Cell layer-specific cell wall modification is associated with exo-mesocarp split in pistachio ( <i>Pistacia vera</i> L.)

Pistachio (Pistacia vera) is a drought- and salinity-tolerant perennial whose fruit features a fleshy exo-mesocarp, or 'hull', that protects the kernel. Hull development and degradation are key to kernel quality, yet the anatomy and mechanisms driving hull breakdown during late-stage development remain largely unknown. Here, we show that the hull contains anatomically distinct layers of hypodermal parenchyma and filler parenchyma. Using a combination of transcriptome analyses and immunohistochemistry, we show that changes in pectin-associated gene expression and modification of this polysaccharide are involved in hull cell size increase, loss of cell-cell adhesion, and hull softening. Anatomical analysis shows that filler parenchyma expands during late-stage hull development while hypodermal parenchyma remains constant in size. Field data suggest that irrigation and humidity affect pistachio hull split, implicating a role for water status in cell expansion. In summary, the complex interplay between molecular, cellular, and environmental changes suggests that cell layer-specific modifications of the cell wall are linked to exo-mesocarp splitting, forming a model for understanding the mechanism of fruit split during ripening in non-berry fruit crops.