Abstract Many cultivars of almond ( Prunus dulcis Mill. D. A. Webb) suffer from a disorder known as non-infectious bud failure, in which preformed meristematic tissue becomes necrotic during dormancy, then fails to emerge in the following growing season. While the cause of the disorder is unknown, previous research has shown possible connections between DNA methylation and symptoms of the disorder. Additionally, non-infectious bud failure has been mitigated through propagation from basal epicormic buds, which show reduced non-infectious bud failure in clonal progeny relative to the axillary canopy buds typically used for propagation. In this study, we explored variation in DNA methylation between epicormic and axillary shoots that may provide insight into the spread and origins of non-infectious bud failure. We hypothesized that shoots derived from different tissues (epicormic and axillary buds) and growing environments would have distinct genome-wide patterns of DNA methylation. We sampled leaf tissue from epicormic and axillary canopy shoots from two almond varieties at four locations, then used whole-genome enzymatic methyl-sequencing to determine global methylation levels and differentially methylated regions between tissues. We found no significant differences in genome-wide methylation levels between different shoot types, locations and varieties. Epicormic and axillary shoots displayed 23 differentially methylated regions in one variety and 45 in the other, with no overlap in these regions between varieties. In addition, we identified 4546 regions that appeared differentially methylated between varieties; however, DNA polymorphism may account for some of these differences. In pairwise comparisons between locations, we identified 312 regions, with overlapping regions between some, but not all, locations. The remarkable stability of DNA methylation patterns across tissues and locations motivates further investigation into possible perturbations of these patterns by non-infectious bud failure and related genetic disorders in perennial plants. Significance of the Study What is known about this subject? Non-infectious bud failure (NBF) of almond is an economically significant production problem of unknown cause. Epicormic propagation has been shown to reduce NBF, and previous studies demonstrated a link between DNA methylation and NBF. What are the new findings? Global levels of DNA methylation are similar across tissues, locations and varieties. We identified a small number of differentially methylated regions between epicormic and axillary shoots in each of the two varieties tested, but there was no overlap in these regions between varieties. What is the expected impact on horticulture? There is still no clear path towards developing a bioassay for NBF in almond. However, genotype-specific differentially methylated sites between epicormic and canopy tissue may be used by propagation specialists to ensure that their propagation material is from epicormic buds, thereby reducing the potential spread of non-infectious bud failure.
Hazzard et al. (Thu,) studied this question.