Gravitropism is an important factor in determining root architecture and directs roots to grow downward in soil and thus may have a great affect on the uptake of water and mineral ions required for plant growth and development.Ammonium profoundly affects root architecture of Arabidopsis thaliana.Despite the inhibitory effects of high ammonium on root length and the number of lateral roots,however,very little is known concerning the influence of ammonium on root growth direction.Experiments were performed with Arabidopsis thaliana using indoor agar media for a simulation to study the effects of high ammonium on root growth direction.It was found that NH4+ stress not only inhibited plant growth,but also affected root growth direction.Kinetics results for a 30-h assay window showed that 10 mmol/L(NH4)2SO4 delayed root gravitropism,while 30 mmol/L(NH4)2SO4 significantly reduced root gravitropic curvature.Reduction the nutrients in basic medium,root agravitropic response induced by NH4+ occurred at a lower concentration.Adding Ca(NO3)2 partially restored the inhibition of NH4+ on root elongation,but had no effects on NH4+ induced root gravitropic angle.When NH4+ was supplied separately to different parts of the plants,the effect of NH4+ on root gravitropism was mainly from the root-supplied NH4+.These results preliminarily indicate that the influence of ammonium on root growth direction is partially independent of its effect on root elongation and is not due to the deficiency of cations such as Ca2+,but may be related with the alteration of root physiological processes when root exposed to high NH4+.

检测拟南芥突变体amos2对外源激素响应的结果表明,该突变体主根伸长对外源生长素有明显的抗性,其主根抗性强于拟南芥野生型(WT),但弱于已知的生长素突变体aux1-7和axr4-2.该突变体的主根伸长对生长素转运抑制剂TIBA也表现出较强抗性,而对一定浓度的ACC表现敏感,但主根伸长对外源6-BA和ABA的响应与野生型一致.外源添加生长素或乙烯抑制剂可同时增加amos2和野生型的可见侧根数量,但突变体的增加百分比要显著高于野生型.虽然amos2突变体的荚果败育率比野生型显著增加,但并未表现出其他生长素突变体特有的胚轴弯钩消失及根系向重性缺失的表型.上述结果暗示突变基因AMOS2在控制拟南芥生长发育的某些方面发挥重要作用.

In this study,we investigated the responses and the mechanisms of the lateral root to root-supplied ammonium in Arabidopsis thaliana seedlings.The results indicated that high concentration of ammonium suppressed the lateral root development through the length of the lateral root rather than the number.Adding MSO showed no rescue of the lateral root length,suggesting that this inhibitory effect was not a systemic effect by the metabolism of excessive ammonium from root but a localized effect.The root tip of the lateral root was the crucial perceived site,which was confirmed by the experiments of supplying ammonium to the different zones of primary or lateral root in Col-0.On the other hand,when ammonium was localized supplied to part of the root,the length of the lateral root on the other agar surface which did not contain ammonium was significantly promoted.According to the analyses of the relative mutants,the lateral root length of mutants both in auxin transport(aux1-7,eir1-1) and insensitive to ethylene(ein2-1,etr1-3) had no tolerance to this repressive effect of root-supplied ammonium on the relative length of the lateral root in Arabidopsis seedlings,which indicated that this effect may be mainly regulated by independent auxin-transport and ethylene pathways.

Gravitropism is one of the important factors in determining the spatial architecture of root system and thus has a great effect on the capacity of plants to anchorage and take up water and nutrients in the soil. Besides controlled by gravity, root gravitropism is also affected by various environmental stimuli. The effects and corresponding mechanism of ammonium on root gravitropism in Arabidopsis were studied with agar medium. The results showed that all concentrations of (NH4)2SO4 used significantly inhibited root gravitropic curvature at earlier time, but the gravitropic angles became smaller along with treatment prolonging. Equal concentrations of NaCl had less inhibitory effect on root gravitropism than that of (NH4)2SO4 and various concentrations of mannitol did not inhibit root gravitropism. Histochemical staining showed that there was no rapid degradation of amyloplast in root columella cells of Col-0 under ammonium stress within 12 h period, and the starchless mutant pgm-1 displayed similar root gravitropic change pattern as that of Col-0 when treated with ammonium. However, the agravitropism of auxin transporter mutants aux1-22 and eir1-1 can be partially rescued by ammonium treatment. These results indicated that the effect of ammonium on root gravitropism might be independent of the amyloplast in gravity sensing cells.

In order to learn about selenium nutrition of village/town inhabitants in the Taihu Lake region, in South Jiangsu, the stratified clustering random sampling method was used in the study. Vegetal and animal foods were procured randomly from free markets for agricultural products hairs of inhabitants gathered for determination of selenium contents therein, with the aid of Atomic Fluorescence Spectrormetry (AF-610A). Results showed that animal food, especially white shrimp, was higher in selenium content than cereal food and vegetable food. The selenium intake of the inhabitants through diet was 34.7 μg/day, which is lower than the bottom limit for selenium intake,50 μg/day suggested by the National Nutrition Association of China. The lower selenium intake might increase the risk of certain diseases caused by selenium deficiency and show harm on human health. The average Se content in the hairs of the inhabitant was 336.0 μg/kg, which is lower than the average of the nation’s and far too lower than that in Guangzhou and Shanghai regions. It is demonstrated that daily diet in that region is far from enough to meet the lowest need of the inhabitants for Se and it is necessary to develop some safe and effective Se-enriched foods for the people therein.