Indol‐3‐ylacetic acid‐ and calmodulin‐regulated Ca2+‐ATPase: A target for the phytotoxic action of hexachlorocyclohexane

In rice seedlings, the organochlorine insecticide hexachlorocyclohexane (HCH) and its four major isomers-alpha, beta, gamma, and delta-have been shown to elicit their phytotoxic action by interacting with indol-3-ylacetic acid (IAA)-regulated growth and Ca2+-ATPase activity. When rice seedlings were grown in the presence of 0.34 mM HCH, seedling vigour was reduced to 33 % of the control. A similar effect was observed when seedlings were grown in the presence of the gamma and delta isomers, but not with the alpha and beta isomers of HCH. This reduced vigour could be restored by treating the seedlings with 100 nm IAA, suggesting that HCH and its isomers (gamma and delta) limit either IAA synthesis or action or both. In a microsomal fraction from rice seedlings grown in the presence of either HCH or its isomers, the calmodulin-regulated Ca2+-ATPase activity was inhibited as follows: technical HCH = 46 %, alpha isomer = 80 %, beta isomer = 72 %, gamma isomer = 65 % and delta isomer = 62 % of the control value, respectively. TLC analysis suggested that the various isomers of HCH are metabolised by the plant, except for the delta isomer, which accumulated. This isomer, along with the degraded products of other isomers, may be responsible for the phytotoxic action of HCH