Mycorrhiza-induced change in root hair growth is associated with IAA accumulation and expression of EXPs in trifoliate orange under two P levels

Root hair growth can be regulated by substrate nutrient levels, phytohormones, and arbuscular mycorrhizal (AM) fungi. However, the information about effects of AM fungi and substrate phosphorus (P) levels on root hair features is scarce. This study evaluated the effect of an AM fungus, Funneliformis mosseae, on root hair morphology, root indole-3-acetic acid (IAA) level, indole-3-acetic acid oxidase (IAAO) activity, and expressions of auxin-related genes and expansins (EXPs) in trifoliate orange (Poncirus trifoliata) seedlings under 0.1 mM (P0.1) and 1.0 mM (P1) P levels for 16 weeks. AM seedlings showed better root colonization under P0.1 than under P1. Under P0.1 and P1, mycorrhizal seedlings recorded higher concentrations of P and IAA and lower IAAO activity in roots than non-mycorrhizal seedlings. Root hair density was significantly higher in AM seedlings under P0.1 but dramatically lower under P1 than non-AM seedlings. Root hair length and diameter of AM seedlings were significantly increased under P1 only, relative to non-AM seedlings. Indole-3-pyruvate monooxygenase (PtYUC2) and auxin transporter-like protein 3 (PtLAX3) expressions in roots were significantly up-regulated by mycorrhizal treatment under both P0.1 and P1. The transcript levels of root auxin efflux carrier component 2 and 8 (PtPIN2 and PtPIN8) were dramatically down-regulated by mycorrhizal inoculation, irrespective of P levels. Root PtEXPB2, PtEXPA2, and PtEXPA4 expressions were notably up-regulated by mycorrhiza under P0.1 but down-regulated under P1. These results indicated that mycorrhiza-induced changes in root hair growth is closely related with up-regulated expressions of EXPs and simultaneous IAA accumulation in roots.