The thread is called plant, not plantBrick
%obj.playThread(2, plant);
Example Code
Weapon State
stateName[#] = "Reload";
stateSequence[#] = "Reload";
stateScript[#] = "onReload";
stateTimeoutValue[#] = 1.00;
stateTransitionOnTimeout[#] = "Ready";
On Load
function YourGunImage::onReload(%this,%obj,%slot)
{
%obj.playThread(2, plant);
}