THE ROLE OF CALMODULIN IN THE GRAVITROPIC RESPONSE OF THE ARABIDOPSIS-THALIANA AGR-3 MUTANT

Calmodulin, a primary plant calcium receptor, is known to be intimatel y involved with gravitropic sensing and transduction. Using the calmod ulin-binding inhibitors trifluoperazine, W7 and calmidazolium, gravitr opic curvature of Arabidopsis thaliana (L.) Heynh, ecotype Landsberg, roots was separable into two phases. Phase I was detected at very low concentrations (0.01 mu M) of trifluoperazine and calmidazolium, did n ot involve growth changes, accounted for about half the total curvatur e of the root and may represent the specific contribution of the cap t o gravity sensing. Phase II commenced around 1.0 mu M and involved inh ibition of both growth and curvature. The agr-3 mutant exhibited a red uced gravitropic response and was found to lack phase I curvature, sug gesting that the mutation alters either use or expression of calmoduli n. The sequences of wild-type and agr-3 calmodulin (CaM-1) cDNAs, whic h are root specific were completely determined and found to be identic al. Upon gravitropic stimulation, wild-type Arabidopsis seedlings incr eased calmodulin mRNA levels by threefold in 0.5 h. On the other hand, gravitropic stimulation of agr-3 decreased calmodulin mRNA accumulati on. The possible basis of the two phases of curvature is discussed and it is concluded that agr-3 has a lesion located in a general gravity transmission sequence, present in many root cells, which involves calm odulin mRNA accumulation.