Profiling of phytohormones in apple fruit and buds regarding their role as potential regulators of flower bud formation

Abstract

Apple (Malus ×domestica Borkh.) cropping behavior, if not regulated, is often manifested by high yields of small-sized fruit in so called ON-years, which are usually followed by strongly reduced crop loads in OFF-years. Such cropping pattern is defined as biennial bearing and causes significant losses in apple production. The growth of apple fruit overlaps with the formation of flower buds, which remain dormant until the following spring. Earlier works proposed that some fruit-derived mobile compounds, as e.g., phytohormones, could suppress flower bud formation that thereby leads to biennial bearing. We addressed this hypothesis by analyzing 39 phytohormones in apple seeds, fruit flesh and by measuring phytohormone export from the fruits of the biennial bearing cultivar ‘Fuji’ and of the regular bearing cultivar ‘Gala’. Moreover, we analyzed the same compounds in bourse buds from fruiting (ON-trees) and non-fruiting (OFF-trees) spurs of both apple cultivars over the period of flower bud formation. Our results showed that apple fruit exported at least 14 phytohormones including indole-3-acetic acid and gibberellin A3, however, their influence on flower bud formation was inconclusive. A gibberellin-like compound, which was detected exclusively in bourse buds, was significantly more abundant in bourse buds from ON-trees compared to OFF-trees. Cultivar differences were marked by the accumulation of trans-zeatin-O-glucoside in bourse buds of ‘Gala’ ON-trees, whereas the levels of this compound in ‘Gala’ OFF were significantly lower and comparable to those in ‘Fuji’ ON- and OFF-trees. Particular phytohormones including five cytokinin forms as well as abscisic acid and its degradation products had higher levels in bourse buds from OFF-trees compared to ON-trees and were therefore proposed as potential promotors of flower bud initiation. The work discusses regulatory roles of phytohormones in flower bud formation in apple based on the novel and to date most comprehensive phytohormone profiles of apple fruit and buds.

Integrative Metabolomic and Transcriptomic Analysis Elucidates That the Mechanism of Phytohormones Regulates Floral Bud Development in Alfalfa

“…Plant hormones are the key regulatory factors during plant morphogenesis [ 16 , 17 ]. Spraying growth regulators on a plant is a conventional cultivation method to increase the crop yield during the flower bud developmental stage; therefore, exploring the accumulation of endogenous phytohormones in floral buds can provide a theoretical support for guiding the production and improving the seed yield.…”

“…The external environment and internal factors commonly influence the development of floral buds [ 13 , 14 , 15 ]. Endogenous phytohormones play an important role in regulating flower bud growth [ 16 , 17 ]. Various endogenous hormones, such as auxins (IAA), cytokinin (CK), abscisic acid (ABA), jasmonate (Ja), gibberellin (GA), and ethylene (ETH), coordinate with each other, thereby regulating flower bud development by jointly controlling the metabolism of plants.…”

Co-regulatory effects of hormone and mRNA–miRNA module on flower bud formation of Camellia oleifera

“…Previous studies on fruit trees had shown that fruit could affect flower bud formation (Munoz-Fambuena et al, 2011;Martinez-Alcantara et al, 2015). In the harvest year, the synthesis of gibberellin from seed embryos also increases significantly (Milyaev et al, 2022). Therefore, it could be inferred that this phenomenon would be more obvious in C. oleifera.…”

Zonal Chemical Signal Pathways Mediating Floral Induction in Apple

“…FI is when chemical signals are received by vegetative buds to induce floral meristem development. The exact time point of the FI phase is unclear; however, it is hypothesised to take place 60-70 days after full bloom (DAFB) in many apple cultivars, approximately 2 weeks prior to floral initiation [1,2] when the first structural changes occur in the bud. Floral initiation is visible microscopically via histological sectioning.…”

“…The tree would then enter an alternating cropping cycle, known as biennial bearing. Trees that bear high crop loads are often described as 'ON' trees and those with relatively lower yields are described as 'OFF' trees [1,2].…”