Auxin modulates root-hair growth through its signaling pathway in citrus

Root-hairs are tip-growing extensions from the root epidermis, and can be affected by phytohormones, such as auxins. A sand culture study was performed to analyze the effects and mechanisms of 3-indolebutyric acid (IBA) and 2-naphthoxyacetic acid (2-NOA, auxin inhibitor) on root-hair growth in trifoliate orange, a widely used rootstock for citrus. The results showed that IBA increased root-hair growth while 2-NOA had the inhibitory effect. A positive correlation was observed between the endogenous auxin level and root-hair number in the root-hair zone. The IBA treatment increased the endogenous IAA content in the root-hair zone of lateral roots by 22.8% while 2-NOA reduced it by 21.4%, compared with control group. Interestingly, 2-NOA increased the endogenous IAA level compared with IBA and control groups in the root tip zone, while it had no distinct difference from the control group in the stele zone of lateral roots. Thus, 2-NOA blocked IAA fluxing from root tips to root-hair zones. Furthermore, IBA up-regulated expression of auxin biosynthesis and transport genes, thus increasing the endogenous IAA level in the root-hair zone which may promote root-hair growth. However, application of 2-NOA had the opposite effect. These results indicated that IBA and 2-NOA regulated auxin biosynthesis and transport toward the root-hair zone, where it achieved a local alteration of the auxin signaling pathway influencing root-hair growth.