Functional Analysis Of Auxin In Arabidopsis Primary Root Elongation In Phosphate Starvation Response

The root system architecture (RSA) of terrestrial plants is involved in the acquisition of water and nutrients, anchorage of the plant, as well as perception of various outside stress. Phosphate, as a kind of nutrient, is required for plant growth, development and reproduction, and it has vital roles in many metabolic activity. Due to slow diffusion rates in rhizospheres and fixation as immobile organic Pi or inorganic complexes which plants can’t utilize directly, plants are often in a Pi starvation condition. In order to adapt the phosphate-deficiency condition, plants have evolved some characteristics such as the increase of the lateral root number, the inhibition of primary root elongation and so on. Phytohormones are crucial elements for plant growth and development, and also determine the root organization. Auxin is a kind of phytohormone which can promote plants development. Our research is mainly on the functional study of auxin to primary root elongation when response to Pi starvation.In this paper, the seedlings of WT and mutant Arabidopsis (arf7, arfl9, arf7arfl9) were used to observe the changes of root morphology under phosphate starvation condition. The results obtained were as follows:1. The primary root length of arf7arf19 was significantly longer than WT, arf7, arf19 under Pi deficiency condition. We took the photos of the root tip of these experimental seedlings using the differential interference microscope, found that the meristematic zone length and elongation zone length of arf1arfl9 were significantly increased in Pi starvation condition, but elongation zone length increased moderately. So the elongation of the primary root of arparf19 was mainly caused by meristematic zone.2. I have further explored the reason why arf7arfl9 shows low sensitivity to Pi-starvation. Founding that the arabidopsis primary root was promoted by 0.001 μmol/L NAA, but it was inhibited by the auxin polar transport inhibitor NPA under Pi-deficiency condition. The results indicated that the inhibition of primary root elongation under Pi starvation condition may caused by the deficiency of auxin in primary root tip.3. The effect of auxin on the elongation of primary root under Pi starvation was further investigated using transgenic plants expressing the β-glucuronidase (GUS) reporter gene under control of DR5,1AA and cyclin B1-type (CycBl) promoter, the results revealed that the GUS signal under Pi starvation condtion were stronger than those under normal Pi condition. Exogenous 1-Naphthaleneacetic acid (NAA) and auxin transport inhibitor (NPA and TIBA) were applied in those transgenic plants, finding that the NAA-treated plants show stronger GUS signal in the root tip, but the signal was suppressed in auxin transport inhibitor-treated plants.4. Through hybridization technique we get the transgenic homozygous arabidopsis of DR5.GUS/WT, IAA.GUS/WT, DR5:GUSIarflarfl9 and lAA:GUSIarf7arfl9. The seeds were germinated on medium containing lmM Pi, transferred the 5-day-old seedlings to lmM Pi medium and luM Pi medium for 2 days, then subjected the seedlings to GUS staining. Histochemical assay revealed that the expression of both DR5 and IAA in arf7arfl9 were stronger than WT, verified that the auxin concentration in arparf19 higher than WT.According to these experiments, we considered that plants perceive the Pi signal by the root tip, and the primary root elongation under Pi-deficiency condition is auxin-dependent. Appropriate auxin application can facilitate the growth of arabidopsis primary root under Pi starvation condition.