Causes loosening and extension of plant cell walls by disrupting non-covalent bonding between cellulose microfibrils and matrix glucans. No enzymatic activity has been found. Plays a major role in control of leaf growth and influences the mechanical breakage behavior of the pedicel. A number of isoforms are produced. According to EST sequences. Most highly expressed in the young leaf petiole and midrib, in trichomes and at the base of the pedicel. Expression is first seen at the base of the emerging first two true leaves but not of the cotyledons. As leaf development progresses expression begins in the base of the petiole and gradually extends toward the whole midrib and later it is restricted to the vasculature of the petiole and leaf blade and disappears as the leaf matures. Belongs to the expansin family. Expansin A subfamily.