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Plant and Cell Physiology Advance Access published online on February 25, 2009
Plant and Cell Physiology, doi:10.1093/pcp/pcp033
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© The Author 2009. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

The Functional Anatomy of Rice Leaves: Implications for Refixation of Photorespiratory CO2 and Efforts to Engineer C4 Photosynthesis into Rice.

Tammy L. Sage and Rowan F. Sage1

Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street Toronto, ON M5S3B2 Canada

1Corresponding Author; Dr. Rowan Sage. Department of Ecology and Evolutionary Biology, The University of Toronto, 25 Willcocks Street, Toronto, ON M5S3B2 Canada, Tel 1-416-978-7660, Fax 1-416-978-5878, E.mail r.sage{at}utoronto.ca


  Abstract

One mechanism to radically enhance global food stocks is to introduce C4 photosynthesis into C3 crops from warm climates, notably rice. To accomplish this, an understanding of leaf structure and function is essential. Chlorenchyma structure of rice and related warm-climate C3 grasses is distinct from cool temperate C3 grasses. In temperate C3 grasses, vacuoles occupy the majority of the cell, while chloroplasts, peroxisomes and mitochondria are pressed against the cell periphery. In rice, 66% of protoplast volume is occupied by chloroplasts, and chloroplasts/stromules cover >95% of the cell periphery. Mitochondria and peroxisomes occur in the cell interior and are intimately associated with chloroplasts/stromules. We hypothesize that the chlorenchyma architecture of rice enhances diffusive CO2 conductance and maximizes scavenging of photorespired CO2. The extensive chloroplast/stromule sheath forces photorespired CO2 to exit cells via the stroma, where it can be refixed by Rubisco. Deep cell lobing and small cell size, coupled with chloroplast sheaths, creates high surface area exposure of stroma to intercellular spaces thereby enhancing mesophyll transfer conductance. In support of this, rice exhibits higher mesophyll transfer conductance, greater stromal CO2 content, lower CO2 compensation points at warm temperature, and less oxygen sensitivity of photosynthesis than cool temperate grasses. Rice vein length per leaf, mesophyll thickness, and intercellular space volume are intermediate between most C3 and C4 grasses, indicating the introduction of Kranz anatomy into rice may not require radical changes in leaf anatomy; however, deep lobing of chlorenchyma cells may constrain efforts to engineer C4 photosynthesis into rice.

Keywords: CO2 concentrating mechanism - C4 photosynthesis - mesophyll conductance parenchyma anatomy - Oryza - stromule

(Received February 1, 2009; Accepted February 23, 2009)
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