Required for neoxanthin biosynthesis, an intermediary step in abscisic acid (ABA) biosynthesis. Involved in an ABA pathway that acts at or downstream of ethylene receptors and positively regulates root ethylene response. In coleoptiles the MHZ4-dependent ABA pathway acts at or upstream of EIN2 to negatively regulate coleoptile ethylene response. In etiolated seedlings, expressed in roots, coleoptiles and vascular tissues of roots. In root apexes, expressed in quiescent center (QC) and root caps. In field-grown plants, expressed in leaf blades, young stem nodes and the base of axillary buds and adventitious roots derived from the nodes. In reproductive organs, expressed in anthers and pistil of young flowers, lemma of mature flowers and parts of developing grains. Induced by ethylene in roots. Mutant etiolated seedlings display reduced ethylene-response in roots and enhanced ethylene-response in coleoptiles. Mature mutant plants have increased shoot length, branches and adventitious roots at high node position on the shoot and impaired grain filling. 'Mao huzi' means cat whiskers in Chinese.