You can use the term reference tree module, this gives you a taxonomy term theme function. Create a subtheme of seven and override the theme function in template.php. In the example below I am also using the tipsy module, which converts the .tipsy class into a tooltip, so the taxonomy term description appears after clicking or hovering on the tooltip on the node add form, but you could of course also just print the $description.
/**
* This function prints a single item in the tree, followed by that item's children
* (which may be another checkbox_tree_level).
*/
function seven_subtheme_checkbox_tree_item($variables) {
$element = $variables['element'];
$children = element_children($element);
$output = "";
$sm = $element['#level_start_minimized'] ? ' term-reference-tree-collapsed' : '';
if (is_array($children) && count($children) > 1) {
$output .= "<div class='term-reference-tree-button$sm'></div>";
}
elseif (!$element['#leaves_only']) {
$output .= "<div class='no-term-reference-tree-button'></div>";
}
foreach ($children as $child) {
// $child is the tid
$term = taxonomy_term_load($child);
$description = trim(strip_tags($term->description));
if (!empty($description)) {
$tooltip = l('ⓘ', '#', array(
'attributes' => array(
'class' => array('tipsy'),
'title' => $description,
)
));
}
$output .= drupal_render($element[$child]) . $tooltip;
}
return $output;
}