<?php

/**
 * Implements hook_form_ID_alter().
 */
function content_taxonomy_form_field_ui_field_edit_form_alter(&$form, &$form_state, $form_id) {
  $field = $form['#field'];
  if ($field['type'] == 'taxonomy_term_reference') {
    // add parent form.
    foreach ($field['settings']['allowed_values'] as $delta => $tree) {
      $options[0] = '---';
      // todo this might break with huge vocs
      $voc = taxonomy_vocabulary_machine_name_load($tree['vocabulary']);
      foreach (taxonomy_get_tree($voc->vid) as $term) {
        $options[$term->tid] = str_repeat('- ', $term->depth) . $term->name;
      }
      
      $form['field']['settings']['allowed_values'][$delta]['parent']  = array(
        '#type' => 'select',
        '#title' => t('Parent'),
        '#options' => $options,
        '#default_value' => isset($tree['parent']) ? $tree['parent'] : 3,
      );
    } 
  }
}