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Sharing a single cell confounds conventional wisdom
It is one of those things that every botany student knows. Terrestrial plants that perform C4 photosynthesis possess Krantz anatomy in which the C4 and C3 phases occur in different types of chloroplast located in different anatomical regions of the leaf. However, recent discoveries with two species of the family Chenopodiaceae show that our botany students will need to update their knowledge. In these species, the C4 and C3 phases of photosynthesis occur in different forms of chloroplast located at opposite ends of individual elongated leaf cells. The potential of this phenomenon for studying the intracellular spatial regulation of gene expression has been recognised by Voznesenskaya et al. (St Petersburg, Russia and Pullman, Washington, USA, pp. 177-187) who have focussed on one of these species, Borszczowia aralocaspica. In this paper they are interested especially in the transition undergone by cotyledons during germination, from a storage organ to a photosynthetic organ. Anatomical investigation of cotyledon development revealed that elongated cells, resembling those of leaves, are already present in mature seeds, but, as expected, they are not green. During germination, Rubisco content, as revealed by immunocytochemistry, starts to increase before emergence into the light (and indeed accumulates in seeds germinated in the dark). However, the presence of two key enzymes of the C4 pathway, phosphoenolpyruvate carboxylase (PEPC) and pyuruvate, Pi dikinase (PPDK) is completely light-dependent. Furthermore, as these chlorenchyma cells develop their full capacity, Rubisco is partitioned into fully developed chloroplasts at one end of the cell, while PPDK is at the other end, contained in less well-differentiated chloroplasts. PEPC, by contrast, is distributed throughout the cytoplasm, possibly reflecting the occurrence of different isoforms with different roles. What an amazing system this is, leading us to ask how the cell achieves this sub-cellular partitioning. Clearly the authors have some fascinating research ahead of them.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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