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Ethylene plays an essential role in the ripening of peach, a climacteric stone fruit. However, several studies have shown that other phytohormones may alter fruit organoleptic properties, nutritional value, and overall quality. Although cytokinin levels are highest during fruit set and early stages of fruit development, exogenous cytokinin application on the late stages of peach fruit development (post-lignification stage) alters the peach fruit ripening indicators and delays ripening. To better understand the molecular mechanisms by which cytokinin alters fruit ripening and fruit quality traits, we identified a conserved set of 26 peach cytokinin-regulated orthologs of the Arabidopsis cytokinin-regulated genes. All but two of these genes show expression in ripe peach fruits. The peach cytokinin-regulated genes map throughout the Prunus persica genome, located on all eight chromosome-scale pseudomolecules. However, eight of these orthologs are located on pseudomolecule 1, while five orthologs are on pseudomolecule 2. Gene Ontology enrichment analyses revealed that many of these genes are associated with metabolic processes. The region upstream of the transcription start site contained Cytokinin Response Elements (CKREs) in 77% (20/26) of these putative cytokinin-regulated genes. The cytokinin responsiveness of eight of these genes (PpeBPH1, PpeCKX4, PpeCPK28, PpeCYP18-2, PpeHSP90-7, PpeNRP2, PpeNIP1-1, and PpePCRP1) was confirmed by RT-qPCR analyses of ripe peach fruits treated exogenously with trans-zeatin for one hour. Therefore, by using a comparative genomic analysis between peach and Arabidopsis, we have identified a conserved set of cytokinin-regulated genes expressed in peach fruits that respond to exogenous application of trans-zeatin and, based upon the conserved cis-regulatory regions, are most likely regulated by the cytokinin Type-B Response Regulators.

Thinning is an agronomic practice that consists in the removal of a certain number of fruits, thereby altering the source–sink equilibrium in the tree and favoring carbon allocation to the remaining fruits. If thinning is performed at the appropriate fruit developmental stage and at an adequate intensity, the remaining fruits will have a larger final diameter and higher soluble solids content. However, the amount of carbon translocated to a fruit depends on the photosynthetic rate in the source leaves, the production of “translocation sugars”, as well as the fruit sink strength, which is directly related to the phloem unloading. It is not clear which of these steps is altered by thinning. In this work, we evaluated the effect of thinning in an early season peach variety. Photosynthetic parameters, quantification of soluble sugars in leaves and fruits, and the expression of sucrose and sorbitol transporters were evaluated in leaves and fruits during whole fruit development. We showed that thinning was effective in improving final fruit size and sugar accumulation without inhibiting photosynthesis. We also observed a 10-day delay in stone lignification when the trees were not thinned. The stone lignification phase was very short in the thinned treatment, while it seems to last for longer under unthinning. Furthermore, it was shown that thinning affects the expression of translocation sugar transporters genes (PpeSUT1 and PpeSUT4) in both leaves and fruits, which could be increasing the fruit sink strength and subsequently improving translocation sugars transport to the fruit when trees are thinned.