how to load stores in treepanel dynamically

Hi friends i am new to extjs3.4 can u please help me....Now i am facing the problem that is i have tree panel in that 3 root nodes are there and each one has its own child nodes and each child nodes are has its own leaf nodes.

Now the problem is how i can call the Ajax request when i am going to load root node? and if json response will come how i can load that into child nodes and if i click on one of the child nodes then leaf node should be refreshed i mean store should be updated how i can do this...

if someone knows this please help me regarding this.....

thanks in advance....