CBF Gene Expression in Peach Leaf and Bark Tissues Is Gated by a Circadian Clock, Poster Board #208

CBF transcription factors regulate a host of genes (CBF-regulon) that respond to low temperature and play a role in freezing tolerance. In peach, (Prunus persica) there are at least 4 CBF genes situated in tandem on scaffold 5 of the peach genome. This is in contrast to apple (Malus x domestica) where there are 5 complete CBF genes that are not in tandem and are distributed on different linkage groups. CBF gene expression, induced by low temperature, has been shown to be gated by a circadian clock in Arabidopsis (Fowler et al., 2005. Plant Physiol 137:961–968). In contrast to herbaceous plants, CBF gene expression patterns in woody plants are more complex.  The present study was conducted to determine if CBF gene expression in peach leaf and bark tissues was also influenced by a circadian clock. One-year-old ‘Loring’ peach trees grafted on ‘Bailey’ rootstocks were moved to a Conviron PGV36 growth chamber for 2 weeks with 12h day/12h night photoperiod. The light level during the day period was approximately 300 mmoles photons m^-2 sec^-1. A constant temperature of 25 °C was maintained.  After two weeks of entrainment, a subset of trees was exposed to 4 °C under  ≤ 100 mmoles photons m^-2 sec^-1 continuous light for up to 48 h. Low temperature exposure was initiated at a Zeitgeber Time (ZT) of either ZT 4 or ZT 16 h (ZT 0 = subjective dawn) and leaf and bark tissues were harvested at various time points during the 48 h exposure.  RNA was extracted from leaf and tissue samples and expression levels of 3 of the peach CBF genes were determined using RT-qPCR. Results indicated a distinct gating of CBF gene expression by a circadian clock for all 3 CBF genes and in both leaf and bark tissues.  The CBF expression in ZT 4 samples showed a clear induction after being placed at 4 °C with expression peaking in leaf samples at 6–12 hours depending on the specific CBF gene, while the CBF expression peaked at 12–24 h in bark tissues.  In contrast to ZT 4 samples, the CBF gene expression in ZT 16 samples was highly attenuated.  These results are in agreement with similar studies in Arabidopsis.  Analysis of expression patterns of two CBF-regulated genes (PpDhn1 and PpDhn3) in ZT 4 and ZT 16 samples is currently being conducted to determine the impact of CBF circadian gating on the expression CBF-regulon genes.