The majority of somatic mutations in fruit trees are layer-specific

Abstract

BackgroundAll plant tissues and organs develop from meristems. Plant meristems are structured organs consisting of distinct layers of stem cells. Somatic mutations occurring in one of these layers can propagate into large sectors of the plant. However, the frequency and characteristics of meristematic mutations that form the basis of somaclonal phenotypic variation remain unclear. ResultsHere, we analysed the frequency and distribution of somatic mutations in an individual Apricot tree. For this, we sequenced the genomes of fruit samples corresponding to distinct meristematic cell layers selected across the entire tree. Most somatic mutations (>90%) were specific to individual layers. Genotyping the somatic mutations in leaves sampled next to the fruits confirmed their meristematic origin. Interestingly, layer 1 (epidermis) had a higher mutation load than layer 2 (mesocarp), implying differential mutational dynamics between the layers. The somatic mutations followed the branching pattern of the tree. These factors led to the unexpected observation that the layer 1 samples from different branches were more similar to each other than to layer 2 samples of the same branch. Further, using single-cell RNA sequencing, we demonstrated that the layer-specific mutant alleles could only be found in the transcripts of the respective, layer-specific cell clusters and could form the basis for somaclonal phenotypic variation. ConclusionsHere, we analyzed the prevalence and distribution of somatic mutations with meristematic origin. Our insights into the yet unexplored layer-specificity of such somatic mutations outlined how they can be identified and how they impact the breeding of clonally propagated crops.