The Physiological Basis Study For Adaptability Responses Of Melon To Low Phosphate Stress

Phosphorus is a macronutrient necessary for crop growth and development.phosphorus is mainly present in the soil in the fixed and inorganic states that are unavailable.The available phosphorus content in the soil is often low.In addition,phosphorus is a non-renewable resource,so the shortage of phosphate fertilizer resources must become a global problem with the progress of agricultural production.Therefore,exploring the genetic potential of crops'phosphorus nutrition efficiency and increasing the absorption and utilization of phosphorus by crops are urgent problems to be studied.In order to find out the adaptive response of melon to phosphorus deficiency,this paper used the muskmelon‘Lvtianshi'as the experimental material to carry out different degrees of phosphate stress hydroponic experiment.Firstly,the different degrees of low-phosphate stress hydroponic experiments were carried out to determine the demand for phosphorus in melon seedlings.Low-phosphorus stress test and short-term absorption test were carried out to explore the adaptive response mode of melon roots to low phosphorus stress and its effect on phosphate absorption and utilization.At the same time,the responses of photosynthesis to low phosphorus stress was explored.Based on the previous physiological response studies,the genes involved in the low-phosphorus responses of melon were further explored by transcriptome sequencing and analyzing of sequencing results.We analyzed the low-Pi signal transduction factors,endogenous hormone signal transduction genes,genes related to Pi absorption,transport and reuse,and low-Pi stress defense genes.The key genes for low-Pi response were further targeted by q PCR.Finally,the physiological basis mechanism of endogenous hormones in regulating response to low-Pi stress were analyzed by combining the morphological,physiological and differential gene transcription level data in melon.The main results of the paper were as follows:(1)adaptive responses of melon roots to low-Pi stressFrom the aspects of root morphology,physiology and molecular level of melon seedlings,the adaptive responses to low-Pi in melon was found.It was found that P_0.025(mild low-Pi stress)could induce low phosphorus adaptive responses of melon,such as the increased growth of roots,the increased types and secretions of organic acids secreted by roots,the increased acid phosphatase activity in root tissue and around the rhizosphere,the up-regulated expression of high-affinity Pi transporter gene,and the increased root vitality.These responses increased the efficiency of phosphate absorption and utilization so that the phosphorus content in the melon seedling tissue was within the normal phosphorus demand range of melon.Mild low-Pi stress did not significantly inhibited seedling growth.P_0.001(severe low-Pi stress)rapidly induced melon showing a significant increase in root-shoot ratio by13.8%compared with the control.Root growth was induced in the early stage of stress,and the total length of roots increased by 14.46%compared with the control.Lateral stress promoted the elongation of the main root increased by 29.53%compared with the control and inhibited the germination and growth of the lateral roots.The activities of the acid phosphatase in the root tissues and around the rhizosphere were significantly increased,and the high-affinity Pi transporter genes were up-regulated and the root vitality was increased.These responses were beneficial to improve the absorption and utilization efficiency of phosphate.The Pi absorption efficiency of P_0.025 and P_0.001 was increased by 65.14%and 309%compared with the control at 7 d.The Pi absorption efficiency was increased by 22.02%and 25.32%compared with the control at 14 d after treatment.However,the long-term severe low-Pi stress inhibited the growth of seedlings.(2)responses of photosynthetic system to low-Pi stressLong-term low-Pi stress caused a significant decrease in phosphorus content in melon leaves,which was 66%(P_0.025)and 85%(P_0.001)lower than the control,which inhibited the expression of ferredoxin and ferredoxin-NADP⁺reductase on the photosynthetic chain,reduced the electron and proton transport efficiency and inhibited the ATP synthase activity.The dark reaction assimilation of ATP and NADPH was further reduced and the photosynthesis efficiency reduced.At the same time,the starch granules accumulated in the chloroplast under low-Pi stress,which formed negative feedback on photosynthesis.These responses led to the decrease of net photosynthesis rate of 31.4%(P_0.025)and 96.4%(P_0.001),respectively.Therefore,long-term severe low-Pi stress inhibited seedling growth and reduced dry matter accumulation.In addition,low-Pi stress reduced the light interception capacity of melon leaves,and excess light energy led to the production of toxic light products such as reactive oxygen species in the leaves,which caused membrane peroxidation and chloroplast endomembrane system damage.Therefore,the redox homeostasis cannot be maintained,and the melon leaves exhibited photooxidative stress symptoms.To alleviate photoinhibition,plants activated NPQ mechanisms,alternative electron transport pathways,and antioxidant systems to protect their chloroplasts.(3)the physiological basis of low-Pi signal transduction,Pi absorption and utilization and defense of low-Pi stress in melonIn order to absorb and efficiently utilize Pi,melon synergistically promoted the absorption of Pi and the recycling of Pi in plants from the root configuration,Pi activation in vivo and the soil,and the expression of PSI gene in roots.After the root sensed the Pi deficiency,the WRKY 43 and WRKY 55 genes were significantly up-regulated after stress for 1 d and 4 d,respectively.The MYB 39 and MYB 108genes were significantly up-regulated after stress 1 d and 7 d,respectively.The genes of SPX domain protein continued to be up-regulated after 1 day of treatment.It can further up-regulate the expression of phosphatase,PHT1 high-affinity transporter and other genes in melon low-Pi rescue system,promoting the activation of organic phosphorus in culture medium and in vivo,phosphate absorption and transport.The reason why short-term low-Pi stress promoted root growth may be that low-Pi inhibited the expression of the negative regulator AUX/IAA gene during AUXs signal transduction in the root system,amplified the AUXs signal,and increases the sensitivity of roots to AUXs.On the other hand,it promoted the expression of the negative regulator A-ARR5 gene in the CTKs signal transduction pathway,inhibited CTKs signaling,and attenuated the inhibition of CTKs on root growth.Therefore,in the early stage of low-Pi stress,root growth was promoted by amplifying AUXs signal and inhibiting CTKs signal without increasing the content of endogenous AUXs content and AUXs/CTKs.With the prolongation of stress time,AUXs redistributed with the help of hormone transporter,which disturbed its polar transport,so that the melon roots showed the primary root elongation,the inhibition of lateral root germination and growth.Based on the results of this study and the comprehensive analysis of existing studies,we hypothesized that the transduction of low-Pi signaling in roots may be transduced to downstream target genes or target proteins by calcium and hormonal signals to response to low-Pi stress in root.Leaf transcriptome results showed that low-Pi stress induced significant up-or down-regulation expression of WRKY,MYB,b HLH,NAC and NAC domain protein genes,further regulating downstream phosphate transport and metabolic gene expression.At the same time,sucrose accumulated in leaves and roots by inducing the synthesis in shoot and transport to the root under the low-Pi stress.This accumulation of sucrose may induce the expression of PSI gene.On the one hand it promoted or inhibited the transport of Pi from root to shoot,and on the other hand promoted the recycling of above-ground phosphate.Low-Pi stress inhibited the signal transduction of AUXs and CTKs by reducing the synthesis of AUXs and CTKs and promoted the synthesis of ABA and SLs and ABA signal transduction.With the prolongation of the stress time,the growth of the shoot was inhibited.The shoot growth inhibition may be the ultimate goal of the low-Pi signal,which redistributed the photosynthetic product and physiologically showed the relative increase of root growth.From these analyses we hypothesized that calcium,sucrose,and endogenous hormone signals may be involved in the low-Pi signals transduction in the shoots.By analyzing the low-Pi signal transduction factors in roots and shoots,Pi,sucrose and SLs may be involved in the response of melon to low-Pi stress as system signal.In order to eliminate or slow down the damage caused by low-Pi stress,the genes of ROS scavenging system in roots and leaves,such as peroxidase and GST,were significantly up-regulated by low-Pi stress,and the immune defense system was activated to protect melon cells.In summary,this study clarified the adaptive responses of melon root to low-Pi stress and the regulation of these responses by low-Pi upstream signal transduction factors.The adaptation responses of photosynthesis to low-Pi stress and stress defense mechanism were explored.The key genes involved in low-Pi stress responses were preliminary explorated.The research fully explored the biological potential of melon to low-Pi stress,which is of great significance for the rational phosphate fertilizer application,screening and the breeding of phosphorus high-efficiency varieties.