Where do Chloroplasts go to die?

Few things please me more than trying to introduce my first year students to the joys of cell ultrastructure (yes, of plant cells. Why? Is there any other kind worth mentioning…?). Especially when appraising them of the superiority of plant’s cells over those of ‘the others’ when listing the extra organelles that the former possess. Even where some organelles may bear a superficial similarity – such as in the case of the digestive enzyme capacity of the plant central vacuole and the enzyme-mediated hydrolytic activity of the lysosome, to animal cells – the distinctiveness of the plant ‘variant’ is impressive because of the number of additional roles it performs within the cell. However, and arguably, that distinction has been eroded by Masanori Izumi et al.

In a process termed chlorophagy [‘chloroplast-eating’], they demonstrate that light-damaged chloroplasts in leaves of Arabidopsis thaliana are transported to the central vacuole where they are digested. In that way, photo-impaired chloroplasts are removed from the cytoplasm and put ‘out of harm’s way’ in the vacuole. Or, and more accurately, potentially harmful, damaged organelles are put ‘out of the way’ in the vacuole, where they can’t harm the delicate metabolism of the rest of the cell, and hence the plant. Zoocentrists may try to use this nugget of information to say that plants and animal cells are therefore not as different as die-hard phytophiles such as Mr Cuttings would maintain. But, they are mistaken. From this phytosupremacist’s point of view, such a revelation actually enhances plant-animal cytological differences because the plant vacuole is now demonstrably even more multi-tasking than considered previously, and certainly much more so than those puny, few-purposed zoolysosomes!

A question this work poses is into what ‘units’ – e.g. chemical compounds – are the chloroplasts digested? And, how are those digestion produce re-used – if at all – by the cell/plant? And – in view of the discovery that chloroplasts from different cells in maize leaves have differences in the lipid components of their thylakoid membranes  – does each cell type digest damaged chloroplasts differently? So, here we have an example of great research, i.e. that which, whilst answering a specific question, raises more and suggests new avenues of investigation.

[Ed. – And, in a pleasingly circular, all-ends-tied-up way, while the above item gives insights into how chloroplasts end, work by Rafael Ponce-Toledo et al. gets us a little closer into finding out how chloroplasts began, in an evolutionary sense.]