Plants utilize light energy for photosynthetic carbon fixation, which occurs through two distinct mechanisms, via a C4 or a C3 pathway. Most grass species use C4 photosynthesis, in which two different layers of photosynthetic cells are wrapped around each leaf vein. The first layer fixes carbon into compartments where the cellular chemistry is equipped to photosynthesise at a faster rate and the second layer then recycles the products back into carbon dioxide before the carbon is fixed once more – both layers therefore working together to concentrate carbon dioxide. By contrast C3 plants, which account for the remaining 95 percent of all terrestrial plant species, possess only a single photosynthetic cell type surrounding the leaf vein, limiting their capacity to fix carbon. C4 photosynthesis is consequently more efficient than C3 photosynthesis under conditions of high light intensity and high temperatures.
Understanding the molecular mechanics of the C4 system has clear implications for future agriculture and food production, yet identifying the regulatory factors involved has proved elusive. In a significant step toward this goal, Shaun Bowman and colleagues in the laboratory of James Berry at the University of Buffalo, USA, have identified a new RNA binding protein that is the first such factor to be implicated in the regulation of an individual photosynthetic gene in a C4 plant, as published in a recent study in BMC Plant Biology.
Both C3 and C4 plants contain mesophyll cells, which contain the apparatus needed for photosynthesis. Here, carbon-dioxide assimilating enzymes fix carbon into a usable form, with each enzyme encoded by a specific pattern of gene expression. The primary workhorse of this process in C3 plants is an enzyme called Rubisco, yet it wastefully also fixes oxygen in the process. C4 plants overcome this wastefulness by housing Rubisco in a different cellular compartment that is surrounded by the mesophyll – known as the bundle sheath. The bundle sheath cells play host to only the second round of carbon fixation using Rubisco; the primary round of fixation instead being carried out in the mesophyll by the enzyme PEPCase, which is more efficient than Rubisco at fixing carbon.
Although inefficient, Rubisco is nevertheless ubiquitous in plants, and essential to their growth and survival. Given this, comparing the two different photosynthetic processes and identifying what regulates the shared components of their enzymatic pathways could hold the key to understanding how they both operate.
Encoding the larger of Rubisco’s two protein subunits is a chloroplast gene known as rcbL, found in both plant types. Expression of this gene in specific cell-types like the bundle sheath is tightly controlled after transcription via specific stretches of sequences within its messenger RNA (mRNA). The proteins that bind these RNA molecules influence patterns of gene expression, and it was these proteins that Bowman and colleagues sought to find.
In a multi-layered analysis using several C4 species and the model C3 plant Arabidopsis, the authors identified a protein called RLSB based on its ability to bind to rcbL mRNA. In both C3 and C4 plants, reductions in the levels of RLSB gene expression in RLSB mutant and gene-silenced plants resulted in reduced photosynthetic function, and visibly yellowed leaves.
Comparison with other plant species indicates that the RLSB protein is highly conserved across many plant species, and co-localises with Rubisco in all C3 plant cells. The localisation of this protein specifically to the bundle sheath cells in C4 plants now adds an extra layer of evidence pointing towards a crucial regulatory role for RLSB in the expression of Rubisco in C4 plants as well. Taken together, this work suggests that the mechanism of RLSB gene expression observed throughout C3 plants may have been modified over evolutionary timescales to provide a more localised, specialised, and ultimately similar function in C4 plants.
C4 plants currently account for around a quarter of global plant primary productivity, yet few crop species utilise this more efficient form of photosynthesis. Tellingly, it is also the competitive and adaptable grass species that are mostly characterised by this system, with C4 plants maize and sugarcane among the crop-plant exceptions. Understanding the full scope of this photosynthetic pathway could therefore pave the way toward more highly efficient varieties of non-C4 crops, and a more efficient answer to future food production.
Written by Simon Harold (@sid_or_simon), Senior Executive Editor for the BMC Series.