Auxin treatment reduces inflorescences number and delays bud development in the alternate bearing Citrus cultivar Murcott mandarin

Specific cultivars of many commercial fruit trees undergo cycles of heavy fruit load (ON-crop) one year, followed by low fruit load (OFF-crop) the next (termed alternate bearing). Fruit load may affect flowering at various developmental stages, and its presence is suggested to generate a flowering-inhibitory signal. In a previous report, we showed that the presence of fruit induces polar auxin transport from the fruit into the stem, interfering with indole acetic acid release from the bud and thus elevating its levels in the bud meristem. To better understand the relationship between auxin homeostasis in the bud and flowering, indole acetic acid or 2,4-dichlorophenoxyacetic acid (2,4-D) was applied with the polar auxin transport blocker 2,3,5-triiodobenzoic acid to OFF-crop 'Murcott' mandarin (Citrus reticulata × Citrus sinensis) trees during the flowering-induction period. The treatment reduced inflorescence number and delayed bud development. Transcriptome analysis following the treatment revealed a reduction in the expression of a few flowering-control genes, including LEAFY and SQUAMOSA PROMOTER BINDING PROTEIN-LIKE. In addition, genes related to carbohydrate metabolism were reduced. We suggest that the elevation of auxin levels in the bud by heavy fruit load directly affects the expression of flowering-control, flower-development and developmental genes.