Add the CSS style to hide the expanding/collapsing icon on the treegrid.
<style>
.mytg .tree-hit{
display: none;
}
</style>
And then listen to the 'onClickRow' event, call the 'toggle' method to expand or collapse the node.
$('#tg').treegrid({
cls: 'mytg',
onClickRow: function(row){
$(this).treegrid('toggle',row.id)
}
})