K ranz Anatomy and the C ₄ Pathway

Abstract

C 4 photosynthesis incorporates novel leaf anatomy, metabolic specialisations and modified gene expression. C 4 plants typically possess a distinctive Kranz leaf anatomy consisting of two photosynthetic cell types. These are bundle sheath (BS) cells that surround the vascular centres, and mesophyll (M) cells that, in turn, surround the BS cells. A more rare form uses compartmentalisation of dimorphic chloroplasts within a single cell type. In C 4 leaves, these structural frameworks functionally separate two sets of carboxylation and decarboxylation reactions. Selective expression of key photosynthesis genes in BS and M cells leads to specific accumulation of key photosynthetic enzymes which catalyse different sets of cell‐type‐specific reactions, enabling these plants to assimilate atmospheric CO 2 with very high efficiency. For some plants, C 4 photosynthesis has facilitated their adaptation to arid conditions, high temperatures and marginal environments. Understanding the basis of this pathway has applications for improvements in agricultural productivity and alternative fuel development. Key Concepts C 4 photosynthesis is a carbon concentration mechanism used by some plants to improve the efficiency of photosynthetic carbon fixation. C 4 photosynthesis incorporates modified leaf morphology, separation of carboxylation and decarboxylation/refixation steps of carbon assimilation, and specialised patterns of cell‐type‐specific gene expression. The leaves of most C 4 plants possess a Kranz‐type anatomy consisting of bundle sheath and mesophyll cells. A more rare form of this pathway, called single‐cell C 4 , uses partitioning of dimorphic chloroplasts to separate different sets of reactions within a single leaf cell type. In C 4 leaves, Rubisco is localised only within internalised bundle sheath cells, or internalised chloroplasts, to protect from atmospheric O 2 and limit the oxygenase activity of this enzyme. The initial carbon assimilation enzyme in these leaves is PEPCase, which incorporates CO 2 but not O 2 . Reactions of C 4 pathway work as a pump to concentrate CO 2 in the vicinity of the internalised Rubisco enzyme. These reduce or eliminate photorespiration, thereby enhancing photosynthetic efficiency. C 4 plants are more efficient than C 3 plants under arid conditions, including high temperatures and water stress. C 4 plants show increased efficiency of water and nitrogen use, allowing them to outcompete C 3 plants in marginal environments. Bioengineering of C 4 characteristics into C 3 species has the potential to improve photosynthetic efficiency in crop plants used for food and biofuel.

Natural Variation within a Species for Traits Underpinning C4 Photosynthesis

“…Calvin-Benson-Bassham cycles are strongly and preferentially expressed in either M or BS cells 57 (Garner et al, 2001;Marshall et al, 2007;Hibberd and Covshoff, 2010). 58…”

“…In C 4 plants, RuBisCO is typically sequestered in bundle 48 sheath (BS) cells that are concentrically arranged around the vasculature. Establishment of a 49 molecular CO 2 pump delivers carbon to RuBisCO from Mesophyll (M) cells via C 4 acid 50 intermediates (Garner et al, 2001). C 4 photosynthesis relies more heavily on the BS for 51 photosynthesis, and less on M cells.…”

Quantitative variation within a species for traits underpinning C4 photosynthesis

“…In C 4 plants, RuBisCO is typically sequestered in bundle sheath (BS) cells that are concentrically arranged around the vasculature. Establishment of a molecular CO 2 pump delivers carbon to RuBisCO from Mesophyll (M) cells via C 4 acid intermediates 8 . C 4 photosynthesis relies on an increased importance of the BS for photosynthesis, reduced dependence on M cells, more chloroplasts in BS cells, increased proliferation of plasmodesmata between M and BS cells, and a higher vein density to increase the volume of the leaf occupied by the BS.…”

“…These morphological alterations to the leaf that facilitate the C 4 cycle are known as Kranz anatomy 9 . Moreover, photosynthesis gene expression is modified such that genes encoding components of the C 4 and Calvin-Benson-Bassham cycles are strongly and preferentially expressed in either M or BS cells 8,10 .…”

MODELING EVAPOTRANSPIRATION FOR C4 and C3 CROPS IN THE WESTERN LAKE ERIE BASIN USING REMOTE SENSING DATA

“…In addition to larger areas covered with corn, the photosynthetic adaptations of corn, as the most widely cultivated C4 plant, affects ET differently than other, more abundant, C3 crops (e.g., soybean, wheat, and alfalfa). It is well known that C4 plants have their photosynthetic mechanism developed to reduce photorespiration and save water (Ajao et al, 2017) and that they are more photosynthetically active than C3 plants (Ueno, 2001;Garner et al, 2016;Ajao et al, 2017). Lundgren and Christin (2016) summarized the findings showing how the anatomical variations in C4 plants also exhibit more efficient uptake of CO2.…”