Zooming into sub-organellar localization of reactive oxygen species in guard cell chloroplasts during abscisic acid and methyl jasmonate treatments.

Regulation of stomata movements is crucial for plants ability to cope with their changing environment. Guard cells' (GC) water potential directs water flux inside/outside this cell, which eventually is causing the stoma to open or close, respectively. Some of the osmolytes which accumulates in the GC cytoplasm and are known to play a role in stomata opening are sugars, arising from chloroplast starch degradation. During stomata closure, the accumulated osmolytes are removed from the GC cytoplasm. Surprisingly little is known about prevention of starch degradation and forming additional sugars which may interfere with osmotic changes that are necessary for correct closure of stomata. One of the early events leading to stomata closure is production of reactive oxygen species (ROS) in various subcellular sites and organelles of the stoma. Here we report that ROS production during abscisic acid (ABA) and methyl jasmonate (MJ) stimuli in Arabidopsis GC chloroplasts were more than tripled. Moreover, ROS were detected on the suborganelle level in compartments that are typically occupied by starch grains. This observation leads us to suspect that ROS function in that particular location is necessary for stomata closure. We therefore hypothesize that these ROS are involved in redox control that lead to the inactivation of starch degradation that takes place in these compartments, thus contributing to the stoma closure in an additional way.