Inheritance of c(4) enzymes associated with carbon fixation in flaveria species

C4 cycles: past, present, and future research on C4 photosynthesis

In the late 1960s, a vibrant new research field was ignited by the discovery that instead of fixing CO(2) into a C(3) compound, some plants initially fix CO(2) into a four-carbon (C(4)) compound. The term C(4) photosynthesis was born. In the 20 years that followed, physiologists, biochemists, and molecular and developmental biologists grappled to understand how the C(4) photosynthetic pathway was partitioned between two morphologically distinct cell types in the leaf. By the early 1990s, much was known about C(4) biochemistry, the types of leaf anatomy that facilitated the pathway, and the patterns of gene expression that underpinned the biochemistry. However, virtually nothing was known about how the pathway was regulated. It should have been an exciting time, but many of the original researchers were approaching retirement, C(4) plants were proving recalcitrant to genetic manipulation, and whole-genome sequences were not even a dream. In combination, these factors led to reduced funding and the failure to attract young people into the field; the endgame seemed to be underway. But over the last 5 years, there has been a resurgence of interest and funding, not least because of ambitious multinational projects that aim to increase crop yields by introducing C(4) traits into C(3) plants. Combined with new technologies, this renewed interest has resulted in the development of more sophisticated approaches toward understanding how the C(4) pathway evolved, how it is regulated, and how it might be manipulated. The extent of this resurgence is manifest by the publication in 2011 of more than 650 pages of reviews on different aspects of C(4). Here, I provide an overview of our current understanding, the questions that are being addressed, and the issues that lie ahead.

Evolution of C4 photosynthesis in flaveria species. Isoforms Of nadp-malic enzyme

NADP-malic enzyme (NADP-ME, EC 1.1.1.40), a key enzyme in C4 photosynthesis, provides CO2 to the bundle-sheath chloroplasts, where it is fixed by ribulose-1,5-bisphosphate carboxylase/oxygenase. We characterized the isoform pattern of NADP-ME in different photosynthetic species of Flaveria (C3, C3-C4 intermediate, C4-like, C4) based on sucrose density gradient centrifugation and isoelectric focusing of the native protein, western-blot analysis of the denatured protein, and in situ immunolocalization with antibody against the 62-kD C4 isoform of maize. A 72-kD isoform, present to varying degrees in all species examined, is predominant in leaves of C3 Flaveria spp. and is also present in stem and root tissue. By immunolabeling, NADP-ME was found to be mostly localized in the upper palisade mesophyll chloroplasts of C3 photosynthetic tissue. Two other isoforms of the enzyme, with molecular masses of 62 and 64 kD, occur in leaves of certain intermediates having C4 cycle activity. The 62-kD isoform, which is the predominant highly active form in the C4 species, is localized in bundle-sheath chloroplasts. Among Flaveria spp. there is a 72-kD constitutive form, a 64-kD form that may have appeared during evolution of C4 metabolism, and a 62-kD form that is necessary for the complete functioning of C4 photosynthesis.

Primary structure of NADP-dependent malic enzyme in the dicotyledonous C4 plant Flaveria trinervia

The primary structure of NADP-dependent malic enzyme (NADP-ME) of the dicotyledonous C4 plant Flaveria trinervia was determined from sequence analysis of a cDNA clone containing the complete coding region. Comparison of the mature F. trinervia NADP-ME with the maize enzyme reveals extensive sequence similarity. In contrast, no significant similarity can be detected between the putative transit peptides of the two enzymes. This suggests that the corresponding parts of the genes arose independently from each other during evolution of mono- and dicotyledonous C4 plants.

Transfer of C(4) Photosynthetic Characters through Hybridization of Flaveria Species

Transfer of C(4) photosynthetic traits was studied through hybridization of Flaveria trinervia (Spreng.) Mohr (C(4)) and Flaveria brownii A.M. Powell (C(4)-like) with Flaveria linearis Lag. (C(3)-C(4)) and the C(3) species Flaveria pringlei Gandoger (C(3)). Fertility was low, based on irregular chromosome pairing and low pollen stainability, except in F. brownii x F. linearis which had bivalent pairing and 76% stainable pollen. Hybrids had apparent photosynthesis values of 71 to 148% of the midparental means, while the CO(2) compensation concentration was similar to the C(4) or C(4)-like parent, except in hybrids having the C(3) species F. pringlei as a parent. Inhibition of apparent photosynthesis by O(2), and phosphoenolpyruvate carboxylase and NADP-malic enzyme activities and subunit levels in the hybrids were closer to the C(3) or C(3)-C(4) parent. The species F. brownii and F. trinervia were equal in their capacity to transfer reduced O(2) inhibition of AP and CO(2) compensation concentration values to hybrids with F. linearis (C(3)-C(4)), although hybrids with F. trinervia had higher PEPC activity. The O(2) inhibition of AP was correlated with the logarithm of activities of phosphoenolpyruvate carboxylase (r = -0.95) and NADP-malic enzyme (r = -0.87). These results confirm that C(4) traits can be transferred by hybridization of C(3)-C(4) and C(4) or C(4)-like species, with a higher degree of C(4) photosynthesis than exists in C(3)-C(4) species, and at least in F. brownii x F. linearis, fertile progeny are obtained.