FRUIT INHIBITS FLOWERING IN ALTERNATE BEARING CITRUS VARIETIES. HORMONAL, GENETIC AND EPIGENETIC REGULATION

In mature citrus trees, flowering is regulated by exogenous signals (i.e., low/high temperature, water shortage/water supply) that control flowering time, and by endogenous signals that control flower intensity. Actually, the ratio of endogenous promoters to inhibitors is considered as responsible for flowering; however, there is evidence suggesting that the inhibitors and their metabolism are the sole factors controlling flowering in Citrus. All the meristems have the information and ability to flower unless a negative factor hampers the process. The main endogenous factor controlling flower intensity is the fruit. Attending to the aforementioned, in this PhD. thesis, the following hypothesis was tested: Fruit inhibits flowering when ripening begins by exporting of hormones that induce epigenetic upregulation of flowering inhibitor genes in the leaves, and interfering in flower bud differentiation. The main findings are: 1. The increase in FLC gene expression in ON-tree leaves coincides with fruit color change and low temperatures in the flower induction period. CiFT2 is expressed in OFF-tree but not in ON-tree leaves. 2. The fruit produces and exports GAs and ABA to the leaves overlapping flower induction inhibition whereas these hormones decrease in OFF-tree leaves. GA3 treatment reduces CiFT2 gene expression but not FLC gene expression. 3. After bud sprouting, the inhibitory signals (FLC, TEM1, SVP) are not expressed in the new leaves. Every bud beside a fruit needs to restart vegetative growth to gain the flowering ability. 4. The fruit activates proteins from the stress responses and the oxidoreductase activity while in OFF-tree the primary metabolism and synthesis of starch is upregulated. 5. During the flower induction period, the fruit modifies the DNA-methylation profile of the FLC gene and increases the expression of methyltransferases that increase FLC gene expression. In defruited trees the process is reverted to the OFF-tree state.