Regulation of coffee flowering time: hormonal crosstalk and transcriptomic analysis of reproductive development and anthesis

Abstract

Flowering is one of the most important steps during the plant´s life cycle; a synchronized interaction of endogenous and environmental signals triggers this process at the right time. In coffee (Coffea arabica L.), coffee flower development is asynchronous resulting in an uneven fruit ripening. Understanding the mechanism by which coffee flowering is regulated may help to adopt some agronomical practices to synchronize coffee production and therefore, to give better final product quality. The objective of this research was to investigate the influence of plant hormones and gene regulation in the control of coffee flowering. In this sense, two experiments were developed. In the first experiment, we evaluated the content of ethylene, Abscisic Acid (ABA), 1-aminocyclopropane-1-carboxylic acid oxidase (ACC), 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) activity, and Lysine Histidine Transporter 1 (LHT1) gene expression, as the ACC transporter, in a different key point of the coffee flower bud development in the Acauã (Late), Oeiras (Early) and Semperflorens (Continuous) coffee genotypes. Moreover, ACC exogenous application compared with 1-methilcyclopropene (1-MCP) and water treatments was supplied in coffee plants. ABA and ethylene content had an inverse profile in the rainy and dry period and, after rain before anthesis, the ACC content increased in all coffee tissues (Leaves, roots, and flower buds). A high relative LHT1 gene expression was observed at the same period for leaves and roots. At this point, increased content of ACC was determined as a modulator of coffee anthesis. In an additional experiment, this activity was corroborated when coffee plants with 300 mM of exogenous ACC supply showed a higher number of G6 flower bud stages. In the second experiment, a transcriptomic analysis (RNA-seq) from leaves, flower buds, and Shoot Apical Meristem (SAM) in the same coffee genotypes of the first experiment was carried out in March and August. In addition, phenotypic (Flower bud development and flowering events) and biochemical analysis (Sugar content) were carried out. Starch content was higher for Semperflorens than in Oeiras and Acauâ, and sucrose content increased from June to August before anthesis in Acauã. Ethylene decreased in leaves from the rainy to dry period. Semperflorens presented more flowering events than Acauã and Oeiras that flowered more than Acauã. A total of 12748 Differentially Expressed Genes (DEGs) were found contrasting different tissues and sampling conditions, associated with hormonal regulation, sugar metabolism, photosynthesis, circadian clock, temperature, endogenous and external stimulus, seed development, and flower development. 280 DEGs for sugar metabolism, 650 DEGs for ethylene biosynthesis and signaling, and 523 DEGs for floral organ identity and photoperiod were selected. Finally, 4 DEGs for sugar metabolism and 6 DEGs for photoperiod and flower development were identified to be congruent with the flowering pattern of coffee genotypes. 28 DEGs were identified for ethylene biosynthesis however, no correlation with ethylene evolution in coffee tissues associated with flowering was found.