The Evolutionary Origin of C₄ Photosynthesis in the Grass Subtribe Neurachninae

Abstract

The Australian grass subtribe Neurachninae contains closely related species that use C 3 , C 4 , and C 2 photosynthesis. To gain insight into the evolution of C 4 photosynthesis in grasses, we examined leaf gas exchange, anatomy and ultrastructure, and tissue localization of Gly decarboxylase subunit P (GLDP) in nine Neurachninae species. We identified previously unrecognized variation in leaf structure and physiology within Neurachne that represents varying degrees of C 3 -C 4 intermediacy in the Neurachninae. These include inverse correlations between the apparent photosynthetic carbon dioxide (CO 2 ) compensation point in the absence of day respiration (C * ) and chloroplast and mitochondrial investment in the mestome sheath (MS), where CO 2 is concentrated in C 2 and C 4 Neurachne species; width of the MS cells; frequency of plasmodesmata in the MS cell walls adjoining the parenchymatous bundle sheath; and the proportion of leaf GLDP invested in the MS tissue. Less than 12% of the leaf GLDP was allocated to the MS of completely C 3 Neurachninae species with C * values of 56-61 mmol mol 21 , whereas twothirds of leaf GLDP was in the MS of Neurachne lanigera, which exhibits a newly-identified, partial C 2 phenotype with C * of 44 mmol mol 21 . Increased investment of GLDP in MS tissue of the C 2 species was attributed to more MS mitochondria and less GLDP in mesophyll mitochondria. These results are consistent with a model where C 4 evolution in Neurachninae initially occurred via an increase in organelle and GLDP content in MS cells, which generated a sink for photorespired CO 2 in MS tissues. 566

Paving the Way for C4 Evolution: Study of C3-C4 Intermediate Species in Grasses

“…It is worth noting that although grasses and sedges dominate C 4 species, ,20% of known intermediate species are monocots, hindering our understanding of C 4 evolution within monocots (Sage et al, 2014). In this issue of Plant Physiology, Khoshravesh et al (2020) compare various anatomical, ultrastructural, and biochemical traits in nine species of the subtribe Neurachninae of the grass family, and identified new C 3 -C 4 intermediate species.…”

“…C 2 anatomy exhibits several C 4like features (although less pronounced than C 4 ), including prominent bundle sheath cells, increased vein density, more abundant and asymmetrically distributed chloroplasts in the bundle sheath, and higher plasmodesmatal frequency ( Fig. 1; Sage et al, 2014;Khoshravesh et al, 2020).…”

“…In addition to typical C 2 photosynthesis, other C 3 -C 4 intermediate species with different intermediate traits may shed light on C 4 evolution. Khoshravesh et al (2020) focused on subtribe Neurachninae, which contains a well-known C 2 species (Neurachne minor), two C 4 species (Neurachne munroi and Neurachne muelleri), and several putative C 3 species (Neurachne alopecuroidea, Neurachne queenslandica, Thyridolepis mitchelliana, Neurachne tenuifolia, Neurachne annularis, and Neurachne lanigera). The Neurachne species possess two layers of bundle sheath, an inner suberized mestome sheath where carbon fixation occurs (functional bundle sheath), and an outer parenchymatous sheath that separates mesophyll cells and mestome sheath.…”

“…The Neurachne species possess two layers of bundle sheath, an inner suberized mestome sheath where carbon fixation occurs (functional bundle sheath), and an outer parenchymatous sheath that separates mesophyll cells and mestome sheath. Khoshravesh et al (2020) measured C* (the CO 2 compensation point) as an indicator of photorespiratory CO 2 refixation. They further comprehensively quantified traits including mestome sheath and mesophyll cell size in three dimensions, the ratio of different cell types, mitochondria and chloroplast number and size, vein spacing, plasmodesmatal frequency, and the abundance of Gly decarboxylase subunit P (GLDP) and Rubisco large subunit by immunolabeling.…”

“…In contrast, N. annularis functions as a C 3 , as its C* is similar to that of other C 3 species. However, it shows subtle C 2 -like characteristics, including slightly wider mestome sheath cells and greater GLDP abundance per mestome sheath chloroplast, indicating its early stage in C 2 evolution (Khoshravesh et al, 2020).…”