Differential activation of four specific MAPK pathways by distinct elicitors

Plant cells respond to elicitors by inducing a variety of defense responses . Some of these reactions are dependent on the activity of protein kinases. Recently, mitogen-activated protein kinases (MAPKs) have been identified t o be activated by fungal and bacterial elicitors as well as by pathogen inf ection. In gel kinase assays of alfalfa cells treated with yeast cell wall- derived elicitor (YE) revealed that 44- and 46-kDa MAPKs are rapidly and tr ansiently activated. Immunokinase assays with specific MAPK antibodies reve aled that YE mainly activated the 46-kDa SIMK and the 44-kDa MMK3 and to a lesser extent the 44-kDa MMK2 and SAMK. When cells mere treated with chemic ally defined elicitors potentially contained in the YE (chitin and N-acetyl glucosamine oligomers, beta -glucan, and ergosterol), the four MAPKs were f ound to be activated to different levels and with different kinetics, Where as SIMK and SAMK have been found to be activated by a number of diverse sti muli, MMK3 is activated during mitosis and was therefore assumed to partici pate in cell division (22). No physiological, process could be associated w ith MMK2 activity so far. This is the first report that MMK2 and MMK3 can b e activated by external stimuli. Overall, our findings indicate that plant cells can sense different cues of a given microorganism through the activat ion of multiple MAPKs.