Phosphoenolpyruvate Carboxykinase Is Involved in the Decarboxylation of Aspartate in the Bundle Sheath of Maize1

Abstract

We recently showed that maize (Zea mays L.) leaves contain appreciable amounts of phosphoenolpyruvate carboxykinase (PEPCK; R.P. Walker, R.M. Acheson, L.I. Técsi, R.C. Leegood [1997] Aust J Plant Physiol 24: 459-468). In the present study, we investigated the role of PEPCK in C 4 photosynthesis in maize. PEPCK activity and protein were enriched in extracts from bundle-sheath (BS) strands compared with whole-leaf extracts. Decarboxylation of [4-14 C]aspartate (Asp) by BS strands was dependent on the presence of 2-oxoglutarate and Mn 2؉ , was stimulated by ATP, was inhibited by the PEPCK-specific inhibitor 3-mercaptopicolinic acid, and was independent of illumination. The principal product of Asp metabolism was phosphoenolpyruvate, whereas pyruvate was a minor product. Decarboxylation of [4-14 C]malate was stimulated severalfold by Asp and 3-phosphoglycerate, was only slightly reduced in the absence of Mn 2؉ or in the presence of 3-mercaptopicolinic acid, and was light dependent. Our data show that decarboxylation of Asp and malate in BS cells of maize occurs via two different pathways: Whereas malate is mainly decarboxylated by NADPmalic enzyme, decarboxylation of Asp is dependent on the activity of PEPCK. C 4 plants have been classified as NADP-ME, NAD-ME, and PEPCK types, according to the major decarboxylase involved in the decarboxylation of C 4 acids in the BS cells Hatch et al., 1975). Maize (Zea mays) belongs to the NADP-ME subgroup of C 4 plants and possesses negligible activity of PEPCK, although suggested that some other NADP-ME species utilize PEPCK as an auxiliary decarboxylase. However, we showed previously that maize leaves contain large amounts of PEPCK (Walker et al., 1997), andFurumoto et al. (1999) identified a PEPCK gene from maize that is specifically expressed in BS cells. Other NADP-ME species such as Echinochloa colona, Echinochloa crus-galli, Digitaria sanguinalis, and Paspalum notatum also contain PEPCK, whereas PEPCK protein was not detectable in Sorghum bicolor, Saccharum officinarum, or Flaveria bidentis (Walker et al., 1997). The reason that the occurrence of PEPCK in these plants has been overlooked may be due to difficulties in measuring PEPCK activity (Walker et al., 1997) or to the lack of an antibody.PEPCK-type C 4 species mainly use Asp as the CO 2 donor and decarboxylate the oxaloacetate formed via the following reaction:In NADP-ME species such as maize, 14 CO 2 is initially incorporated into the C-4 position of malate and Asp (about 75% and 25%, respectively), and the C-4 of both is subsequently incorporated into other metabolites (Hatch, 1971; Morot-Gaudry and Farineau, 1978). Using isolated BS strands, Chapman and Hatch (1981) showed that BS cells of maize have a significant capacity to decarboxylate Asp, and they assumed that NADP-ME was responsible. In view of the occurrence of PEPCK in maize, the involvement of PEPCK in the decarboxylation of Asp appears more likely, because PEPCK can directly catalyze the decarboxylation of oxaloacetate formed from Asp without previous...

Systems Analysis of a Maize Leaf Developmental Gradient Redefines the Current C4 Model and Provides Candidates for Regulation

“…Within the age gradient in a single leaf, there is no evidence in the enzyme activities, transcript abundance, or metabolite accumulation pattern to suggest that operation of the cycle switches from one transfer acid to another ( Figure 5). The presence of higher PEP-CK activity in older maize plants with older leaves (Wingler et al, 1999), however, points to a developmental regulation between leaves rather than within a leaf, similar to what has been recently observed in the dicotyledonous C 4 plant Cleome gynandra (Sommer et al, 2012). Environmental adaptation of Asp metabolism based on N availability in maize leaves with regard to pool size and turnover has also been demonstrated (Khamis et al, 1992).…”

“…For example, bundle sheath strands can efficiently decarboxylate not only malate but also the amino acid Asp (Chapman and Hatch, 1981). Older maize leaves, at least, harbor a second decarboxylation enzyme, PEP carboxykinase (PEP-CK), which releases carbon dioxide from OAA, producing PEP (Wingler et al, 1999). Furthermore, approximately one-quarter of radioactively labeled carbon dioxide that was fed to maize leaves was found to be rapidly incorporated into Asp (Hatch, 1971).…”

Individual Maize Chromosomes in the C3 Plant Oat Can Increase Bundle Sheath Cell Size and Vein Density

“…Traditionally, this has led to the classification of C 4 plants into one of three biochemical subtypes, depending on whether they primarily use NADP-dependent malic enzyme (NADP-ME), NAD-dependent malic enzyme (NAD-ME), or phosphoenolpyruvate carboxykinase (Hatch et al, 1975;Hatch, 1987;Leegood, 2002). However, there appears to be reasonable mechanistic flexibility that is determined both temporally and environmentally (Walker et al, 1997;Wingler et al, 1999;Furumoto et al, 1999), which suggests that the distinctions between these three biochemical subtypes are less rigid than originally thought (Furbank, 2011). As well as this diversity within the biochemistry of C 4 photosynthesis, there is also variation in terms of the leaf anatomy associated with the C 4 pathway.…”

The Operation of Two Decarboxylases, Transamination, and Partitioning of C4 Metabolic Processes between Mesophyll and Bundle Sheath Cells Allows Light Capture To Be Balanced for the Maize C4 Pathway

“…Recently, the maize C 4 subgroup has been shown to be complicated by the presence of two BS decarboxylation enzyme systems (NADP-ME and phosphoenolpyruvate carboxykinase [PEPCK]), presumably both acting as CO 2 delivery pathways (via malate [MAL] and Asp, respectively; Furumoto et al, 1999Furumoto et al, , 2000Wingler et al, 1999;Eprintsev et al, 2011;Furbank, 2011;Pick et al, 2011). There is also an extensive overlap between BS and M functions, since both cell types can synthesize starch (Spilatro and Preiss, 1987;Kanai and Edwards, 1999) and reduce phosphoglyceric acid (PGA; Majeran and van Wijk, 2009; see the overall scheme in Fig.…”