Resumen de A dynamic snapshot of bud dormancy in peach
The general aim of this thesis is to study the dormancy process from a molecular point of view identifying mechanisms and targeting genes that control it. In order to do that we have focused on the study of three genes that are differentially expressed during reproductive bud development within the conceptual framework of the three major processes that converge spatially and temporally in a reproductive bud: dormancy, stress tolerance and flower development.
The first gene is down-regulated in dormancy release flower buds and encodes a STRESS ASSOCIATED PROTEIN (PpSAP1) that contains Zn-finger domains A20 and AN1. SAP proteins have been related to stress tolerance response in both plants and animals and in fact, we have shown that drought stress induces its expression in buds, resembling other SAP genes in plants. Moreover, the constitutive expression of PpSAP1 in plum increases its tolerance to water stress by increasing water retention. Likewise, transgenic plum plants show leaf alterations related to reduced cell size concomitant with the down-regulation of genes involved in cell growth. All these studies suggest a dual role of PpSAP1 in stress tolerance response and cell growth during peach dormancy.
The second gene is PpeS6PDH, coding for an enzyme with sorbitol-6-phosphate dehydrogenase activity. PpeS6PDH is differentially regulated during bud development, highly expressed in dormant buds consistently with sorbitol accumulation. Concomitantly with PpeS6PDH down-regulation in dormancy-released flower buds, chromatin around the translation start site of the gene shows changes in the methylation state of specific residues of histone H3 (H3K4 and H3K27). These data suggest the transcriptional regulation of PpeS6PDH expression by chromatin modification mechanisms. Moreover, abiotic stresses affect PpeS6PDH expression. Low temperature treatments induce gene expression in buds and leaves, whereas desiccation up-regulates PpeS6PDH in buds and represses the gene in leaves. These data suggest the participation of PpeS6PDH in tolerance against cold and water deficit stresses in buds.
Finally, the third gene is PpeDAM6, one of the major regulators of bud dormancy in peach. PpeDAM6 is sharply down-regulated during bud development concomitantly with dormancy release events. This repression is in part due to the direct binding of PpeBPC1, a BASIC PENTACYSTEINE PROTEIN, to the GAGA motifs present in an intronic regulatory region of PpeDAM6 gene that becomes enriched in H3K27me3 chromatin modification after dormancy release. In addition, the ectopic expression of PpeDAM6 in Arabidopsis shows abnormal flower phenotypes resembling 35S::SVP plants. On the other hand, overexpression in plum causes stunted growth in the transgenic lines due to an altered hormonal homeostasis. The changes in hormone content are mediated by the modulation of genes involved in jasmonic acid, cytokinins and gibberellic acid metabolism and signalling pathways. These results suggest that PpeDAM6 works as a master growth repressor maintaining dormancy, stress tolerance response and flowering inhibition by mainly modulating hormone homeostasis.
