Natural variation in metabolism of the Calvin-Benson cycle.

The Calvin-Benson cycle (CBC) evolved over 2 billion years ago but has been subject to massive selection due to falling atmospheric carbon dioxide, rising atmospheric oxygen and changing nutrient and water availability. In addition, large groups of organisms have evolved carbon-concentrating mechanisms (CCMs) that operate upstream of the CBC. Most previous studies of CBC diversity focused on Rubisco kinetics and regulation. Quantitative metabolite profiling provides a top-down strategy to uncover inter-species diversity in CBC operation. CBC profiles were recently published for twenty species including terrestrial C3 species, terrestrial C4 species that operate a biochemical CCM, and cyanobacteria and green algae that operate different types of biophysical CCM. Distinctive profiles were found for species with different modes of photosynthesis, revealing that evolution of the various CCMs was accompanied by co-evolution of the CBC. Diversity was also found between species that share the same mode of photosynthesis, reflecting lineage-dependent diversity of the CBC. Connectivity analysis uncovers constraints due to pathway and thermodynamic topology, and reveals that cross-species diversity in the CBC is driven by changes in the balance between regulated enzymes and in the balance between the CBC and the light reactions or end-product synthesis.

The genome sequence of Hirschfeldia incana, a new Brassicaceae model to improve photosynthetic light-use efficiency.

Photosynthesis is a key process in sustaining plant and human life. Improving the photosynthetic capacity of agricultural crops is an attractive means to increase their yields. While the core mechanisms of photosynthesis are highly conserved in C3 plants, these mechanisms are very flexible, allowing considerable diversity in photosynthetic properties. Among this diversity is the maintenance of high photosynthetic light-use efficiency at high irradiance as identified in a small number of exceptional C3 species. Hirschfeldia incana, a member of the Brassicaceae family, is such an exceptional species, and because it is easy to grow, it is an excellent model for studying the genetic and physiological basis of this trait. Here, we present a reference genome of H. incana and confirm its high photosynthetic light-use efficiency. While H. incana has the highest photosynthetic rates found so far in the Brassicaceae, the light-saturated assimilation rates of closely related Brassica rapa and Brassica nigra are also high. The H. incana genome has extensively diversified from that of B. rapa and B. nigra through large chromosomal rearrangements, species-specific transposon activity, and differential retention of duplicated genes. Duplicated genes in H. incana, B. rapa, and B. nigra that are involved in photosynthesis and/or photoprotection show a positive correlation between copy number and gene expression, providing leads into the mechanisms underlying the high photosynthetic efficiency of these species. Our work demonstrates that the H. incana genome serves as a valuable resource for studying the evolution of high photosynthetic light-use efficiency and enhancing photosynthetic rates in crop species.