts prepared from discs cut from the first leaf. Later, this phenotype was confirmed on extracts prepared from second or third leaves. As shown in Figure  2B and Table I Fig. 2B). As shown in Figure 2C, wild-type plants accumulated GO mRNA in contrast to the homozygous mutant plants with the low GO activity, consistent with the Figure 2. Molecular analysis of the maize go1::Ac allele. A, Representation of the maize GO locus (on BAC clone AC187560.5) and location of the Ac insertion. Black boxes signify exons (numbered), while white boxes denote intron sequences. Enlarged region depicts locus from exon 6 through 8 detailing the insertion of an Ac element in the beginning of exon 7. Arrows depict oligonucleotide primers in genomic and Ac-DNA that amplify wild-type sequences (430-bp fragment) or Ac-junction fragments (209 bp). B, Genotypic characterization by PCR analysis of maize seedlings displaying a high or moderate GO activity (lanes 2-14) or low GO activity (lanes 15-23). C, GO gene expression. Transcripts from maize seedlings: wild-type (lanes 1 and 5), heterozygote (lanes 2 and 6), or mutant (lanes 3, 4, 7, and 8) seedlings analyzed by reverse transcription-PCR for GO expression (lanes 5-8) or actin expression (lanes 1-4). Ac transcripts amplify a 436-bp fragment and GO transcripts amplify a 236-bp fragment. severe mutant phenotype of the homozygous insertion. When go1 mutant seedlings were transferred from high CO 2 to normal air, leaves became necrotic within 7 d, and the seedlings died within 15 d under either growth chamber or greenhouse conditions (Fig. 3B). Taken together these data indicate that the seedling lethality of the mutants is due to the cytotoxic accumulation of glycolate in homozygous individuals due to an Ac insertion in the maize GO1 gene. We therefore named the mutant allele go1-m1::Ac. A phylogenetic analysis of known GOs from Oryza sativa and Arabidopsis reveals two separate protein subgroups (Fig. 4A) in which the maize GO1 (derived from mRNA PCO065133 gb AY108197) is most similar to proteins encoded by Os07g0152900 (91.0%/96.8% identity/similarity) and At3g14415 (83.7%/94.8%; Fig. 4B).

CO 2 Assimilation in Low GO Mutant Is Strongly Inhibited in High O 2
To examine the consequences of the loss of GO activity on A, wild-type and go1 mutant leaf tissue was examined under low (1%) and high (50%) O 2 conditions (Table II). Representative results are shown in Figure 5. Values of A attained relatively stable levels after 20 min in high light and low O 2 (Fig. 5). The average value of A attained after 40 to 50 min in 1% O 2 was lower for the mutant, but this effect was not statistically significant (P . 0.05; Table II). A similar trend was noted for effect of genotype on intrinsic quantum yield of PSII (F v /F m in Table II). Though not visible by eye, leaf-to-leaf variation in F v /F m was unusually high among mutant leaves, consistent with the occasional occurrence of mild cytotoxic symptoms. An immediate 3% to 4% reduction in A occurred upon increasing the O 2 level from 1% to 50% for both genotypes (Fig. 5). During the remainder of the exposure to 50% O 2 , the magnitudes of A declined on average by 16% (10% in the example illustrated in Fig. 5) for wild type. A similar inhibition of A at high O 2 was observed in maize leaves by Dai et al. (1993). In go1 mutants, A declined by an average of 93% (92% in the example given in Fig. 5) relative to high CO 2 conditions. The decline in A in the go1 mutants in 50% O 2 was essentially linear (Fig. 5), and A was almost completely abolished after 50 min at this higher O 2 level. Low GO 0.66 6 0.08 (n = 39) Intermediate GO 2.86 6 0.14 (n = 88) Wild-type GO 6.00 6 0.30 (n = 65) a GO activity was determined on extracts of leaf disks from plants grown in 1% to 5% CO 2 in a chamber in a growth room as described in "Materials and Methods." Samples were taken from 192 plants in seven separate growth experiments. The mean activity values are given 6SE. . Phylogenetic and amino acid alignment analyses of maize GO1. A, Phylogenetic relationship of maize GO1 with four glycolate oxidases encoded by Oryza sativa loci Os07g0152900 (protein ID BAF20823.1), Os03g0786100 (protein ID BAF13401.1), Os04g0623500 (protein ID BAF15839.1), and Os07g0616500 (protein ID BAF22190.1), and with five GOs encoded by Arabidopsis loci At3g14415 (protein ID NP 188059), At3g14420 (protein ID NP 850585), At4g18360 (protein ID NP 193570), At3g14130 (protein ID NNP 188029), and At3g14150 (protein ID NP 188031). The tree represents an unrooted maximum parsimony derived from PAUP 4.0 (Swofford, 2003). B, Alignment of amino acid sequences from maize GO1, Os07g0152900, and At3g14415. Black boxes indicate amino acid identity, gray boxes indicate similar amino acids, and white boxes show no amino acid identity or similarity as determined by ClustalW (Thompson et al., 1994) and Boxshade 3.2. To examine the consequences of a disruption of GO activity on additional reactions of the photorespiratory cycle, go1 mutants were further characterized by providing [1-14 C]glycolate to leaf discs in darkness and measuring the 14 CO 2 produced (Table III). These results are consistent with parallel assays made on leaf extracts showing that go1 mutants do in fact have a low but measurable activity compared with intermediate and normal plants. An important later step in photorespiration, after oxidation of glycolate, involves the conversion of two Gly to Ser and CO 2 (Fig. 1). Leaf discs taken from go1 mutants were tested to determine whether any later step in the pathway was also affected by supplying [1-14 C]Gly in darkness. Table IV shows that the rate of formation of [ 14 C]Ser and 14 CO 2 was similar for all three genotypes. The mean ratio of CO 2 produced:Ser formed was 1.02, close to the expected stoichiometry, and an ANOVA showed there was no significant difference in the mean ratios among the three genotypes. Thus, plants with low GO activity were not likely altered in their ability to otherwise complete metabolism of photorespiratory Gly.

Glycolate Accumulates in Normal Air in the Low GO Mutant
Though considerable phosphoglycolate is produced by the Rubisco oxygenase reaction, a high GO activity keeps the free glycolate concentration at low steadystate levels in leaves, and even when glycolate concentration increased greatly in short-term experiments in tobacco treated with an inhibitor of GO, the phosphoglycolate concentration was unaffected (Zelitch, 1965). The low concentration of glycolate is likely due solely to the function of GO, as there is no other known mechanism for metabolizing glycolate without it first being converted to glyoxylate. By means of isotope dilution, a glycolate concentration of 0.471 6 0.038 (SE) mmol/g fresh weight was deduced for maize leaves in light (Jolivet et al., 1985), and by use of HPLC, a value of 1.2 mmol/g fresh weight was obtained (González-Moro et al., 1997). A similar mean steady-state value for wildtype leaves can be obtained from Figure 6 (n = 33) of 0.93 6 0.12 (SE) mmol glycolate/g fresh weight (1.0 cm 2 leaf area = 14 mg fresh weight).
When seedlings were transferred from growth in high CO 2 to normal air under the same growth conditions, glycolate concentration in the leaves increased rapidly and linearly for at least 6 h in go1 mutants to a level of 6.3 mmol glycolate/g fresh weight, a 7-fold increase (Fig. 6). After a total of 25 h in air, the glycolate concentration increased to 10.0 mmol glycolate/g fresh weight, an 11fold increase over the initial concentration. An increase in glycolate concentration in maize leaves induced by photorespiratory inhibitors was accompanied by a decrease in A (González- Moro et al., 1997), and a level of 4.0 mmol glycolate/g fresh weight was sufficient to completely abolish A. This inhibitory glycolate concentration was reached after about 3 h in normal air in mutant leaves (Fig. 6). The linear increase in glycolate in go1 mutants (Fig. 6) is consistent with the linear loss of A capacity in 50% O 2 and high light (Fig. 5).

DISCUSSION
Consistent with lower rates of oxygenation of ribulose bisphosphate, it has often been noted that in C 4 plants, the enzymes of the photorespiratory pathway, including GO, are present at much lower levels than in C 3 plants (Edwards and Walker, 1983;Dai et al., 1995). For example, the GO specific activity in C 3 grasses is approximately 10-fold greater than in maize (Ueno et al., 2005), and we have also found (I. Zelitch, unpublished data) that on a leaf area basis, maize has about 10% as much GO activity as the C 3 tobacco leaf. Nevertheless, no mutant has previously been described in a C 4 plant in which GO, or any other gene underlying an enzymatic step of the photorespiratory pathway, was defective.
We have shown here that a homozygous disruption of the GO1 gene produces a conditional lethal mutation. This mutant is analogous in its phenotype to other mutants that disrupt enzymatic steps in photorespiration, as previously described for Arabidopsis and other C 3 species (Ogren, 1984(Ogren, , 2006 and shows that a functional photorespiratory pathway is essential for maize seedling development. As shown in Figure 1, glycolate is an early intermediate following the oxygenation of ribulose-1,5-bisphosphate. To examine the effects of glycolate accumulation on carbon assimilation in maize, González-Moro et al. (1997) excised maize leaves in light and supplied them with inhibitors of photorespiratory enzymes. Following the treatments, leaf glycolate concentrations rose from 1.2 mmol/g fresh weight to 4.0 mmol/g fresh weight, at which time CO 2 assimilation was completely blocked, Rubisco activity decreased, and the ribulose bisphosphate concentration increased. They later showed that adding 10 or 20 mM glycolate to excised maize leaves together with low levels (1 mM) of phosphinothricin, an inhibitor of Gln synthetase, greatly increased the inhibitory effect on CO 2 assimilation within several hours (González-Moro et al., 2003). These inhibitory effects were attributed to the higher leaf glycolate concentrations.
Three isoforms of GO have been identified in maize. Two isoforms are localized to the BS, representing 80% of the total activity, and one to the M cells (Popov et al., 2003). Given the restriction of Rubisco to the BS cells, it is perhaps not surprising that the majority of the GO activity is restricted to this compartment. Several lines of evidence support the contention that the majority of this activity is associated with GO1. First, a single lesion at the GO1 locus eliminates all but 5% to 10% of the activity of wild-type leaves as assayed in leaf extracts (Table I) and shows a low rate of metabolism of [1-14 C] glycolate in intact leaf tissue (Table III). Importantly, this residual activity in the mutant is insufficient to prevent rapid accumulation of glycolate in normal air and seedling lethality. Second, molecular analyses correlated genotype with enzymatic phenotype. Plants with intermediate or high GO activities had at least one wild-type GO1 locus, whereas all plants with low GO activities were homozygous for the Ac insertion in the GO1 locus (Table I; Fig. 2). Third, the maize GO1 is most similar to Oryza and Arabidopsis GO enzymes, which are highly expressed in leaves (Fig. 4B). Although we attempted to examine the cell-specific expression of GO1 in separated BS and M cells, it was not possible to conclusively determine cell-specific expression as the transcript is transcribed at a lower rate or rapidly degraded during the isolation of maize M cell protoplasts (data not shown).
As we have discussed, photorespiration in maize is normally low, but our results confirm that it is not The mean GO activities assayed from leaf disc extracts are given for each plant material 6SE (n = 5). Enzymatic 14 CO 2 production during the 2-h incubation is likewise reported as the corrected count rate (n = 3). entirely absent, so we must conclude that the CO 2concentrating mechanism in the BS cells does not completely suppress Rubisco-catalyzed oxygenation of ribulose bisphosphate, the primary source of glycolate ( Figs. 1 and 6).
This conclusion is consistent with previous studies (Dai et al., 1995;González-Moro et al., 1997) that suggested photorespiration is operational in maize. In 50% O 2 and high light, we also observed that A declined rapidly and linearly with time in the low GO mutant (Table II; Fig. 5). Moreover, and despite residual low levels of GO activity in GO1 mutants, there are indications of chronic glycolate toxicity even under the high CO 2 conditions used to rescue these mutants. This is not surprising when considered in the context of the Rubisco mechanism (see below). We note the modest tendency toward lower values of A in 1% O 2 and especially F v /F m in the mutants (Table II; Fig. 5). The latter is a sensitive indicator of stress. This suggests that even minute traces of glycolate production exert cumulative detrimental effects that become visible only in the later stages.
The reaction catalyzed by GO is the only known route for processing of glycolate. Hence, our results show that accumulation of glycolate is a causal agent leading to inhibition of metabolic processes associated with A in C 4 photosynthesis. The results strongly support the view that an important function of GO and the photorespiratory pathway is the avoidance of buildup of toxic levels of glycolate, as has previously been suggested (González-Moro et al., 1997. The pattern of necrosis in mutant leaves exposed to normal air does not first appear at the leaf/ligule base where cells display a more C 3 -like character, but rather begins at the leaf tip consistent with a persistent requirement for high GO activity throughout leaf development (data not shown). The need to rapidly dispose of glycolate is thus as important to C 4 physiology as it is to C 3 physiology.
The question has often been posed: does photorespiration have a function? There is some evidence that glycolate can strongly inhibit Q A /Q B electron transfer by displacement of bicarbonate bound to the non-heme iron in PSII (Petrouleas et al., 1994). However, these authors doubted a physiological role for glycolate based on weak binding of this anion at pH 7.5, a value close to that existing in the stroma. It has often been suggested that photorespiration serves as a sink to dissipate excess reducing power that accumulates under high light conditions (Kozaki and Takeba, 1996). However, it should be pointed out that photorespiration utilizes photochemical energy as does the photosynthetic carbon reduction cycle, so that any interference with either process could potentially cause photoinhibition resulting from overreduction. For example, transgenic tobacco (C 3 ) plants with decreased activity of the photorespiratory cycle enzyme Gln synthetase were reported to exhibit greater susceptibility to photoinhibition (Kozaki and Takeba, 1996). On the other hand, all higher plants possess an effective array of H +and carotenoid-dependent mechanisms to cope with excess light (Ruban et al., 2007;Ahn et al., 2008). Thus, a redundant contribution to photoprotection by such a slow rate of photorespiration as occurs in maize is unlikely to explain the rapid and inescapable lethality of GO deficiency under mild conditions in air. Ogren (1984) emphasized that the photorespiratory pathway is necessary only when the Rubisco oxygen- Figure 6. Glycolate levels in wild-type (WT) and low GO maize leaves versus time in light and normal air. Two experiments were conducted with plants after 32 and 21 d of growth in high CO 2 , respectively. Three plants with wild-type GO activity and three with low activity were used in one experiment and two different plants with wild-type activity and one with low activity in the second experiment. The mean GO activity as micromoles glyoxylate formed (grams fresh weight) 21 hour 21 (6SE) for low GO plants was 0.46 6 0.12 (n = 9) and for wild-type GO plants was 5.92 6 0.52 (n = 10). The plants were first sampled after 1 h in darkness in normal air and then kept in light for 14 h, in darkness for 6 h, then in light again and leaf samples taken after 5 h. The mean glycolate concentrations are shown (n = 5 for each time point for wildtype GO plants and n = 4 for low GO plants; error bars indicate 6SE).  AcGO1  ACGGGATTTTCCCATCCTAC  GO 3#UTR F  GATACTCAGCTCGTGCATGGAT  GO 3#UTR R  CAAATACAGAGAGATCACCATGTGC  GOA1  GCCGTCAATGAAACCAAACCAGAGACC  GOA2  GTCCTTGCAGCGGCGTACCATCTGG  GOB1  CTGAATCAGCAGCCTGCCCATG  GOB2  TCGACGAAGCTCCATATGTACTGC  GOC1  AAGTCGACCGCACCCTGAGCTG  GOC2  AGACGTCAAGTGGTTGCAGACCA  GO IN2 F  AACTTCAAGCGTCGAGGAGGTTGC  GO IN2 R  ACCTTCGAAGTTCTTGAGCGTCAG  GO IN3 F  CTGACGCTCAAGAACTTCGAAGGTC  GO IN3 R  TGGTCTGCAACCACTTGACGTC  GO OUT F  ACACCTGACGCTCAAGAACTTCG  GO OUT R  CACTGGAATCGGGGAGCTG  GO OUT R2  GGACATCTGCAGACCATTATGG  GO OUT R3  TGAAGGCAGGGTAACCA  GOP1  CGCTCGCTCTTGGCGCTTCTG  GOP2  CATGTGCAGACAAGGCAGCTCTGG  ZGO1 CACACCTGACGCTCAAGAACT ZGO2 GTGGTTCGACACGATGATCC ase reaction occurs. The presence of the photorespiratory pathway is even more imperative insofar as the oxygenase activity of Rubisco is ubiquitous (Jordan and Ogren, 1981) and is, in effect, an intrinsic feature of the interaction of the enzyme with ribulose bisphosphate (Lorimer and Andrews, 1973). Thus, glycolate will inevitably be synthesized if O 2 is present. Regardless of the mechanisms of glycolate toxicity, the necessity of avoiding glycolate accumulation presents sufficient selective pressure to account for the universal presence of the photorespiratory pathway in oxygenic photosynthesis. This is consistent with initial findings demonstrating lethal effects of photorespiratory pathway lesions in Arabidopsis (Ogren, 2006). This article offers strong genetic and biochemical support for the importance of photorespiration in maintaining low (nontoxic) levels of glycolate.

Ac Mutagenesis and Cloning of GO1
The go1 mutant was identified in sandbench screens of lines segregating newly transposed Ac elements. To identify an Ac-containing restriction fragment polymorphism, genomic DNA was fractionated using EcoRI and resolved on a 0.8% agarose gel prior to transfer to nylon membranes as previously described (Kolkman et al., 2005). A 3.2-kb Ac-containing restriction fragment that cosegregated with the mutant phenotype was identified using the internal 700-bp EcoRI/HindIII fragment of Ac as a molecular probe. Inverse PCR reactions were performed using the iPCR-1 protocol as previously described (Kolkman et al., 2005) to amplify an approximately 800 bp of DNA flanking the Ac insertion. Ac casting reactions were conducted using two rounds of PCR with the Ac-specific primers TBp34, TBp35, JGp2, and JGp3 as previously described (Singh et al. 2003). GO (Table V). Casting products were assembled into a contig using Sequencher Software (Gene Codes Corporation) and used in BLAST searches against maize (Zea mays) GSS reads.

Growth of Plants in High CO 2
Plants were grown in an environmental control chamber under an 18-hlight/6-h-dark photoperiod and 250 mmol photons m 22 s 21 light provided by sodium vapor lamps. The temperature regime was 30°C light/20°C dark. Eight plastic pots (each 2.2 L in volume) were filled with moistened sterile Promix-BX (peat-based medium containing perlite, vermiculite, and limestone), and the pots were placed in a tray containing water. Four seeds of a population segregating for GO activity were placed in each pot. A portable chamber (total gas volume 219 L) covered with clear plastic was placed over the pots and 1% to 5% (v/v) CO 2 in air was continuously pumped into the middle of the chamber at 1 L min 21 . Care was taken to avoid simultaneous exposure to actinic light and normal air during plant growth. A complete nutrient solution containing 800 mg L 21 nitrogen, Jack's Fertilizer (J.R. Peters Co.), and 7 mg L 21 chelated iron was added to each pot weekly, and at the same time the foliage was sprayed with the same nutrient solution containing 200 mg L 21 disulfosuccinic acid (Na salt) to "wet" the leaves. After replacing the portable chamber, the pots were flushed with high CO 2 for 2 h in darkness before the lights were turned on again.

GO Assay
Four 0.8-cm discs (2.0 cm 2 ; 28 mg fresh weight) were cut with a sharp punch in near darkness from a single leaf from plants grown in high CO 2 . The discs were transferred to microfuge tubes that were quickly placed in liquid N 2 and then stored at 270°C. Four samples at a time were thawed in an ice bath for the GO assays. A small amount of washed sand was placed in each tube, and 0.8 mL of cold extraction buffer was added (0.05 M potassium phosphate buffer, pH 8.0, containing 1 mM mercaptoethanol). The leaf discs were ground with a plastic pestle and the tubes centrifuged under refrigeration at 16,000g for 10 min. The supernatant was used for the enzyme assays.
The reactions were carried out in 12-3 100-mm test tubes on samples from four plants at a time. For each sample, two separate tubes were used, one without glycolate and the other containing the complete reaction mixture. Each tube had 100 mL of extraction buffer, 100 mL cold (freshly prepared) 1.6% phenylhydrazine hydrochloride, 100 mL freshly prepared FMN (4.8 mg 10 mL 21 ), 300 mL of leaf extract, and water to make the final volume 1.5 mL. The tubes were shaken at 30°C for 10 min, then at 1-min intervals, 50 mL 0.1 M potassium glycolate was added to every other tube and the reaction continued for 20 min. The reaction was stopped by the addition of 500 mL 12 N HCl. Color development of the glyoxylate phenylhydrazone was induced by the addition of 100 mL 8% potassium ferricyanide to each tube (Jameel et al., 1984), and the A 535 was determined in a spectrophotometer after 3 to 4 min. For each sample, a correction was made for any A 535 without glycolate. Under these conditions in a 1-cm cuvette, 0.01 mmol of glyoxylate phenylhydrazone gave an A 535 of 0.186.

Genotype Analysis
Genomic DNA from leaves was prepared according to Dellaporta (1993). RNA isolation was carried out using TRIzol (Invitrogen) and reverse transcription of RNA with a RCM-RACE kit (Ambion) according to the manufacturer's recommendations. The oligonucleotide primers ZGO1 and ZGO2 flank the Ac insertion site and amplify a 430-bp DNA fragment from wild-type maize genomic sequences and a 236-bp DNA fragment from reverse transcribed wild-type mRNA. Primers ZGO1 and AcGO1 amplify a 209-bp DNA fragment from genomic sequences corresponding to the Ac insertion allele. PCR parameters employed were: 94°C 3 min; (94°C 30 s; 55°C 30 s: 72°C 1 min) 30 times repeated followed by 72°C 7 min.

Metabolism of [1-14 C]Glycolate and [1-14 C]Gly by Leaf Tissue
Radioactive calcium [1-14 C]glycolate was obtained from American Radiolabeled Chemicals. It was dissolved in water and first passed through a column of Dowex 50 (H + form, Bio-Rad) and eluted with water. Potassium glycolate (0.1 M) was added to make the final concentration 40 mM. A portion was added to a column of Dowex 1 X8, 100 to 200 mesh, anion-exchange resin (Bio-Rad) 0.7 cm in diameter and 6 cm in height. The column was eluted with 10 mL of water, then with 4 mL of 4 N acetic acid, and these fractions were discarded. Glycolic acid was eluted by collection of the next 10 mL of 4 N acetic acid (Zelitch, 1972). Recovery of glycolic acid was 95% to 98%. Portions of the radioactive glycolic acid were placed in microfuge tubes and concentrated to dryness in a rotary evaporator under vacuum at room temperature to remove the acetic acid.
For metabolic experiments, 12 0.8-cm leaf discs (6.0 cm 2 ) were cut with a punch from a leaf obtained from plants growing in high CO 2 and previously identified as having wild-type, intermediate, or low GO activity. The discs were floated on water and transferred to 50-mL Erlenmeyer flasks containing center wells. Potassium [1-14 C]glycolate (1.0 mL, 40 mM, 2.41 3 10 6 dpm) was added to each flask, and a control flask omitting leaf discs was used in every experiment. A paper wick moistened with 5 M ethanolamine was placed in the centerwell to trap 14 CO 2 . The flasks were covered with aluminum foil, closed with a rubber serum stopper, and shaken for 2 h at 20°C. The paper wicks were removed to scintillation vials and the radioactivity determined by scintillation spectrometry.
Experiments on the metabolism of [1-14 C]Gly, 7.1 3 106 dpm, were carried out in a similar manner as those with [1-14 C]glycolate. At the end of the experiments, the 14 CO 2 was determined as before. The flask was filled with water to dilute the radioactive substrate, and the leaf discs were killed by placing them in a Ten Broeck homogenizer containing 5 mL of boiling 20% ethanol and kept in a boiling water bath for 5 min. The leaf tissue was ground in the homogenizer, centrifuged, the residue washed with water and centrifuged again, and the combined supernatants further analyzed for radioactive metabolites. A portion of each sample was placed on a Dowex-1 acetate column, and the neutral and basic compounds, including Gly and Ser, were eluted with water. A 1.0-mL portion of each sample was concentrated to 400 mL in a rotary evaporator under vacuum, and 50-mL samples were placed on a HPLC cation exchange Na+ column (PRP 3 200, 4.2 3 250 mm, Hamilton) and eluted at 30°C with 25% [NaCl:HCl 0.05 m] and 75% water. Ser and Gly eluted separately in this system. Eluates containing each compound were collected in scintillation vials and their radioactivity determined by scintillation spectrometry.

Photosynthesis Measurements
All measurements were conducted using a computer-controlled, dual channel flow-through apparatus previously described (Peterson et al., 2001). The GO phenotypes of the test plants were confirmed by enzyme assay prior to the gas exchange measurements. Plants were first darkened for 12 h to fully reverse photoprotective thermal dissipation processes. Steady-state chlorophyll fluorescence levels (F o and F m ) were then measured for the dark-adapted leaves using a PAM 101 system equipped with an ED101BL emitter-detector unit (H. Walz). The maximum quantum yield of PSII was estimated as (F m 2 F o )/F m (= F v /F m ) after correction of measurements for fluorescence emission from PSI (Peterson et al., 2001). Rates of A by attached maize leaves were measured at 360 mL L 21 CO 2 and 25°C. Photosynthesis was induced by providing white light from a tungsten-halogen source at an irradiance of 1,290 mmol photons m 22 s 21 . The increase in A was recorded continuously in a gas phase O 2 concentration of 1% (balance N 2 ) for 40 to 50 min. The O 2 level was then raised to 50%, and recording of A continued for a similar time period.

Determination of Glycolate Levels in Leaf Tissue in Normal Air
Plants were grown in high CO 2 under temperature-and light-controlled conditions as described above. Plants were kept in normal air in darkness for 1 h. Leaf segments were cut from a leaf (zero time), and the same light and temperature conditions were then maintained for the plants as before but in normal air. Successive segments (2-6 cm 2 ) were cut from the same leaf during the time course, and the segments were placed in microfuge tubes and plunged into liquid nitrogen. The tubes were stored at 270°C. Leaf tissue was extracted in boiling 20% ethanol as described above. The samples were ground and the suspension centrifuged for 10 min at 15,000g under refrigeration. The residue was suspended in water and centrifuged again. The combined supernatants were added to columns of Dowex-1 X8 acetate, 100 to 200 mesh, anion-exchange resin (Bio-Rad) 0.8 cm in diameter and 6.0 cm in height. The columns were washed with water, elution continued with 4 mL of 4 N acetic acid that was discarded, and the glycolic acid fraction (also known to contain glyceric acid in leaf extracts) was eluted by addition of 10 mL 4 N acetic acid (Zelitch, 1972). A portion (1.0 mL) of the glycolic acid fraction was concentrated to 200 mL in a rotary evaporator under vacuum at room temperature with quantitative recoveries. Isolation and determination of glycolic acid (and its separation from glyceric acid) was carried out on 20-mL samples of the concentrated fraction by HPLC on an anion exclusion column (Supeloge 610H, 7.4 3 300 mm; Supelco) at 40°C with 0.1% v/v H 3 PO 4 in water as the eluent.