Integrative characterization of physiological and molecular regulation of hop (Humulus lupulus l.) development: photoperiod and hormonal signaling

Abstract

Hop cultivation has expanded into tropical and subtropical regions due to its high added value and importance to the industry, particularly the brewing industry. However, large-scale production in these regions remains challenging, mainly because premature flowering limits the vegetative growth required to achieve high yields. Competitive production is estimated to require approximately 3 kg of female inflorescences, known as strobiles or cones, per plant, which develop mainly on reproductive lateral shoots. Under short-day conditions (<16 h of light), these shoots fail to develop, and plants transition prematurely to flowering. Extending the photoperiod to approximately 17 h delays flowering and maintains vegetative growth. However, the need for continuous artificial light supplementation over several months represents an environmental and socioeconomic limitation, particularly in rural regions affected by unreliable electricity supply, while the use of fossil fuels is not a sustainable alternative. The ability of hop plants to maintain vegetative growth under extended photoperiods suggests the existence of endogenous mechanisms capable of integrating environmental signals and regulating the reproductive transition. In this context, this thesis aimed to provide an integrative characterization of the physiological and molecular mechanisms involved in the response of hop to photoperiod extension. In the first study, we demonstrated that exogenous GA₃ application promotes lateral shoot development and delays flowering under short-day conditions. In the second study, we demonstrated that photoperiod extension alters the expression of genes involved in gibberellin biosynthesis and signaling, and that these responses are modulated by nocturnal temperature, revealing an interaction between photoperiodic and thermal signals. In the third study, we characterized a family of transcription factors with expression patterns associated with different tissues and developmental stages, whose members have been implicated in the regulation of plant architecture in other species. Together, these results contribute to understanding how hormonal regulation, gene expression, and plant architecture participate in maintaining vegetative growth and delaying flowering. Therefore, these findings provide a basis for breeding programs aimed at developing cultivars that are less dependent on artificial light supplementation, potentially enabling more efficient and sustainable hop production in tropical and subtropical regions.