Auxin polar transport as a signaling mechanism for fruit presence in fruit trees

Many commercial fruit trees cultivars undergo cycles of heavy fruit load (ON-Crop) one year, followed by low fruit load (OFF-Crop) the following year (alternate bearing, AB). Fruit load might affect flowering at various developmental stages, such as, flowering induction, and subsequent stages of flower development and bud break. It is commonly accepted that the fruit or an organ-sensing fruit presence generates an inhibitory signal that moves into the bud and inhibits flowering. Indeed, partial or complete fruit removal induces back flowering. Previous comparative transcriptomic analysis of buds isolated at various fruit load states identified the induction of Ca²⁺-dependent and -independent polar auxin transport (PAT) in OFF-Crop buds and following de-fruiting. Indeed, IAA levels within the bud were lower following fruit removal, and in OFF-Crop buds during flowering induction period, as compared to ON-Crop buds. Using radiolabeled IAA both in citrus and in olive, demonstrated that regardless of fruit position to the bud, apical or basal, its presence generates relatively strong PAT in the stem, which inhibits hormone release from the bud. This suggests that stem PAT and IAA levels within the bud are associated with the perception of flowering-inducing signal by fruit load. Here we demonstrate that external application of auxin to OFF-Crop trees during flowering induction period, resulted in reduced numbers of flowers, and inhibition of bud break. Transcriptomic analysis identified processes which were altered upon auxin treatment, including alternation in the expression of a few flowering control genes. These results are discussed in relation to possible inhibition of flowering induction by auxin.