Synergistic Effect Of Potassium And Nitrate On Ammonium Toxicity Alleviation Of The Seedling Roots In Rice

Ammonium is the preferred nitrogen form for rice and other similar crops.Whereas,unsuitable ammonium concentration will exert significant toxic effects on plants.A lot kinds of substances existed in soil can alleviate this toxicity,among which potassium and nitrate are the most extensively studied.However,previous researches on the toxicity alleviation of the two substances have been carried out independently,and there is no systematic study on the synergistic effects of potassium and nitrate on plant ammonium toxicity alleviation.In this research,we found that the coexistence of K⁺and NO₃^-obviously had a better alleviation effect on plant growth than that under the separate existence under the ammonium toxic stress.Based on this phenomenon,it is speculated that K⁺and NO₃^-have synergistic effects in the alleviation of ammonium toxicity.The phenotype,physiology,metabolism,auxin distribution and gene expression difference of rice seedling roots had been investigated under different alleviation treatments,which results proved the existence of this synergistic effect,and proved that Os CIPK18 is a node molecule of this synergistic effect.The main results are as follows:1.Under ammonium toxic stress,the synergistic alleviation effect of K⁺and NO₃^-increased the number and volume of coleoptiles crown roots and adventitious roots of rice seedlings,as well as the length of embryonic primary roots and its lateral roots.This synergistic alleviation effect could restore the shape of root tip cells,the length of meristem and elongation zones,and the auxin distribution at the root tip zone.2.Under ammonium toxic stress,the coexistence of K⁺and NO₃^-did not increase the activity of GS/GOGAT metabolic system in rice seedling roots,but increased the expression of Os GDH1.The presence of NO₃^-could promote the absorption of K⁺by rice seedling roots,and the presence of K⁺could balance the distribution of NO₃^-between roots and shoots.Thus,the content of K⁺and NO₃^-accumulated in roots,keeping the nutrition,electricity and redox state in a better balanced condition.This may be one of the mechanisms contributing to the synergistic alleviation effect to ammonium toxicity.3.Under ammonium toxic stress,the coexistence of K⁺and NO₃^-inhibited the acidification of rhizosphere of rice seedling roots,which may be another mechanism contributing to the synergistic alleviation effect to ammonium toxicity.Compared with K⁺or NO₃^-treatment,the transcriptome status of rice seedling roots in KNO₃ treatment was similar to that in non-ammonium toxic treatment,which demonstrated the existence of the synergistic effect of K⁺and NO₃^-in alleviating ammonium toxicity at the transcriptional level.The synergistic effect involves in many pathways,such as cytoskeleton and cell wall synthesis,multiple ion and sugar transport,antioxidant,multiple hormone synthesis and regulation,calmodulin,transcription and translation regulation etc.4.The absence of Os CIPK18 gene weakened the alleviation effect of KNO₃ on ammonium toxicity at rice seedlings,but it did not affect the alleviation effect of KCl or Na NO₃,suggesting that Os CIPK18 may be a node molecule in the synergistic alleviation of K⁺and NO₃^-in rice.