Effect Of Boron Deficiency On Root Growth And Expression Of Related Genes In Trifoliate Orange [Poncirus Trifoliata (L.) Raf]

Boron(B) is an essential microelement for higher plants and has important physiological functions in plant growth and development. The symptom of boron deficiency is very common, because the soil B levels are low in most citrus orchards. It s widely known that the acquisition of B and other mineral nutrients were mainly relying on the root system of citrus rootstocks. Research the effects of B deficiency on rootstock growth and development are very important in resolving the B deficiency problems incitrus production.In this study, Poncirus trifoliate(L.) Raf. was selected as research material. Hydroponics was used to investigate the physiological and molecular mechanisms of B deficiency on root growth and developmentof trifoliate orange. The main contents and results are as follows: 1. Effects of B deficiency on root morphology and anatomical structure of trifoliate orangeAfter 90 days of B deficiency treatment, primary roots and lateral roots elongation of trifoliate orange were significantly inhibited. Root inhibition in B deficiency treatment is envidenced by typical morphological changes including radial cell enlargement and root swelling near the root tip, precocious maturation of tissues near the root apex, as well as stimulation of lateral root formation near the apical. The lateral root density and lateral root primordia density in B deficiency treatment were significantly higher than that in control. Light microscopy of slitting of transverse sections of root tips showed considerable anatomical alterations due to B deficiency. B deficiency resulted in apical enlargement, cells arranged disorderly, the cells necrosis and seriously shedding were observed, and visible cavity formation was also finally observed in apical growing point. The root vascular bundles were increased in number and size, root epidermal cracked. Slitting scanning electron microscope showed that, in B deficiency treatment, apical cell wall thickened, cells gap increased, cells arranged loosely. Transmission electron microscope was used to investigate the ultrastructural changes in trifoliate orange root tip cells in B deficiency treatment. The results showed that smaller nucleiwas closed to cell wall, vacuole swelled, organelles disappeared, cell gap junction was disconnected, a large number of substances accumulated in cell wall and vessel. 2. Effects of B deficiency on expression levels of several key genes involved in cell cycle and root elongation.Root growth requires two conditions: One is the constant division of cells in the root meristem;The other one is the elongation of cells in the root elongation zone. Therefore, we investigated the effects of B deficiency on expression levels of several key genes involved in cell cycle and root elongation. The results showed that B deficiency has little effect on the cell cycle related genes. The expression level of Pt LRP1 was significantly induced by B deficiency before 18 h and then it decreased significantly to the end. The expression levelof Pt SWP1 was obviously increased after 18 h in B deficiency treatment, and its expression level was significantly higher than that of in the control, and the 3 times higher than that of in the control. 3. Effects of B deficiency on IAA content in the roots of trifoliate orange.There are a large number of new roots emerged under B deficiency with 0.3mg/L exogenous IAA. Like control roots, the new root tips contain distinct quiescent center and root cap. However, the maturation region was closed to root tip in new roots. In short term B deficiency treatment, the concentration of soluble B decreased gradually in the roots of trifoliate orange, but the B deficiency treatment had little effect on the concentration of insoluble B. The IAA content was at 5.8 ng.g-1 FW or there about in control root tips, while in the B deficiency treated root tips, it increased gradually after 12 h, peaked at 24 h, and then decreased dramatically to the end of this experiment. The content was obviously higher than that in the control root tips from 12 h to 24 h of treatment and was significantly lower than that in the control root tips from 48 h to 72 h. 4. Effects of B deficiency on the expression levels of genes involved in IAA metabolism.In order to further clarify the role of B in IAA metabolism, we chosed some key genes involved in IAA metabolism to investigate the effects of B deficiency on the expression levels of these genes. The results show that B deficiency significantly induced the transcript levels of genes of TAA1, TAR2, YUC3 and YUC8 before 12 h and then these genes decreased significantly to the end. The trend of those genes’ transcript profiles was consistent with the change of IAA content. Besides, the expression levels of rootward transport genes, including AUX1, PIN1 and PIN 4 were down-regulated significantly, but the expression levels of shootward transport genes, including LAX1, ABCB1, and PIN3 were increased significantly in B deficiency treatment, suggesting that influx of IAA to root tips was decreased and efflux of itfrom root tips was increased.