C4 cycles: past, present, and future research on C4 photosynthesis

Abstract

In the late 1960s, a vibrant new research field was ignited by the discovery that instead of fixing CO(2) into a C(3) compound, some plants initially fix CO(2) into a four-carbon (C(4)) compound. The term C(4) photosynthesis was born. In the 20 years that followed, physiologists, biochemists, and molecular and developmental biologists grappled to understand how the C(4) photosynthetic pathway was partitioned between two morphologically distinct cell types in the leaf. By the early 1990s, much was known about C(4) biochemistry, the types of leaf anatomy that facilitated the pathway, and the patterns of gene expression that underpinned the biochemistry. However, virtually nothing was known about how the pathway was regulated. It should have been an exciting time, but many of the original researchers were approaching retirement, C(4) plants were proving recalcitrant to genetic manipulation, and whole-genome sequences were not even a dream. In combination, these factors led to reduced funding and the failure to attract young people into the field; the endgame seemed to be underway. But over the last 5 years, there has been a resurgence of interest and funding, not least because of ambitious multinational projects that aim to increase crop yields by introducing C(4) traits into C(3) plants. Combined with new technologies, this renewed interest has resulted in the development of more sophisticated approaches toward understanding how the C(4) pathway evolved, how it is regulated, and how it might be manipulated. The extent of this resurgence is manifest by the publication in 2011 of more than 650 pages of reviews on different aspects of C(4). Here, I provide an overview of our current understanding, the questions that are being addressed, and the issues that lie ahead.

Figure 1.

Schematic of C₃ Calvin-Benson and NADP-ME C₄ Cycles. Calvin-Benson (A) and NADP-ME C₄ (B) cycles. The green box represents the chloroplast. Blue dots represent active transport steps.

Figure 2.

Comparison between Photosynthetic Pathway and Mean Annual Temperature for 1200 Grass Species Representing 20 Origins of C₄. (Reprinted with permission from Edwards and Smith [2010], Figure 2.)

Figure 3.

Transverse Leaf Sections and Corresponding Schematics of C₃ Rice and C₄ Maize. Rice (left) and maize (right). Bars = 30 μm.