The Characteristics Of Al-induced Organic Acid Anions Secretion In Polygonums And Rice Bean

Aluminum (Al) toxicity is one of the most significant constaints limiting plant growth and crop production in acid soils, which comprise approximately 30% of arable land worldwide. Al at micromolar concentrations rapidly inhibits root elongation and the uptake of water and nutrients, leading to reduced plant growth and increased susceptibility to environmental stresses. However, some plant species or genotypes, especially native plant species, grow vigorously in acid soils, indicating these species or genotypes have adapted well to high levels of Al in the growth medium. The mechanisms of Al tolerance are not fully understood despite extensive research efforts have contributed to our understanding of Al resistance in plant. For example, the role of organic acids in detoxifying Al both externally and internally in some species has been well documented in the past decade, but it cannot explain high Al resistance in others. To date, most studies of Al-resistance mechanisms have focued on Al-resistant genotypes of crops that have a long breeding history. Therefore, it is very likely that some Al resistance genes have been lost when selection for other agronomic features. In the present study, three native Polygonum species were selected to study Al resistant mechanisms and the relationship between their Al resistance and the soil characteristics of their orgins. On the other hand, although Al-induced organic acid anions secretion has been proven as one of the most important Al resistance mechanisms in many plant species and cultivars, the process of how Al regulated the organic acid anions secretion has not been completely understood. In the present study, Polygonums and rice bean {Vigna umbellate [Thunb.] Ohwi & Ohashi) were selected as the plant materials to study the physiological basis of Al-inuduced oranic acid anions secretion. Because we have proven that Polygonum aviculare L exude oxalate rapidly under Al stress and fitted into Pattern â…  type, but rice bean exude citrate delayed by several hours in response to Al stress and fitted into Pattern â…¡ type. Thus, the two species must differ greatly in the mechanisms of Al-induced organic acid anions efflux. The results were as follows:1. A comparison of aluminum resistance among Polygonum species originating onstrongly acidic and neutral soilsPolygonum aviculare L and Polygonum lapathifolium L grown in tea garden soils at pH around 3.5 showed high Al resistance, but Polygonum bungeanum Turcz grown in neutral soils was extremely sensitive to Al. Hydrophonic experiments showed that the root elongation was only inhibited by 15% in P. aviculare and 35% in P. lapathifolium after exposure to 50 uM Al for 24 h. Their Al resistance was respectively higher than and similar to that in an Al resistant buckwheat {Fagopyrum esculentum Moench) cultivar. In contrast, the root elongation of P. bungeanum was inhibited by 80% under the same condition. The differences in Al resistance among Polygonum species were confirmed in a 10-d intermittent Al treatment experiment, the root biomass of the former two species were unaltered but decreased by 50% in the latter species. However, high Al accumulation was not found in the leaves, indicating these species were not Al accumulators. Oxalate efflux was detected in root exudates of both Al resistant species. No organic acid anions were detected in the root exudates of the Al sensitive species. The anion channel inhibitor phenylglyoxal (PG) inhibited the oxalate efflux greatly. Inhibition of root elongation was greater in the presence of PG, confirming that oxalate efflux was associated with the Al resistance. However, since the efflux rate was much lower than their related species buckwheat, other mechanisms must be involved in Al resistance and these need to be studied further.2. A comparative study on Al induced oxalate secretion in P. aviculare and citrate secretion in rice beanIn order to clarify the processes of Al induced organic acid anions secretion in different plants with different patterns, the characteristics of Al induced organic acid anions efflux in the roots of rice bean and P. aviculare was investigated. Oxalate secretion in P. aviculare roots occurred within 30 min of Al exposure, while citrate secretion in rice bean roots was induced by at least 4 h Al exposure. Therefore, the former was classified to pattern I, and the latter pattern II. A broad protein-synthesis inhibitor cyclohexide had no obvious effect on the Al-induced oxalate secretion in P.aviculare, but inhibited the Al-induced citrate secretion in rice bean roots by about 84%. This suggested that novel protein synthesis didn't involved in the Al-induced oxalate secretion in P. aviuculare, but was essential in regulating the Al-induced citrate secretion in rice bean. Al exposure did not affect the oxalate content in P. aviculare root apices, but stimulate citrate synthesis in rice bean. The oxalate secretion in P. aviculare was sensitive to anion channel blocker PG and the citrate secretion in rice bean was sensitive to anthrancene-9-carboxylic acid, indicating anion channels involved in organic acid anions secretion in both plants but their pharmacological properties were different from each other.3. Al in symplast played very important role in regulating oxalate secretion in P. aviculare rootsIn order to determine whether Al in the apoplast or symplast activated the organic acid anion channels in plasma membrane, P. aviculare was grown in nutrient solution with or without NaCl to modify the root cell wall properties. Results showed that neither root plasma membrane intergrity nor oxalate content in root tips was affected by NaCl pretreatment. However, NaCl pretreatment resulted in about 36 % increase in total pectin content of P. aviculare root apices, and enhanced the Al adsorption rate and total Al accumulation in the extracted root cell walls. The oxalate efflux occurred after 3 h of Al exposure in the NaCl pretreatment P. aviculare roots although that occurred within 30 mins Al exposure in the control roots. This was consistent with the delay of Al entry into root symplasm. Therefore, it was suggested that Al must enter into symplast to induce oxalate secretion from P. aviculare roots. An alternative explanation for the present result is that there is a critical Al content in root symplsam to active oxalate efflux. However, the possibility remains that NaCl pretreatment delayed the oxalate secretion by affecting the activity of enzymes or signal substance involved in the opening of anion channels. This needs further investigation in the future.4. The physiological mechanism of Mg alleviating Al toxicityMagnesium (Mg) can alleviate Al toxicity, but the physiological mechanisms arestill not clear. Here we found that micromolar level of Mg obviously alleviated Al-induced but not lanthanum-induced root elongation inhibition in rice bean. The increase of Al-induced citrate efflux paralleled with the decrease of root elongation inhibition with the addition of Mg at either dose (10-50 uM) or time-course (0-4, 4-8, 8-12 and 12-24 h) manner, suggesting that the ameliorative effect by Mg might be closely associated with Mg-enhanced citrate efflux from rice bean roots. Vanadate (an effective H+-ATPase inhibitor) decreased the Al-induced citrate efflux, while addition of Mg partly restored the efflux;Mg addition obviously increased the activity of Al-reduced plasma membrane H+-ATPase, and Mg and calcium contents in root apices significantly. Taken together, we proposed the following scheme: Mg addition compensated the Al-induced Mg decrease, the increased intracellular Mg content up-regulated the activity of plasma membrane H+-ATPase, and finally resulted in the increase of Al-induced citrate efflux.