Effects Of Mineral Nutrients On Root Hair Development And Arbuscular Mycorrhizal System Establishment In Poncirus Trifoliata L. Raf.

The effects and primary mechanism study of mineral nutrients on root hair development and Arbuscular mycorrhizal system establishment in Poncirus trifoliata L. Raf were investigated in the study. Firstly, the root hair morphology and distribution were illustrated in 4 different citrus rootstocks; Secondly, the effects of different mineral nutritions on root hair development of Trifoliata orange were evaluated either in water or in sand media; thirdly, root architecture alterations caused by deficiencies of P, K and Ca were thus identified, respectively; finally, Arbuscular mycorrhizal system establishment and root hair architecture was measured under the conditions of different levels of P and K in combination with the inoculation of the fungus onto the Trifoliata orange root system, transcription levels of a few genes governing the root hair development were thus quantified, and the correlationship between the root hair and Arbuscular mycorrhizal system establishment was discussed. Results showed that:(1) Root hair morphology in 4 different citrus rootstocks. The root hair of all citrus rootstocks were not forked, and distributed asymmetrically with the density of lateral root hair greater than that of taproot hairs. The root hair morphology and distribution of varied largely with different citrus rootstocks:the densities of either taproot or lateral root hair of Red tangerine(Citrus reticulata) and Xiangcheng (C.junos) were significantly higher than that of others; the root hair density of Trifoliata orange dropped gradually from the top to the apical, which was in contrary to the distribution of the red tangerine; however, the root hair density of Xiangcheng was high in the Xiangcheng middle and tip, low in the top of the roots.(2) The effect of different nutrient deficiency on the root hair development and root growth. Root hairs of Poncirus trifoliata seedlings growth in sand and hydroponics condition were distributed asymmetrically, and were condesed in the root base but scattered in root tip, which might be one of the natural characteristics of the citrus root hair distribution. The root hair density on lateral roots was significantly larger than that on primary root, but the root hair length was on the contrary. Furthermore, the root hair density on primary root varied from 55.0 to 174.3 mm-2 under hydroponics condition while it was 73.3～340.6 mm-2 under sand culture, suggesting the sand culture was better for Poncirus trifoliata root hair development. The deficiency of each test nutrient had greater effects on the growth and development of root hairs under sand and hydroponics condition. The density and length of root hair were much higher in P, Ca deficiency than in the control, however, lower density of root in K-deficiency treatment, implied the critical role of those nutrients in root hair and elongation. Additionally, root growth was hampered at Ca, Mg, Fe, or Mn deprivation condition, suggesting that it is necessarity of the mineral nutrients at seedling stage to ensure better root architecture.(3) Effects of P, K and Ca deficiencies on the root morphology and nutrient absorption of Poncirus trifoliata seedlings. The results showed that the total root length, surface areas and volumes were reduced significantly in the nutrient deficiency media. When the root system was classified into fine, middle and coarse roots according to their diameters, the effects of nutrient deficiencies on the three fractions were varied with treatments. Compared with the control and P deficiency, the root length, surface area and volume of fine and coarse roots were reduced, but the surface area and volume of middle roots were increased remarkably; under K and Ca deficiency, the root length, surface area and volume of fine and middle roots were reduced. The nutrient deficiencies induced significant decreases of shoot and root biomass with stronger inhibition effects on shoot than on root under the P deficiency, but opposite effects was found under K deficiency, while the minimum effects were detected under Ca deficiency. However, the Zn concentrations were significantly increased under three nutrient deficiency media, the Fe concentration were increased under the Ca deficiency but decreased under the K deficiency, Ca concentrations were increased under the P and K deficiencies but decreased under the Ca deficiency.(4) The effect of P, K nutrient level on root hair development and arbuscular mycorrhizal colonization. The plant biomass was significant increased in arbuscular mycorrhizal inoculated Poncirus trifoliata seedlings compared with non-arbuscular mycorrhizal plants under P, K deflcency condition and control, but no difference was investigated under high P condition. First lateral root hair density of control plants and 2nd lateral root hair length of plants under high P and K deficency condition were increased by arbuscular mycorrhizal fungi colonize. However, the root hair density and length of first lateral roots was reduced in arbuscular mycorrhizal plants under P deficency condition. Under P-deficient conditions, the total AM colonization, Arbuscular colonization rate and Colonization frequency were notably elevated, but were declined under high P condition. The colonization frequency was reduced, while the total AM colonization and Arbuscular colonization rate of 2nd lateral root were increased on K deficency condition. The epidermis cell elongation was impaired at P and K deficency condition, and the epidermis cell number increased at P deficency condition but reduced at K deficiency. There was no influence on number and elongation of epidermis cell when plant grown in high P condition.(5) The effects of P, K nutrient level on the expression of root hair development, AM system signaling and auxin related genes.The P, K deficiency and high P triggered the up-regulaion of root hair initiation related gene ROP], ROP2,ROP9 and tip growth gene LRX1. The expression of atrichoblast special gene WER and TTG were increased under high P condition, and it was down-regulated by AM colonization. The expression of AM system signaling gene was stimulated by P deficiency but was inhibited by high P, and the relative expression of DMI5, CASTOR, NUP133 were higher at P deficiency condition than other treatments. The expression of CASTOR gene was suppressed by K deficiency while another 6 signaling gene were promoted, and the relative expression of DMI1, NUP85, CYCLOPS were higher than in other treatments. The expression of auxin transport gene PIN3 was suppressed by P, K deficiency and high P, while FER was induced. P deficiency up-regulated the expression of ARX2 while down-regulated the expression of GH3, and high P up-regulated the GH3 and TIR1 expression and down-regulated the expression of ABCB expression, while the expression of ABCB and TIR1 was increased and GH3 was descended by K deficiency.