| Tubular-shaped outgrowths from root epidermal cells,root hairs not only vastly increase surface area of root to help water and nutrients acquisition,but also help anchorage and in plant-edaphon interactions.Citrus are generally believed to possess few or even no root hair in citrus orchards.This paper compared the development of root hairs of Poncirus trifoliata in sand culture,and studied the growth-cycle,distribution,growth stages and development ways.The paper also studied the impacts and function mechanisms of auxin and ethylene on the development of root hairs.The main findings are as follows:1.Impacts of culture substrate on root hair growthStudy conducted in four culture types showed no root hair of seedling growth in yellow-soil culture;a few uneven root hairs growth on epicotyl-root of tap-root in Hoagland solution culture;notable amount of root hairs growth on tap-and lateral root in sand culture;and significant amount of even root hairs growth on tap-root in MT Agar culture,but no lateral roots and much smaller diameter of tap-root than that of tap-root in sand and yellow-soil cultures.2.Growth cycle,distribution,growth stages and development ways of root hairThe growth cycle of root hairs of Poncirus trifoliata was 4 days in sand culture.Root hairs on tap-root and lateral roots grew mainly above maturation zone;fewer and shorter root hairs grew in maturation zone and grew in blocks;and root hairs close to tap-root base were longer and grew more evenly.SEM and paraffin section clearly showed the growth stages of root hairs are unoutgrowth,initiation,tip growth,and maturation.It was also shown that growth mode of root hairs was random.Any epidermal cell could develop into root hair.3.Impacts of IBA and ETH on the growth of tap-root,lateral root and root hairExperiments in sand culture showed that IBA at all concentration levels improved the growth of tap-root and increased the length and number of lateral roots.ETH was also shown to increase the length and number of lateral root,but have no major impact on tap-root growth.The optimum concentration for the growth of root hair of P.trifoliata was 1.0 mol.L-1for both IBA and ETH as at this concentration,the length,diameter,and density of root hair on tap-root and density of root hair on lateral root were statistically higher than those of CK.Furthermore,low concentration of either IBA or ETH inhibited elongation of root hair while high concentration of either IBA or ETH blocked root hair initiation.4.The mechanism of auxin on root hair development.The content of endogenous auxin in root-hair zone has a positive correlation with root hair growth by NMT and ELISA.IBA increased the density of root hairs by increasing the number of epidermal cells and the proportion of epidermal cells outgrew developing into root hairs.After inducing the expression of auxin synthesis and transport carrier genes（TAR2/YUC3/YUC4/YUC6/YUC8;AUX1/LAX1/LAX2/PIN1/PIN3/PIN4/ABCB1/ABCB19）,IBA also increased the content of auxin on root-hair zone.The auxin transport inhibitor 2-NOA inhibited the function of auxin transport carrier genes（AUX1/LAX1/LAX2/LAX3/PIN3/ABCB1）,which blocked auxin transfer from root tip to root-hair zone,and inhibited the development of root hairs.5.The mechanism of ethylene on root hair development.ETH also increased the density of root hairs by increasing the number of epidermal cells and the proportion of epidermal cells outgrew developing into root hairs.ETH promoted root hair growth and induced ethylene synthesis and receptor genes（ACS2/ACO1/ACO2;ETR1/ETR2/ERF）that enhanced ethylene effects.The ethylene inhibitor Ag NO3 reduced the expression of ethylene synthesis and receptor genes（ACS1/ACS2/ACO1/ACO2;ETR1/ETR2/EIN2）and blocked root hair initiation and elongation.6.The interaction of auxin and ethylene on root hair development.The IBA+ETH treatment showed higher root hair density,length and diameter than other treatments.Compared with CK,the IBA+ETH treatment increased the content of endogenous auxin on root-hair zone by 33.0%and up-regulated the expression of auxin synthesis and transport carrier genes（TAR2/YUC4/YUC8;AUX1/LAX1/ABCB1）.The IBA+ETH treatment also enhanced ethylene effects by inducing the expression of ethylene synthesis and receptor genes（ACS2/ACO2;ETR1/ETR2/CTR1/ERF）.The2-NOA+Ag treatment significantly blocked the development of root hairs.Compared with CK,The 2-NOA+Ag treatment reduced the expression of auxin synthesis and transport carrier genes（TAR2/YUC3/YUC4/YUC6/YUC8;AUX1/LAX1/LAX2/LAX3/PIN1/PIN3/PIN4/ABCB1/ABCB19）,and reduced the level of endogenous auxin in the root-hair zone by 46.3%.The 2-NOA+Ag treatment also significantly reduced the expression of ethylene synthesis and receptor genes（ACS1/ACS2/ACO1;ETR1/ETR2/CTR1/EIN2）,which weakened ethylene effects.These results showed the collaborative effect between auxin and ethylene on root hair development.The IBA+Ag treatment reduced the expression of auxin synthesis and transport carrier genes（TAR2/YUC3/YUC4/YUC6/YUC8;AUX1/LAX1/LAX2/LAX3/PIN1/PIN3/PIN4/ABC B1/ABCB19）,and suppressed the effect of IBA on root hair development.The2-NOA+ETH treatment significantly decreased the expression of auxin synthesis（TAR2）and reduced transport carrier gene（PIN1/PIN4）,lowered the content of endogenous auxin in root hair zone by 20.8%.The 2-NOA+ETH treatment significantly induced ethylene receptor genes（EIN2/ERF）,but it had no significant effects on root hair density and length,nor is the level of endogenous auxin in root hair zone significantly different from CK.These results showed that auxin plays a major role in root hair development.7.EXPs regulated root hair development downstream of auxin and ethylene.Analysis using q RT-PCR showed that the EXPA4 and EXPA5 regulations of root hair development and their expressions were affected by IBA/ETH and its inhibitor.However,there were no such effects for EXPA7.8.The effects of auxin and ethylene on root biomass and mineral nutrient of seedlings.The IBA and ETH treatments not only increased the length of taproot and lateral root and lateral root number by 20.4%,21.4%,9.1%and 5.8%,7.1%,22.5%,respectively,but also dramatically increase the density of root hair on tap root and lateral root by143.3%,42.4%and 70.6%,33.6%,respectively.The IBA and ETH treatments significantly increased the root surface area,enhancing the mineral nutrients acquisition.The growth regulator affect mineral nutrients（N/P/K/Fe etc.）in plants partially by root hairs.This study also showed that root hairs,especially those of lateral roots,play important role in acquisition of N/P/K/Fe. |
