DIFFERENTIAL EXPRESSION OF 2 CALMODULIN GENES IN RESPONSE TO PHYSICALAND CHEMICAL STIMULI

Two different calmodulin (CaM) cDNAs (MBCaM-1 and MBCaM-2) were isolat ed from a vigna radiata lambda gt 11 library by screening with a heter ologous Arabidopsis cDNA probe (TCH-1). Both cDNAs are 85% homologous inside the coding region but are highly divergent outside this region. The polypeptides encoded by MBCaM-1 and MBCaM-2 are identical except for two conservative substitutions at positions 7 and 10. Southern ana lysis revealed that both cDNAs are encoded by different genes. Express ion studies revealed different patterns of expression of both genes. M BCaM-1 mRNA exhibited a dramatic transient increase in response to tou ch, while MBCaM-2 expression showed a steady but small increase as com pared to MBCaM-1. When plants were grown in complete darkness MBCaM-1 was undetectable and MBCaM-2 exhibited very low levels of expression. One hour after exposure of etiolated seedlings to light MBCaM-1 showed no change, while MBCaM-2 expression was increased. After a 6 h exposu re to light there was an induction of both MBCaM-1 and MBCaM-2; howeve r, the magnitude of this increase was much greater for MBCaM-2. When p lants were grown under a 16 h light/8 h dark cycle the mRNA levels for MBCaM-1 were lower during the light period and increased during the b eginning of the night cycle, while MBCaM-2 showed no change. Plants tr eated with indole-3-acetic acid had a peak in MBCaM-1 expression 6 h a fter treatment initiation with a slight decline 3 h after the peak, wh ile MBCaM-2 showed a steady but small increase over time as compared t o MBCaM-1. When plants were subjected to salt stress they showed an in crease in MBCaM-1 expression 2 h after treatment initiation reaching a maximum after 4 h with no further increase after 6 h, while MBCaM-2 r emained unchanged over the time course.