Jasmonic acid (JA) induced-calcium elevation in Arabidopsis is highly variable due to time of day and conversion to JA-Ile

Abstract

Plants have evolved mechanisms to effectively anticipate environmental changes via diurnal rhythmicity (day/night) maintained by the circadian clock. Jasmonic acid biosynthesis and signalling are known to be under the control of the circadian clock. Both JA and its bioactive form jasmonoyl-l-isoleucine (JA-Ile) when externally added can induce a cytosolic Ca2+ influx in Arabidopsis thaliana. JA and JA-Ile induced Ca2+ is poorly understood and often used inter-changeably to study Ca2+ regulation of jasmonates. We attempted to understand if they are similar and if diurnal rhythms or time of day regulate them. JA induced Cacyt2+ signature is variable according to time-of-day in Arabidopsis. JA is sensed in two ways according to the time-of-day (a) directly sensed as JA and induces Cacyt2+ elevation (b) JA gets converted into the JA-Ile by JAR1 and is sensed as JA-Ile, which we proved using jar1-1*aequorin. This twin sensing mode is responsible for variability in JA induced Cacyt2+ signature. We further suggest caution when using JA as a stimulant for Cacyt2+ elevation measurements to compare wild-type (Col-0 transformed with pMAQ2; transgenic aequorin) and effect of different mutations. On the other hand bioactive JA-Ile induced Cacyt2+ signature is constant diurnally with maximum amplitude at dawn which coincides with maximum sensitivity of JA-Ile receptor, COI1 and increased VSP2 expression. From the above study we conclude that JA-Ile induced Cacyt2+ elevation is a better read-out than the highly variable JA-induced Cacyt2+ elevations to study the output pathways.