INVESTIGATION INTO PROPERTIES OF C4 PHOTOSYNTHESIS FROM SINGLE-CELL AND KRANZ TYPE SPECIES IN SUAEDOIDEAE (CHENOPODIACEAE)
Rosnow, Joshua Jeffrey
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Although the majority of C4 plants have an anatomy that separates carboxylation and decarboxylation activities between two cell types (Kranz anatomy), some terrestrial plants Bienertia cycloptera, B. sinuspersici and Suaeda aralocaspica, can conduct C4 photosynthesis within an individual chlorenchyma cells. These single-cell C4 species are part of the Suaedoideae subfamily, where four independent gains of C4 photosynthesis are documented. Single-cell C4 species provide an interesting opportunity to study the developmental transition of chloroplasts from C3 to C4 photosynthesis and investigate if enzymatic differences exist between the anatomies of C4 photosynthesis. Using B. sinuspersici several methods were used to investigate the extent of dimorphic chloroplast differentiation and possible mechanisms for how this is accomplished. Chloroplast differences in content of photosystem II and the protein RLSB were shown, as well as differences in stromal redox potential. The results suggested that protein localization and the differential chloroplast redox state play a role in post-transcriptional regulation of rbcL. To determine how enzymes are optimized for C4 photosynthesis, the two carboxylases phosphoenolpyruvate carboxylase (PEPC) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), were investigated in Suaedoideae. Branch-site tests for positively selected codons in PEPC showed that, residues 364, 368, and 733 (maize numbering) are under positive selection with a posterior probability >0.99 using Bayes Empirical Bayes (BEB). The signature C4 substitution of an Ala for a Ser at position 780 in the carboxy-terminal end (which is considered a major determinant of affinity for PEP) was only found in 4 of the C4 species sampled, while 8 of the C4 species and all the C3 species have an Ala residue. This is the first report that shows PEPC from C4 plants without a serine at position 780 have C4 like PEPC kinetics. This is the also the first investigation which shows that Rubisco from C4 plants that lack any change in the rbcl gene still show C4 like Rubisco Kcat values. Thus, the major carboxylases in C4 Suaedoideae species lack the same mutations as other C4 systems but still have similar convergent kinetic properties, showing that there are multiple molecular routes to a convergent phenotype.