Studies On The Mechanisms Of Melatonin Seed Priming Enhanced Tolerances Of The Glandless Cotton To The Stresses Of Salt And Cadmium

Cotton is an important cash crop,its fiber is a vital textile raw material,and its seeds,the most basic production materials,contain a huge amount of high-quality oil and protein which are crucial source of edible oil and protein.The pigment gland is a unique structure of plants in Gosspium and related species,which contains gossypol and its derivatives,the toxic materials to human beings and monogastric animals.As such,the glanded cottonseeds with higher gossypol contents cannot be fully utilized due to the pigment glands in cotton seeds.The gossypol content in glandless cotton is also lower due to no pigment glands in the entire plant,which has broad application prospects.However,gossypol is also a phytoalexin of cotton plant,which plays an important role in plant tolerances to both biotic and abiotic stress.Therefore,it is of important theoretical and practical significance to improve the resistance for glandless cotton to biotic and abiotic stress.Seed priming technology plays a considerable role in crop production,which can increase the germination rate of crop seeds,promote crop growth and stress tolerance,and increase crop yield and quality.Melatonin can regulate plant growth and developments,promote plant growth and reduce biotic and abiotic stress in plant.In this study,three pairs of glanded-glandless cotton NILs were used as materials to study the effect of melatonin priming to germination rate,seedling growth,yield,and fiber quality,as well as the tolerances to salt and cadmium stress of glandless cotton,to analyze the physiological and biochemical mechanism including the activities of antioxidant enzymes,contents of malondialdehyde（MDA）,chlorophyll,and endogenous melatonin,and revealing its molecular mechanism through transcriptome analysis.The main results are as follows:（1）There was no significant difference in the main production properties between glanded and glandless cotton.The results showed that the average germination rate,germination potential,and germination index of the three glandless cotton materials,ZMS12,ZMS16,and ZMS17,were96.78%,80.67%,and 53.73,respectively,which were better than that of corresponding glanded cotton NILs,in which the difference of germination potential was significantly.The seedling growth,biomass,main agronomic characters,and fiber quality of glandless cotton materials were not significantly different from those of glanded cotton materials,but the rotten boll was significantly higher than that of glanded cotton.（2）Melatonin priming could improve seed germination potential,plant growth,and the performance of cotton production.With three concentrations of treatments,melatonin priming could enhance germination potential and germination index of cotton significantly,the Mean relative germination potential to control（CK1）reached 112.21%,and the germination index was16.93%higher than that of CK1.Melatonin priming could also promote the growth and development of seedlings,increasing root-shoot ratio by 19.67%.It also improved the field agronomic traits,increased the number of bolls per plant,boll weight,and lint percentage,and ultimately the fiber yield.For most traits studied,the effect of melatonin priming on glandless cotton was better than that of their glanded cotton NILs.Among three treated concentrations,50μM melatonin was the best one in results.However,there was no significant difference of melatonin priming on fiber quality.（3）Melatonin priming could improve the tolerances to salt and cadmium stress of glandless cotton.The results shown that the stress of salt and cadmium severely inhibited the germination and seedling growth,which significantly reduced the germination rate and germination potential,plants grow slowly and dwarf,and biomass accumulation decreases significantly.The effect of salt stress on glandless cottons was stronger than that of glanded cotton NILs,while the opposite was observed under cadmium stress,but the difference between the two did not reach the significant level.Melatonin priming could significantly improve the salt and cadmium tolerance of the tested cotton materials.Compared with the unprimed control under salt stress（CK1+ST）,the relative germination rate and relative germination potiential of the glandless cottons after melatonin priming reached 157.43%and 290.28%,respectively,and the germination index increased by 14.60%;compared with unprimed control under cadimum stress（CK1+Cd）,the relative germination rate and germination potential were 104.62%and 106.87%,respectively,and and the germination index increased by 14.61%.The effects of melatonin priming on the growth and biomass parameters of the roots,stems and leaves of the tested materials were similar to those of seed germination.Although there were some differences among the materials and priming concentrations in the experiment,melatonin priming on the improvement of salt tolerance in glandless cottons was better than that in their corresponding glanded cotton NILs,while the cadmium tolerance varied with the trait parameters.。（4）Melatonin priming improved the photosynthesis and activities of antioxidant enzymes of glandless cotton.As the results showed that the photosynthesis parameters,antioxidant enzyme activities,and content of chlorophyll and endogenous melatonin of glandless cotton NILs were all higher than glanded cotton,while the MDA content was lower than that of glanded cotton NILs in the condition without any stress,although the differences were not significantly.Salt stress and cadmium stress would seriously affect the photosynthesis and biochemical characteristics of cotton plants,with significant reduction of photosynthetic parameters and antioxidant enzyme activities,and significant increases of MDA contents.The effects of salt stress were more serious on glandless cotton,while cadmium stress was stronger effect on glanded cotton conversely.Melatonin priming improved the various physiological and biochemical indexes of the experimental materials,especially of the glandless cotton under salt and cadmium stress conditions.The results of transcriptome sequencing also proved that under the conditions of salt and cadmium stress,the expression of photosynthesis and antioxidant enzymes related genes of ZMS12W were significantly up-regulated under melatonin priming.Among them,the photosynthesis rate,antioxidant enzyme activity and MDA content of plants primed by 50μM melatonin were not significantly different from those of the control.（5）Melatonin priming optimized the absorption and metabolism of nutrient elements in cotton plants.The 11 elements determined in different treatments of three pairs of glanded-glandless cotton NILs showed that salt stress and cadmium stress would cause significant changes in most nutrient elements,which would affect the growth and development of cotton plant.Cadmium stress caused a large amount of cadmium accumulation in cotton plants(6237.80μg·g^-1),which lead to unbalance of the essential nutrients in plants.Melatonin priming could improve the absorption,metabolism,and accumulation of most nutrient elements in plants,thereby alleviating the effects of salt and cadmium stress on cotton growth.Meanwhile,Melatonin priming could reduce the absorption and accumulation of unnecessary elements such as cadmium,alleviate the unbalance of other elements caused by salt and cadmium stress.（6）Melatonin priming improved the tolerance of the cotton plant via the increase in synthesis of ethylene,jasmonates,and brassinolide,and slow down plant growth and senility by decrease in synthesis of abscisic acid and auxin.Under salt stress and cadmium stress,melatonin priming led to a large number of significant differential expression genes in eight plant hormone signaling pathways,which indicated that melatonin participates in plant hormone signal pathways in response to the stresses.The expression of plant hormone-related signaling pathway genes among the different materials was speciic to salt stress and cadimium stress.（7）The genes in the phenylpropane synthesis pathway were significantly expressed in glanded cotton cotton under stress and melatonin priming,which could resistance to salt and cadimum stress by synthesizing more lignin.While on glandless cotton under salt or cadmium stress,more gossypol and its derivatives were synthesized through significantly up-regulating genes related to the sesquiterpenoids and triterpenoids synthesis pathway.Although,melatonin priming could up-regulate the expression of genes related to gossypol synthesis,the expression levels of genes under salt stress after priming,especially the number of up-regulated genes were not significantly higher than those in the non-stress control.In summary,melatonin priming could increase the germination rate and germination potential of cotton,promote cotton growth and development,and increase fiber yield.This treatment might also pre-regulate the expression of various types of genes to regulate photosynthesis,enhance the active oxygen scavenging system,promote the up-regulated expression of genes related to the synthesis of gossypol with its derivatives and lignin,and coordinate plant hormone signaling pathways to improve the stress tolerance of cotton plants.Therefore,melatonin priming could be used as a production practice to enhance the stress tolerance and improve the production of glandless cotton.