Controlling floral transition with Gibberellin A7 in peach (Prunus persica L. Batsch): a transcriptomic characterization of its effects

Peach (Prunus persica L. Batsch) is a temperate fruit that has great value, whether consumed fresh or processed. Due to the estimated increase in the world’s population, a variety of methods for increasing the yield in many of the crop species is inevitable while at the same time, due to the importance of fruit quality to consumers in today’s market, emphasizing the quality of products is equally important. In many fruit crops, thinning is an indispensable practice to achieve a desirable yield, both quality and quantity-wise, while preventing alternate bearing. This method is usually done by the mechanical removal of the excessive flower and/or fruitlets, or by the application of chemicals. In peach, thinning flowers lead to a controlled fruit load and were shown to be more effective than fruit thinning. The number of flowers can be controlled either at the time of bloom, through flower thinning methods such as mechanical ones, or at the time of floral induction/transition the year before. Flowering is a complex and hardly known procedure, made of different stages. Floral induction, the first step, is the production of the signals, which leads to the second stage, the beginning of the transition from the vegetative phase to the reproductive. Within this context, a new gibberellin A7-based formula, SevenGib®, has been recently registered in Italy, aimed at reducing the return bloom in peaches and nectarines. However, the molecular mechanism of this flowering inhibition is still poorly understood and further investigations are needed to implement the biological dossier of such an important plant bioregulator. Despite the very high rate of success of SevenGib® treatments (close to 87% in 38 trials carried out from 2019 to 2021), a failure occurred in the Springcrest cultivar application in 2022 (called the “old trial”), which resulted in no significant flowering inhibition with respect to the untreated trees. Despite, this failure an RNA-seq analysis on bud samples collected from this trial was carried out anyway, although resulting into a very low number of differentially expressed genes in the treated vs the untreated samples. These results were then compared with those coming from a “new trial” carried out similarly on cv Fercluse, in order to shed light on the probable causes of the failure. A confirmative multiple approach was carried out, utilizing several techniques such as qPCR, RNA-seq, bioinformatics with different types of analysis, and statistics. For the final effect of the treatment, return to bloom measurement in the following year after the treatment was also used. According to these comparative analyses, the experiment on the Fercluse cultivar was more aligned with the expected effects of the treatment by SevenGib®, based on the regulated genes and pathways. Interestingly, the control groups of the two experiments showed significant variations with more than 1,000 DEGs identified. Enrichment analysis of these changes suggested many pathways aligning ideally with the flowering procedure. The control group of the Springcrest cultivar demonstrated a downregulation in many of these procedures such as auxin signaling, flavonoid biosynthesis, and cell cycle, suggesting a probable natural inhibition in the flowering procedure of this cultivar by the excessive fruit load, which is also consistent with the return to bloom measurements and the temporary halt in manual thinning of trees due to the lack of manpower in the same year. As a general concluding remark, this study allowed us to point out for the first time the physiological changes associated with an inhibition of floral transition in peach caused by GA7 and to highlight the importance of fruit load in the natural inhibition of the same process through the regulation of the same gene functions triggered by the exogenous inhibition.