Setting comment text as required

I want to set also text comment as required… but in the backend only email and author are possible.
Without using a javascript validation, how it is possible?
Thank you

You can hook into 'pre_comment_on_post' and inspect $_POST['comment']:

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: T5 Require comment text
 * Description: Dies if there is no comment text.
 * Plugin URI:  http://wordpress.stackexchange.com/q/55679/73
 * Version:     2012.07
 * Author:      Thomas Scholz
 * Author URI:  http://toscho.de
 * License:     MIT
 * License URI: http://www.opensource.org/licenses/mit-license.php
 */

if ( ! function_exists( 't5_require_comment_text' ) )
{
    add_action( 'pre_comment_on_post', 't5_require_comment_text' );

    function t5_require_comment_text()
    {
        if ( ! isset ( $_POST['comment'] ) or '' === trim( $_POST['comment'] ) )
        {
            print 'Please hit your back button and write something useful.';
            exit;
        }
    }
}

See wp-comments-post.php for more actions and a way to create a useful back link. I didn’t include one because some browsers restore all filled form fields when the user clicks the back button. A link would prevent this rather useful behavior.