Gene level responses of shortleaf pine and loblolly pine to top removal

Young shortleaf pine sprouts prolifically after death of the stem due to top removal or fire damage. Considerable interest has been shown in shortleaf pine restoration due to continually declining native populations. Shortleaf pine's strong sprouting ability has tremendous potential in promoting its regeneration. However, little is known about its sprouting mechanisms at the molecular level. We designed a microarray experiment to study genes responsible for sprouting ability. In this study, 1-year-old shortleaf pine and loblolly pine seedlings' tops were removed mechanically and the remaining stem tissues collected just before sprouting. Shortleaf pine showed an extraordinarily strong sprouting ability. Large numbers of sprouts were seen within 2 days after top removal. However, loblolly pine developed only a few sprouts about 1 week following top removal. Using microarray gene profiling with about 2,400 cDNA clones obtained from suppression subtractive hybridization, 139 differentially expressed genes were found to be associated with the sprouting response, including genes functioning in reserve (carbohydrates, protein, and fatty acid) mobilization, transcriptional regulation, stress response, plant development, signal transduction, and hormone regulation. Shortleaf pine responds actively to top removal at the molecular level, while loblolly pine apparently does not. As reported for dormancy release of buds of other perennial plants, oxidative stress might be the major cause of dormancy release of axillary buds of shortleaf pine. Apparent cross talk between plant hormones (gibberellins, auxins, and ethylene), carbohydrates, and other players of signal transduction leads to the cooperative promotion of sprouting of shortleaf pine after top removal.