Exploring the role of tryptophan in biennial bearing : A mobile molecule that is exported from growing apples but does not suppress flowering

<em>Background of the study</em> – <strong>Apple (<em>Malus × domestica</em> Borkh.) production faces significant challenges, with biennial (or alternate) bearing being a major obstacle to stable yields. Natural high crop load (ON-year) considerably reduces return bloom in the following year resulting in low yields (OFF-year) that significantly diminish the profitability of apple production. Previous studies suggested that growing apples may export some unknown signaling molecule(s) to the adjacent bourse buds and thereby inhibit flower bud formation. </strong><em>Objectives</em> – <strong>This study addresses the role of tryptophan as a potential signal that triggers biennial bearing. Previously, we found that bourse buds of high-cropping 'Fuji' and 'Gala' trees contain more tryptophan compared to bourse buds of the trees with no crop load. This led to the hypothesis that tryptophan is exported from the growing apples and accumulates in the adjacent bourse buds where it potentially inhibits flower initiation, or further metabolizes to melatonin, which may act as an inhibitor of flower bud formation.</strong> <em>Methods</em> – <strong>We tested our hypothesis by measuring the export of tryptophan from 'Fuji' and 'Gala' apples. Besides, we carried out foliar applications with tryptophan on 'Nicoter' and 'Gala' trees. In addition, we checked whether tryptophan or melatonin affect the continuous-flowering habit of RNAi transgenic apple plantlets with silenced expression of <em>MdTFL1</em> cultivated <em>in vitro</em>. </strong><em>Results</em> – <strong>Our data shows that growing apple fruits of 'Fuji' and 'Gala' export tryptophan. However, foliar applications with this compound did not affect return bloom in 'Nicoter' or 'Gala'. Finally, neither tryptophan nor melatonin suppressed flowering of <em>MdTFL1-1</em> RNAi transgenic apple plantlets <em>in vitro</em>. </strong><em>Conclusions</em> – <strong>Although the transport of tryptophan from apple fruits to adjacent bourse buds cannot be excluded, we reject the hypothesis on an inhibitory effect of tryptophan and its downstream metabolic product melatonin on flower bud formation in apple.</strong>