JAXB: skip parent tag name

Question

As a summary: I need to place some fields directly into the root tag, skipping the tag of their direct parent.

Detailed description:

I have these classes:

@XmlRootElement
class Root{
    protected Group1 group1;
    protected Group2 group2;
}

@XmlRootElement
class Group1 {
    protected String field1;
    protected String field2;
    ...
    protected String field99;
}

@XmlRootElement
class Group2 {
    protected String field101;
    ...
    protected String field199;
}

Default JAXB marshalling will produce this XML:

<root>
   <group1>
     <field1></field1>
     <field2></field2>
     ...
     <field99></field99>
   </group1>

   <group2>
     <field100></field100>
     ...
     <field199></field199>
   </group2>
</root>

I need the output to skip the parent tags (group1 and group2), and to incorporate the fields directly into the root element:

<root>
     <field1></field1>
     <field2></field2>
     ...
     <field99></field99>
     <field100></field100>
     ...
     <field199></field199>
</root>