Influence of stem and bud auxin levels on bud release and flower meristem formation in Citrus

Abstract

Flowering is crucial for the productivity of fruit trees. In Citrus, the presence of fruit and the position of the bud on the shoot influence meristem fate in the following spring. However, the endogenous signals from the fruit or apical bud that prevent flower meristem formation remain unknown. Auxin, the main hormone synthesized by dominant organs, regulates plant architecture, but its role as the fruit signal that prevents flowering is unclear. This uncertainty arises because auxin modulates bud initiation, which coincides with floral meristem differentiation in Citrus. Our working hypothesis is that auxin synthesis in the meristem is necessary for initiating floral differentiation. Our experiments covered two dominance conditions, apical dominance and fruit-meristem dominance, and show that meristems unable to reactivate cell division (CYCB2), auxin synthesis (YUCCA4, TRN2), and transport (PIN3) fail to activate LEAFY (LFY) expression during floral differentiation. In the apical dominance model, although all leaves can express FLOWERING LOCUS T (CiFT3) relative to node position, high polar auxin transport from the most developed buds inhibits bud release in basal buds, indirectly affecting floral differentiation. Gibberellin (GA₁, GA₄, GA₂₀, GA₉) and cytokinin (IP) content in the stem and buds did not correlate bud release inhibition. In the fruit-meristem model, the fruit also induced strong auxin transport in the stem and inhibited bud release, but it is concluded that the fruit inhibition of flower induction requires an additional mechanism beyond auxin flux.