A Study On The Roles Of Jasmonic Acid And Salicylic Acid Signaling Pathways In Mediating Defenses Against Whitefly In Tomato

Plants are often attacked by multiple herbivores simultaneously in nature.In response to different herbivores,especially those with different feeding guilds,plants usually activate distinct defense responses.Jasmonic acid(JA),salicylic acid(SA),and ethylene(ET)are recognized as key signaling pathways in mediating plant defense responses.Crosstalk between different signaling pathways was thought to equip plant with a powerful regulatory capacity to finely tune its defense responses,and consequently,enhance its fitness.However,recent studies show that some herbivores can manipulate or interfere with the crosstalk between different signaling pathways to suppress plant effective defenses,which is one of strategies employed by herbivores to disarm plant defense.Bemisia tabaci has been recently recognized as an important invasive pest in China.Previous studies show that B.tabaci has the capability of disarming plant defense.Bemisia tabaci can activate SA-dependent responses and suppress JA-regulated defense responses to their own advantage when feeding on the host plant,such as lima beans and Arabidopsis.Although research on physiological,biochemical and molecular mechanisms of host plants in response to B.tabaci has made a promising advances recently,however,the underlying mechanisms involved in the suppression of plant defenses by B.tabaci still lack conclusive evidence.In this study,we used B.tabaci and its host plant tomato as modle system to analyze the dynamic changes in the defensive response of tomato in response to B.tabaci feeding.Meanwhile,by using tomato mutants that were silenced in different signaling pathways,we further examine the roles of JA-regulated defenses in defending against B.tabaci crosstalk,and determine the role of JA-SA crosstalk in mediating the suppression of plant defenses induced by B.tabaci in laboratory and field.The results are listed as follows: 1.The role of JA-regulated defenses in the defending against Bemisia tabaciThe fecundity and survival rate of B.tabaci adults did not significantly differ on wild-type plants,mutants silenced in JA signaling(def-1 and Spr-2),and mutant in which JA signaling is consitutively expressed(35S).In contrast,the developmental rate of B.tabaci nymphs was significantly accelerated on def-1 and Spr-2,but significantly delayed on 35 S than on wild-type plants.In field,the developmental rate of nymphs was significantly accelerated on def-1,but significantly delayed on 35 S than on wild-type plants.The developmental rate of nymphs on Spr-2 did not differ from wild-type plants.On the other hand,we treated plants with exogenous JA and SA solution to elicit the corresponding defense responses,and thereby,examine the developmental rate of numphs on JA-and SA-treated plants.Our results showed that the developmental rates of B.tabaci nymphs on JA-treated plants were significantly slower than control plants.In contrast,the developmental rates of B.tabaci nymphs on SA-treated plants did not differ from control plants.Taken together,we believe that JA signaling pathway plays a key role in mediating plant defenses against B.tabaci nymphs,but not adults.2.The changes of SA and JA in response to whitefly feedingWe analyzed the kenetic changes of JA and SA levels in undamaged and B.tabaci-infested plants.Our data showed that at the initial stage of infestaion,B.tabaci induced a JA induction,with a peak at 12 h.Thereafter,JA levels in infested plants gradually declined,and reached the level as similar as control plants.In contrast,SA levels in infested plants was consistantly and significantly accumulated along with the increasement in B.tabaci feeding time.In addition,we also quantified the expression levels of three defense genes(Lox-D,PI-I,and PI-II)which are regulated by JA signaling pathway.LoxD showed a significant induction at 12 h after infestation.Thereafter,expression level of LoxD declined along with the increasement in B.tabaci feeding time.Likewise,PI-I and PI-II showed significant induction at 12,48 h after infestation,respectively.Thereafter,he expression levels of PI-I and PI-II declined along with the increasement in B.tabaci feeding time,and finally reached the level as similar as control plants.Phytohoemone and gene-expression data indicate that B.tabaci feeding induces the SA-dependent defenses,but suppresses JA-dependent defenses.3.The role of SA signaling pathway in the suppression of plant defenses induced by Bemisia tabaciIn field,we examined the developmental rate of B.tabaci nymphs reared on wild-type(MM)plants and on NahG plants that are silenced in SA signaling pathway.The developmental rate of B.tabaci nymphs was significantly delayed on NahG plants than on MM plants.Phytohormone analysis showed that B.tabaci feeding induced an enhanced SA level,but a reduced JA level in infested MM plants,which was consistent with phytohormone results in laboratory.In contrast,B.tabaci feeding induced an enhanced JA level in infested NahG plants.Bioassay and phytohormone data demonstrated that the induction of SA signaling activated by B.tabaci feeding play a crucial role in the suppression of JA defenses in tomato.