var treeControl = new dijit.Tree({
        model: xyz,
        id: 'abc',
        showRoot: false,
        openOnClick: true,

// dependencies:
// var on = require("dojo/on");
// var mouse = require("dojo/mouse");
// var registry = require("dijit/registry");

on(tree.domNode, on.selector("img.dijitTreeExpandoClosed", mouse.enter), function(event) {
    var treeNode = registry.getEnclosingWidget(event.target);
    treeNode.tree._expandNode(treeNode);
});