Ethylene modulates root hair development in trifoliate orange through auxin-signaling pathway

Root hair, tubular outgrowths from root-specific epidermis, is strongly affected by plant growth regulators, such as ethylene and auxin, while the interaction between them is more attractive. A sand culture study was performed to analyze the mechanisms of exogenous ethylene (ETH, ethephon) and silver nitrate (Ag, ethylene effect inhibitor) interaction with auxin on root hair development of trifoliate orange (Poncirus trifoliata), a widely used rootstock for citrus production. The results showed that ETH up-regulated the genes of ethylene synthesis, receptor and signal transduction that leading to increased root hair density and length while Ag presented the opposite effects. Root hair initiation was attributed to epidermal cell development as well as the proportion of epidermal cell bulge to root hair when treated by ETH and Ag. Furthermore, ETH up-regulated the expression of auxin biosynthesis and transport genes, thus, increasing the endogenous auxin content in the root hair zone and subsequently enhancing root hair growth dramatically. However, application of Ag represented opposite effect, namely an inhibited effect on relevant genes expression. These results indicated that ethylene stimulated auxin biosynthesis and transport toward the root hair zone, where it achieved a local activation of the auxin signaling pathway leading to promotion of root hair development.