| As a root holoparasitic angiosperm,sunflower broomrape(Orobanche cumana Wallr.)specifically affects sunflower(Helianthus annuus L.)crops in many countries like Spain,Russia,Turkey,and China.O.cumana can cause severe growth retardation and yield loss to sunflowers by subtracting water and nutrients.A series of experimental trials has been conducted to acquire information on chemical,biological and integrated control.However,productive and tiny seeds with long-lasting viability contribute to a large seedbank of broomrapes in soil.The yield losses are observed after infection and before the emergence of parasites,which prevent the predication of critical control period.These characteristics of broomrapes make the current control methods ineffective.Although,the genetic resistance in plants against O.cumana is the most realistic,useful,economically and environmentally friendly strategy to solve the problems,the occurrence of new adapted races of O.cumana can easily overcome this advantage.Thus,there is a require to explore the genetic resistance mechanisms at a molecular level in sunflowers against the O.cumana infection for assistance in breading of resistant cultivars.Meanwhile,To cope with the intractable issue,one approach is to prime the seeds with SA to induce plant defense in sunflowers against O.cumana.The main results were as follows:(1)ROS levels in roots of sunflower cv.JY207 and TK0409 inoculated with O,cumana were increased as compared to their corresponding controls.Inoculation with O.cumana significantly enhanced activities of all antioxidant enzymes including SOD,POD,CAT,APX and GR in the roots of both cultivars.However,the extent to which POD and APX activities increased were close between the two cultivars,whereas more elevation of SOD activity in cv.TK0409 and CAT and GR activities in cv.JY207 were observed.In compatible interaction,abrupt generation of ROS and lack of proper ROS scavenging system leads to oxidative stress while in the case of incompatible interaction,ROS was successfully scavenged by the antioxidant defense system.Therefore,ROS generation and its detoxification by activated antioxidant defense system act as signaling molecules that communicate downstream with defense signals.In addition,lignin content in sunflower roots might be induced by the inoculation with O.cumana and more lignin content was accumulated during resistant interaction,suggesting induction of lignin might be involved in the resistance mechanism.Meanwhile,the germination stimulants in root exudates of sunflowers may also affect the resistance mechanisms.However,the authentic roles of different types of germination stimulants in resistance need to be determined by identification of chemical structures and their corresponding contents in sunflowers.(2)More than 3500 proteins were identified in the roots of both cultivars by iTRAQ analysis,in which 706 proteins were significantly up-or down-regulated by over twofold in cv.JY207 after inoculation with O.cumana as compared to the non-inoculated control,in comparison with total 866 proteins in cv.TK0409.Of the 706 proteins detected in resistant cv.JY207,363 proteins were up-regulated and 343 down-regulated in response to O.cumana,while in susceptible cv.TK0409,more proteins with 361 up-regulated and 505 down-regulated were observed.The main biological functional categories in both cultivars contain small molecule metabolic process,biosynthetic process,cellular nitrogen compound metabolic process,transport,carbohydrate metabolic process and catabolic process.The differentially accumulated proteins were subsequently processed by access to KEGG database,displaying the metabolic pathways including carbon metabolism,oxidative phosphorylation,biosynthesis of amino acids,ribosome,RNA transport etc.The selected up-regulated proteins in resistant cultivar showed stronger associations with the ribosome and oxidative phosphorylation pathways and the proteasome related proteins.While,predicted protein-protein interaction(PPI)for down-regulated proteins of resistant cultivar showed strong association with secondary metabolite pathways.Similarly,in susceptible cultivar,predicted protein-protein interaction was mainly related to ribosomal proteins.Whereas,down-regulated proteins’ predicted PPI revealed strong association between secondary metabolites pathways and carbohydrates metabolism.The resistance of sunflower against O.cumana is probably involved in different signaling pathways and due to an integration of several mechanisms,including the regulation of germination stimulants for broomrape seed,successful recognition of invading parasites,accumulation of defense-related proteins,biosynthesis of lignin,detoxification of toxic metabolites and regulation of plant growth regulators.In addition,whether a plant is susceptible or resistant is probably also determined by the speed,extent and effectiveness to which the resistant mechanisms are activated.The proteins identified in the present investigation are useful tools for reverse genetic studies and their further functional characterization will provide deep insight into resistance mechanism of sunflower during O.cumana infection and also will aid in developing new breeding programs for sunflower.(3)Infection of O.cumana imposed adverse effects on sunflowers.Contents of H2O2 and-OH in the O.cumana-infected sunflower plants were elevated by SA treatment with different concentrations(0.5 and 1 mM)by 7%to 14%and 42%to 114%respectively,and no change in O2-content was found.Activities of APX and CAT were significantly reduced in the roots of O.cumana-infected sunflower plants by SA treatment with different concentrations(0.5 and 1 mM)by 38%to 49%and 18%to 42%,respectively.No significant effects of SA treatment were observed on the activities of SOD in O.cumana-infected sunflowers.Most of the genes such as PR genes,several antioxidant enzyme-related genes and phytohormone-related genes were up-regulated in response to the O.cumana infection,whereas pal,chs and NPR1 were down-regulated by 55%,56%and 34%,respectively and no changes were observed in the expression of Cu/Zn-SOD,GST and hsr.The SA treatment significantly enhanced expression of most genes in the roots of O.cumana-infected sunflower plants,whereas CAT,lox,PR5-1,HACS.1 and chs were not affected by the S A treatment.The results of the present study demonstrate a critical role for S A in the activation of defense responses in sunflower against O.cumana infection,which might be involved in regulation of ROS levels and both enzymatic and non-enzymatic antioxidant systems.The induction of NPR1,PR and other defense related genes suggests high immune response implicated in SAR and HR as well as phytohormones against O.cumana infection with SA application.Seed priming with signaling elicitors provides a promising approach for the better adaptation to biotic and abiotic stresses in crop plants. |
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