The genus Neurachne: Key to Understanding the Evolution of C4 Photosynthesis

Title: The genus Neurachne: Key to Understanding the Evolution of C4 Photosynthesis

Speaker: Prof Martha Ludwig

Affiliation: School of Molecular Science, University Western Australia

Abstract: The Australian grass tribe Neurachninae is the only known monocot group with distinct, closely related species using C₃, C₄ or C₂ photosynthesis. C₂ photosynthesis is recognised as the evolutionary bridge from the ancestral C₃ pathway to the more derived C₄ pathway. To gain insights into the evolution of C₄ photosynthesis in the monocots, we examined gas exchange, leaf anatomy and cellular location of glycine decarboxylase subunit P (GDC-P) in Neurachne species previously described as C₃ (N. alopecuroidea, N. annularis, N. lanigera), C₂ (N. minor), and C₄ (N. munroi, N. muelleri). Our results demonstrate that progressive changes occurred in organelle numbers and GDC-P location as C₂ metabolism became engaged in the Neurachne, with scavenging of photorespiratory CO₂ within a vascular sheath tissue being a critical early stage in the evolution of the C₄ pathway.

An important enzyme in the CO₂ concentrating mechanism of C₄ plants is carbonic anhydrase (CA), which catalyses the first reaction in the C₄ pathway. We have used the photosynthetic diversity in the Neurachne to identify the C₄-associated CA and the sequence changes in the C₃ orthologue that enabled the evolution of the C₄ form of the enzyme. Different splice variants of the ca1 gene encode the C₄-associated CA in the two C₄ Neurachne species, supporting phylogenetic information for two independent C₄ origins in the genus. Remarkably, the evolutionary mechanism used by one of the C₄ species is homoplastic with the evolution of the C₄-associated CA in a dicot genus, suggesting that there may be molecular limitations / constraints on the evolutionary path to C₄.