Chloroplasts: what are they for?

Taken at face value that is a pretty daft question. After all, it is well known that chloroplasts are for photosynthesis. True, but what about the chloroplasts in the guard cells (GCCs) of stomata?

Chloroplasts in those highly specialised epidermal cells are quite far removed from those of mesophyll cells and presumably not involved in generation of photosynthate destined for export to sinks within the plant – an important role of those mesophyll-located organelles. So presumably any role of GCCs is local to their position in the guard cells, and is an area that has many questions and few answers. But work by Tamar Azoulay-Shemer et al. sheds some light on this topic.

Using transgenic arabidopsis plants – with chlorophyll-deficient guard cells – their data support the view that guard cell photosynthesis is critical for energization and guard cell turgor production (which leads to stomatal opening). So, hopefully, another piece of the complex jigsaw that is stomatal physiology and behaviour can be slotted into place.

Related to drought (a situation of extremely low water availability whose effects are lessened by stomatal closure, which the phenomenon promotes, and whereby photosynthesis is consequently considerably reduced…), news of a more traditional photosynthetic role for chloroplasts – albeit those in a location that is often overlooked – comes from Maurits Vandegehuchte et al.

The arborichloroplastic quartet is concerned about forest decline and tree mortality that is occasioned by global-change-associated droughts and the concomitant reduction in long-distance photosynthate transport from the leaves to the sinks under such drought conditions. Recognising the existence of chloroplasts in woody tissues (‘bark’) of trees, they aim to encourage researchers to investigate the role of woody tissue photosynthesis during drought stress and mortality events.

A case of barking up the right tree or the tree’s own version of ‘think global, act local’?