The Role Of Sugar In Waterlogging-induced Adventitious Root Formation Of Cucumber

Cucumber(Cucumis sativus L.)is one of the main vegetable crops cultivated in China.In 2018,China’s cucumber cultivation area is 104624 hectares,with an annual output of 56.29351 million tons.With the global warming and the frequent occurrence of waterlogging disasters,waterlogging stress has become a serious adverse factor to the growth and development of crops and even death.Cucumber is a fibrous root system,with underdeveloped aeration tissue,weak absorption capacity and poor regeneration capacity,so it is very vulnerable to waterlogging stress.Under waterlogging stress,the primary roots of cucumber die because of lack of oxygen,and the waterlogged parts are prone to adventitious roots.The newly formed adventitious roots have strong activity,which can replace the damaged primary roots,enhance gas exchange and energy transfer,and maintain plant growth.Therefore,adventitious root formation is the main morphological mechanism of cucumber response to waterlogging stress.With the continuous waterlogging stress,the light interception ability of the plant decreased,leading to the gradual yellow of chlorophyll degradation leaves and the decrease of photosynthesis efficiency.In the early stage,we found that strong light can promote the occurrence of adventitious roots,but its specific regulatory mechanism is poorly understood.The purpose of this study is to explore the role of carbohydrate in the formation of adventitious roots induced by waterlogging stress and to analyze the mechanism of adventitious roots.The main results are as follows:1.It was found that there was no adventitious roots in dark by regulating the duration of light.With the extension of light time(8,16 and 24 hours/day),the number and length of adventitious roots in cucumber also increased significantly.Secondly,by regulating carbon dioxide,the substrate of photosynthesis,It was found that the occurrence of adventitious roots was completely inhibited in the absence of CO2,and the occurrence of adventitious roots increased with the supply of CO2.Light time and CO2 are the necessary conditions for photosynthesis.Therefore,the above study shows that under waterlogging stress,photosynthesis is positively related to the ability of adventitious root development of cucumber.2.In order to further explore the effect of photosynthesis product sugar on the formation of adventitious roots induced by waterlogging stress,it was found that waterlogging stress significantly induced the accumulation of sugar in hypocotyls(waterlogged parts)by comparing the contents of sucrose,stachyose,glucose and fructose in hypocotyls(waterlogged parts)at 0,1,3,5 and 7 day after treatment with the control.The sucrose and stachyose content of waterlogging stress plants were significantly higher than that of the control in the first three days;the glucose and fructose content of waterlogging stress plants were significantly higher than that of the control in the fifth and seventh days,indicating that carbohydrate was an important energy material to promote adventitious root development.Secondly,exogenous sugars(sucrose,stachyose,glucose,fructose)were used to remove the photosynthetic organs(true leaves)of cucumber seedlings.It was found that four kinds of sugars could induce the occurrence of adventitious roots to a certain extent,among which sucrose induced the largest number and the longest length of adventitious roots.3.Sugar is the necessary energy source for plant growth and development,and it can also be used as a signal molecule to regulate various growth and development processes.The above physiological evidence can fully prove that carbohydrate,as an energy substance,plays an important role in the regulation of adventitious roots in cucumber.However,the occurrence of adventitious roots is a complex biological process.Whether there are other factors involved in the regulation still needs to be explored.Therefore,we used transcriptome sequencing technology to compare the different expression genes in hypocotyls of Cucumber under waterlogging stress with different light duration(0,8,16,24 hours/day).The results showed that:(1)The genes regulating the synthesis and metabolism of sugar were up-regulated with the increase of light duration,among which cssus3 was up-regulated under waterlogging stress;(2)Auxin transport and signal transduction related genes were induced by light,among which PIN,ARP and ARF were all up-regulated Induced expression.These results indicate that carbohydrate is not only an energy substance,but also a signal molecule to activate auxin transport and signal transduction pathway.4.Through the experiments of auxin transport inhibitor combined with sugar induction treatment,it was found that:(1)Auxin content did not change significantly under waterlogging stress and different light conditions,but auxin transport inhibitor NPA significantly inhibited the occurrence of adventitious roots;(2)Auxin transport vector PIN,auxin response factor ARF and downstream target gene LBD significantly induced expression under sugar treatment;(3)NPA significantly inhibited the adventitious root formation induced by stachyose and sucrose.The above studies showed that auxin mediated the adventitious root formation induced by sugar,and auxin transport and signal transduction played a key role in the process of adventitious root formation.The present results indicate that waterlogging-induced AR formation is regulated by an interaction of sugars(especially stachyose and sucrose)and auxin signaling.Sugars act as molecular signals that are mediated by auxin transport and response signaling to induce AR emergence,and as energy resources for AR elongation.Further studies are needed to clarify how these signals are coordinated.