Flower bud dormancy in temperate fruit tree species, such as almond [Prunus dulcis (Mill.) D.A. Webb], is a survival mechanism that ensures that flowering will occur under suitable weather conditions for successful flower development, pollination and fruit set. Dormancy is divided into three sequential phases: paradormancy, endodormancy and ecodormancy. During the winter, buds need cultivar-specific chilling requirements (CRs) to overcome endodormancy and heat requirements to activate the machinery to flower in the ecodormancy phase. One of the main factors that enables the transition from endodormancy to ecodormancy is transcriptome reprogramming. In this work, we therefore monitored three almond cultivars with different CRs and flowering times by RNA sequencing during the endodormancy release of flower buds and validated the data by quantitative real-time PCR in two consecutive seasons. We were thus able to identify early and late flowering time candidate genes in endodormant and ecodormant almond flower buds associated with metabolic switches, transmembrane transport, cell wall remodeling, phytohormone signaling and pollen development. These candidate genes were indeed involved in the overcoming of the endodormancy in almond. This information may be used for the development of dormancy molecular markers, increasing the efficiency of temperate fruit tree breeding programs in a climate-change context.

DNA methylation and histone post-translational modifications have been described as epigenetic regulation mechanisms involved in developmental transitions in plants, including seasonal changes in fruit trees. In species like almond (Prunus dulcis (Mill.) D.A: Webb), prolonged exposure to cold temperatures is required for dormancy release and flowering. Aiming to identify genomic regions with differential methylation states in response to chill accumulation, we carried out Illumina reduced-representation genome sequencing on bisulfite-treated DNA from floral buds. To do this, we analyzed almond genotypes with different chilling requirements and flowering times both before and after dormancy release for two consecutive years. The study was performed using epi-Genotyping by Sequencing (epi-GBS). A total of 7317 fragments were sequenced and the samples compared. Out of these fragments, 677 were identified as differentially methylated between the almond genotypes. Mapping these fragments using the Prunus persica (L.) Batsch v.2 genome as reference provided information about coding regions linked to early and late flowering methylation markers. Additionally, the methylation state of ten gene-coding sequences was found to be linked to the dormancy release process.

Water scarcity is one of the greatest concerns for agronomy worldwide. In recent years, many water resources have been depleted due to multiple factors, especially mismanagement. Water resource shortages lead to cropland expansion, which likely influences climate change and affects global agriculture, especially horticultural crops. Fruit yield is the final aim in commercial orchards; however, drought can slow tree growth and/or decrease fruit yield and quality. It is therefore necessary to find approaches to solve this problem. The main objective of this review is to discuss the most recent horticultural, biochemical, and molecular strategies adopted to improve the response of temperate fruit crops to water stress. We also address the viability of cultivating fruit trees in dry areas and provide precise protection methods for planting fruit trees in arid lands. We review the main factors involved in planting fruit trees in dry areas, including plant material selection, regulated deficit irrigation (DI) strategies, rainwater harvesting (RWH), and anti-water stress materials. We also provide a detailed analysis of the molecular strategies developed to combat drought, such as Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) through gene overexpression or gene silencing. Finally, we look at the molecular mechanisms associated with the contribution of the microbiome to improving plant responses to drought.

For fruit tree (Prunus) species, flower bud dormancy completion determines the quality of bud break and the flowering time. In the present climate change and global warming context, the relationship between dormancy and flowering processes is a fundamental goal in molecular biology of these species. In almond [P. dulcis (Miller) Webb], flowering time is a trait of great interest in the development of new cultivars adapted to different climatic areas. Late flowering is related to a long dormancy period due to high chilling requirements of the cultivar. It is considered a quantitative and highly heritable character but a dominant gene (Late bloom, Lb) was also described. A major QTL (quantitative trait loci) in the linkage group (LG) 4 was associated with Lb, together with other three QTLs in LG1 and LG7. In addition, DAM (Dormancy-Associated MADS-Box) genes located in LG1 have been largely described as a gene family involved in bud dormancy in different Prunus species including peach [P. persica (L.) Batsch] and Japanese apricot (P. mume Sieb. et Zucc.). In this work, a DAM transcript was cloned and its expression was analysed by qPCR (quantitative Polymerase Chain Reaction) in almond flower buds during the dormancy release. For this purpose two almond cultivars (‘Desmayo Largueta’ and ‘Penta’) with different chilling requirements and flowering time were used, and the study was performed along two years. The complete coding sequence, designated PdDAM6 (Prunus dulcis DAM6), was subjected to a phylogenetic analysis with homologous sequences from other Prunus species. Finally, expression dynamics analysed by using qPCR showed a continuous decrease in transcript levels for both cultivars and years during the period analysed. Monitoring almond flower bud dormancy through DAM expression should be used to improve almond production in different climate conditions.

Simple sequence repeats (SSRs) are defined as sequence repeat units between 1 and 6 bp that occur in both coding and non-coding regions abundant in eukaryotic genomes, which may affect the expression of genes. In this study, expressed sequence tags (ESTs) of eight Prunus species were analyzed for in silico mining of EST-SSRs, protein annotation, and open reading frames (ORFs), and the identification of codon repetitions. A total of 316 SSRs were identified using MISA software. Dinucleotide SSR motifs (26.31 %) were found to be the most abundant type of repeats, followed by tri- (14.58 %), tetra- (0.53 %), and penta- (0.27 %) nucleotide motifs. An attempt was made to design primer pairs for 316 identified SSRs but these were successful for only 175 SSR sequences. The positions of SSRs with respect to ORFs were detected, and annotation of sequences containing SSRs was performed to assign function to each sequence. SSRs were also characterized (in terms of position in the reference genome and associated gene) using the two available Prunus reference genomes (mei and peach). Finally, 38 SSR markers were validated across peach, almond, plum, and apricot genotypes. This validation showed a higher transferability level of EST-SSR developed in P. mume (mei) in comparison with the rest of species analyzed. Findings will aid analysis of functionally important molecular markers and facilitate the analysis of genetic diversity.

Peach (Prunus persica (L.) Batsch) is one of the most produced and studied stone fruits. Many genetic and genomic resources are available for this species, including a high-quality genome. More recently, a new high-density Illumina peach Single Nucleotide Polymorphism (SNP) chip (9+9K) has been developed by an international consortium as an add-on to the previous 9K array. In the current study, this new array was used to study the genetic diversity and population structure of the National Peach Germplasm Collection of the Agrifood Research and Technology Centre of Aragon (CITA), located in Zaragoza (northern Spain). To accomplish this, 90 peach accessions were genotyped using the new peach SNP chip (9+9K). A total of 9796 SNPs were finally selected for genetic analyses. Through Identity-By-Descent (IBD) estimate analysis, 15 different groups with genetically identical individuals were identified. The genetic diversity and population structure elucidated a possible exchange of germplasm material among regions, mainly in the northern regions of Spain. This study will allow for more efficient management of the National Peach Germplasm Collection by classifying valuable individuals for genetic diversity preservation and will benefit forthcoming Genome-Wide Association Studies (GWAS) of commercially important fruit traits in peach.