Mango mesocarp fibers are a continuum of fibrous endocarp structures that are co-lignified during seed coat hardening.

Mango fruit is a drupe containing a single seed enclosed in a hard, fibrous endocarp surrounded by an edible fleshy mesocarp which contains significant fibrous structures. We performed morphological, histological and gene expression analyses of mango fibers during fruit maturation and endocarp hardening of "fibrous" and "non-fibrous" accessions, to characterize the developmental process and explore the molecular mechanism of fiber formation and hardening. All studied accessions contain fibers, but they differ in their length and rigidity. Mesocarp fibers develop as braid-like fibrous structures of the endocarp, early in fruit development and lignified in parallel to the hardening of the endocarp fibers during fruit maturation and seed hardening. Polyphenol and lignin biosynthesis pathways and secondary cell wall deposition are the major processes involved in fiber hardening in both endocarp and mesocarp fruit tissues. Mango mesocarp fibers are a structural continuum of the fibrous endocarp. Lignification plays a major role in fibrousness. The resemblance to fibrous mesocarp tissues of other drupe species suggests a common mechanism with both evolutionary and horticultural importance. The results offer new insight into the evolution of fruit structure in mango and may provide a path to breed "freestone" varieties with reduced fibrousness.