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Abstract

Calcium is an important second messenger in plants that is released into the cytosol early after recognition of various environmental stimuli. Decoding of such calcium signals by calcium
sensors is the key for the plant to react appropriately to each stimulus. Several members
of Calmodulin-like proteins (CMLs) act as calcium sensors and some are known to
mediate both abiotic and biotic stress responses. Here, we study the role of the Arabidopsis
thaliana CML9 in different stress responses. CML9 was reported earlier as defense regulator
against Pseudomonas syringae. In contrast to salicylic acid-mediated defense against
biotrophic pathogens such as P. syringae, defenses against herbivores and necrotrophic
fungi are mediated by jasmonates. We demonstrate that CML9 is induced upon wounding
and feeding of the insect herbivore Spodoptera littoralis. However, neither different CML9
loss-of-function mutant lines nor overexpression lines were impaired upon insect feeding.
No difference in herbivore-induced phytohormone elevation was detected in cml9 lines. The
defense against the spider mite Tetranychus urticae was also unaffected. In addition, cml9
mutant lines showed a wild type-like reaction to the necrotrophic fungus Alternaria brassicicola.
Thus, our data suggest that CML9 might be a regulator involved only in the defense
against biotrophic pathogens, independent of jasmonates. In addition, our data challenge
the involvement of CML9 in plant drought stress response. Taken together, we suggest that CML9 is a specialized rather than a general regulator of stress responses in Arabidopsis.