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JXB Advance Access published online on March 28, 2008
Journal of Experimental Botany, doi:10.1093/jxb/ern054
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© The Author [2008]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Opinion Paper

On the mechanism of C4 photosynthesis intermediate exchange between Kranz mesophyll and bundle sheath cells in grasses

Pawel Sowinski1,2,*, Jaroslaw Szczepanik1 and Peter E. H. Minchin3

1University of Warsaw, Institute of Plant Experimental Biology, Department of Plant Growth and Development, Miecznikowa 1, 02-096 Warszawa, Poland
2Plant Breeding and Acclimatization Institute, Plant Biochemistry and Physiology
Department, Radzików, 05-870 Blonie, Poland
3The Horticulture and Food Research Institute of New Zealand Ltd, 412 No 1 Road, RD 2 Te Puke 3182, New Zealand

* To whom correspondence should be addressed. E-mail: pawes{at}biol.uw.edu.pl

C4 photosynthesis involves cell-to-cell exchange of photosynthetic intermediates between the Kranz mesophyll (KMS) and bundle sheath (BS) cells. This was believed to occur by simple diffusion through plentiful plasmodesmatal (PD) connections between these cell types. The model of C4 intermediates’ transport was elaborated over 30 years ago and was based on experimental data derived from measurements at the time. The model assumed that plasmodesmata occupied about 3% of the interface between the KMS and BS cells and that the plasmodesmata structure did not restrict metabolite movement. Recent advances in the knowledge of plasmodesmatal structure put these assumptions into doubt, so a new model is presented here taking the new anatomical details into account. If one assumes simple diffusion as the sole driving force, then calculations based on the experimental data obtained for C4 grasses show that the gradients expected of C4 intermediates between KMS and BS cells are about three orders of magnitude higher than experimentally estimated. In addition, if one takes into account that the plasmodesmata microchannel diameter might constrict the movement of C4 intermediates of comparable Stokes’ radii, the differences in concentration of photosynthetic intermediates between KMS and BS cells should be further increased. We believe that simple diffusion-driven transport of C4 intermediates between KMS and BS cells through the plasmodesmatal microchannels is not adequate to explain the C4 metabolite exchange during C4 photosynthesis. Alternative mechanisms are proposed, involving the participation of desmotubule and/or active mechanisms as either apoplasmic or vesicular transport.

Key words: C4 photosynthesis, grasses, modelling, plasmodesmata, symplasmic transport

Received 10 October 2007; Revised 4 February 2008 Accepted 5 February 2008


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