i write a visualforce page with source code

<apex:page controller="MyController1">
<apex:form>
<apex:pageBlock >
<apex:pageBlockSection  id="search">
<apex:commandLink action="{!commandLinkAction}" value="Advance Search"  reRender="thePanel" id="theCommandLink"/>
<apex:outputPanel id="thePanelWrapper">
<apex:outputPanel id="thePanel" rendered="{! rend}" layout="block">My div</apex:outputPanel>
</apex:outputPanel>
</apex:pageBlockSection>
</apex:pageBlock>
</apex:form>
</apex:page>

and the MyController1 class is

public class MyController1{
public Boolean rend{get;set;}
public PageReference commandLinkAction(){
rend=true;
return null;
}

}

when i click on Advanced Search link nothing happens but i was expecting outputPanel with id "thePanel" should render.why it is not rendering please someone explain??