The Role Of Melatonin In Alleviating Aluminum Toxicity In Wheat(Triticum Aestivum L.)Genotypes Differing In Aluminum Tolerance

Aluminum(Al)toxicity is a major limiting factor of crop production on acid soil,which comprise 30%of the global ice-free land or 50%of the worlds arable land.Plants suffering from Al toxicity manifest obvious and typical symptoms in root morphology(stubby,thick,swelling or brown roots)and cell ultrastructure.Inhibition of root growth is a common symptom of Al toxicity in plants,and is generally considered as the ultimate consequence of many damaging reactions in cells,including lesion of cell walls,disruption of plasma membranes,damage to microtubules,mitochondria dysfunction,disruption of signal transduction,reduction of DNA duplication activity.Melatonin(N-acetyl,5methoxytryptamine)is an amphiphilic low-molecular-weight compound found in evolutionary distant living organisms,from bacteria to mammals.It can be synthesized by plants and acts as a potent antioxidant and/or a regulator of plant growth and development.Most importantly,melatonin(MT)protects plants against biotic and abiotic stresses by activating various physiological systems and mechanisms,including those that involve antioxidants,the ascorbate-glutathione cycle,ascorbic acid,and photosynthesis.However,the details of how melatonin regulates Al stress responses in wheat are unknown.In the present study,two wheat genotypes with different Al tolerance Xi Aimai-1(Al-tolerant)and Yangmai-5(Al-sensitive)are employed to investigate the content changes of MT in root tips of wheat,the role of exogenous MT application in Al tolerance strategies,possible association with MT and antioxidation capability in regulating Al tolerance,the effect of MT on cell wall composition as well as its subsequent Al-binding capacity under Al stress.Main results are as follows:(1)Melatonin,an evolutionarily highly conserved molecule,plays important roles in the regulation of plant growth and development and defense against biotic and abiotic stresses.Here we investigaed the responses of MT content in root tips of wheat to Al stress,the effects of MT pretreatment on H2O2 and O2·-production,oxidative damage and root elongation in wheat when seedlings were exposed to Al stress.The results showed that after 24 h of exposure to 30 ?M Al,endogenous MT content elevated significantly in two wheat genotypes,especially in Al-tolerant Xi Aimai-1.An application of 10 ?M MT significantly elevated endogenous MT content and reduced Al-induced root inhibition and H2O2,O2·-accumulation.The addition of melatonin also lessened the amount of oxidative damage brought on by Al stress.These results are the first evidence that MT has an important role in alleviating the inhibitory effect of aluminum stress.(2)The possible association with MT and antioxidation capability in regulating aluminum(Al)tolerance of wheat was investigated using two genotypes with different Al tolerance.Exposure to Al triggered ROS generation in wheat roots,however,exogenous MT pretreatment significantly reduced high ROS accumulation in root tips under Al stress.Increased melatonin levels was capable of increasing GSH,AsA levels and subsequently depressed Al-induced production of high levels of DAH and GSSG,thus maintained higher levels of GSH/GSSG and AsA/DAH ratio in both wheat genotypes under 6 h Al exposed.Moreover,pretreatment with MT was linked with much higher SOD,CAT and POD activities compared with seedlings receiving no such pretreatment after 6 h Al treatment,MT pretreatment significantly enhanced the acitivity of SOD,POD,CAT in Al-tolerant Xi Aimai-1;POD,CAT in Al-sensitive Yangmai-5.These results clearly suggest that MT protects wheat root against Al-induced oxidative stress,possibly through stimulating antioxidant enzymes or augmenting the activities of other antioxidants.(3)Using two wheat genotypes Xi Aimai-1(Al-tolerant)and Yangmai-5(Al-sensitive),we provide insight into the role of MT in regulating the composition and propeties of cell wall under Al stress.We found that Al exposure caused more Al accumulation in Yangmai-5 than Xi Aimai-1,MT treatment significantly reduced Al accumulation.Application of 10 ?M MT,however,have little effect on Al-induced malic acid efflux.The decrease in Al accumulation in root tips after MT application was mainly due to decrease bindings sites for Al in the cell wall.Al exposure triggered more remarkable increasing of cell wall polysaccharides(pectin and hemicellulose)and pectin methylesterase(PME)activity in Yangmai-5 than Xi Aimai-1,whereas MT pretreatment contributed to decrease cell wall polysaccharides(pectin,hemicellulose 1 and hemicelluloses 2)and the activity of PME,thus significantly decreased the levels of Al bound to pectin and hemicellulose and depressed Al accumulation in wheat root tips.These results revealed that MT pretreatment decreased cell wall polysaccharides content and PME activity and subsequently reduced its sequestration of Al,and positively regulated Al tolerance in wheat.