Epigenetic modifications can yield information about connections between genotype, phenotype variation and environmental conditions. Bud dormancy release in temperate perennial fruit trees depends on internal and environmental signals such as cold accumulation and photoperiod. Previous investigations have noted the participation of epigenetic mechanisms in the control of this physiological process. We examined whether epigenetic modifications were modulated in MADS-box genes, potential candidates for the regulation of bud dormancy and flowering in sweet cherry (Prunus avium L.). We identified and cloned two MADS-box genes homologous to the already-characterized dormancy regulators DORMANCY-ASSOCIATED MADS-box (DAM3 and DAM5) from Prunus persica (L.) Batsch. Bisulfite sequencing of the identified genes (PavMADS1 and PavMADS2), Methylated DNA Immunoprecipitation and small RNA deep sequencing were performed to analyze the presence of DNA methylations that could be guided by non-coding RNAs in the floral buds exposed to differential chilling hours. The results obtained reveal an increase in the level of DNA methylation and abundance of matching small interference RNAs (siRNAs) in the promoter of PavMADS1 when the chilling requirement is complete. For the first intron and 5′ UTR of PavMADS1, de novo DNA methylation could be associated with the increase in the abundance of 24-nt siRNA matching the promoter area. Also, in the second large intron of PavMADS1, maintenance DNA methylation in all cytosine contexts is associated with the presence of homologous siRNAs in that zone. For PavMADS2, only maintenance methylation was present in the CG context, and no matching siRNAs were detected. Silencing of PavMADS1 and PavMADS2 coincided with an increase in Flowering Locus T expression during dormancy. In conclusion, DNA methylations and siRNAs appear to be involved in the silencing of PavMADS1 during cold accumulation and dormancy release in sweet cherry.

Genetic engineering based on Agrobacterium-mediated transformation has been a desirable tool to manipulate single or multiple genes of existing genotypes of woody fruit crops, for which conventional breeding is a difficult and lengthy process due to heterozygosity, sexual incompatibility, juvenility, or a lack of natural sources. To date, successful transformation has been reported for many fruit crops. We review the major progress in genetic transformation of these fruit crops made in the past 5 years, emphasizing reproducible transformation protocols as well as the strategies that have been tested in fruit crops. While direct transformation of scion cultivars was mostly used for fruit quality improvement, biotic and abiotic tolerance, and functional gene analysis, transgrafting on genetically modified (GM) rootstocks showed a potential to produce non-GM fruit products. More recently, genome editing technology has demonstrated a potential for gene(s) manipulation of several fruit crops. However, substantial efforts are still needed to produce plants from gene-edited cells, for which tremendous challenge remains in the context of either cell’s recalcitrance to regeneration or inefficient gene-editing due to their polyploidy. We propose that effective transient transformation and efficient regeneration are the key for future utilization of genome editing technologies for improvement of fruit crops.

Recently, the plant hormone abscisic acid has been involved as key player in the regulation of endodormancy in grapevine buds (Vitis vinifera L). In this study, we show that in the vine, the expression of genes related to the biosynthesis of ABA (VvNCED1; VvNCED2) and the content of ABA are significantly higher at the latent bud than at the shoot-apex, while the expression of ABA catabolic gene (VvA8H3) showed no significant differences between both organs. A negative correlation between the content of ABA and transcript levels of the cell cycle genes (CCG) was found in both tissues. This result, suggested that ABA may regulate negatively the expression of the CCG in meristematic tissues of grapevines. To test this proposition, the effect of ABA on the expression of the CCG was analyzed in two meristematic tissues of the vine, somatic embryos and shoot-apex. The results indicated that cell-cycle progression is repressed by ABA in both organs, since it down-regulated the expression of genes encoding for cyclin-dependent kinases (VvCDKB1, VvCDKB2), for genes encoding cyclins type A (VvCYCA1, VvCYCA2, VvCYCA3), B (VvCYCB) and D (VvCYCD3.2a), and up-regulated the expression of VvICK5, a gene encoding for an inhibitor of CDKs. During the endodormancy the content of ABA increased and the expression of the CCG decreased. Moreover, the dormancy-breaking compound hydrogen cyanamide (HC) reduced the content of ABA and up-regulated the expression of the CCG. Taken together, these results suggest that ABA by repressing the transcription of the CCG may modulate the development of endodormancy in grapevine buds.