The Effects Of Brassinosteroid Application And Receptor Overexpression On Drought Tolerance And Fruit Quality In Tomato

Drought is a major environmental factor that influences the growth and development of plants,reduces quality and yield of fruit.Tomato?Solarium lycopersicum?is one of the most important and popular vegetable and fruit in worldwide.During its life cycle,a large amount of water is needed for irrigation.Therefore,improvement the drought tolerance and fruit quality of tomato varieties will have great significance.Previous study reported that exogenous application Brassinosteroids?BRs?could relieve the drought damage of plant under drought stress.However,rare research is about the relationship of changed BR level and BR signaling intensity on tomato drought tolerance.This study uses tomato with different BR levels and BR signaling?Micro-Tom and altered brassinolide sensitivity1?to study regulation mechanism of drought tolerance by analyzing plant growth,stomatal characteristics,hormone metabolism,photosynthesis,antioxidant enzyme activities and drought responsive gene expression.Furthermore,using improved BR signaling plants effects on plant growth and development,quality and yield of fruit were investigated.The main results of the study are as follows:1.The effect of exogenous application 2,4-epibrassinolide?EBR?and Brassinazole?Brz?on tomato seedlings under drought stress.This research was about the relationship of BRs level and tomato drought tolerance by exogenous application EBR and Brz,respectively.Application EBR could alleviate plants wilting degree and decrease of leaves relative water content,P_n,F_v/F_m,?PSII and qP.Furthermore,application EBR also improved SOD,CAT,APX and POD activity,and increased expression levels of stress-related and antioxidant-related genes,and finally mitigated electrolyte leakage,hydrogen peroxide?H₂O₂?and superoxide?O₂^-?accumulation under drought stress.In contrast,application Brz accelerated plants wilting degree and damage,and finally decreased drought tolerance of tomato seedling.2.BRs receptor SlBRI1?Solanum lycopersicum BRASSINOSTEROID INSENSITIVE 1?overexpression decreased drought tolerance of tomato.This research was about the regulation mechanism of increased BR signaling intensity and tomato drought tolerance by SlBRI1overexpression.SlBRI1 overexpression reduced water conservation and more electrolyte leakage in leaves.Drought stress significantly enhanced ABA content of all plants.Application EBR up-regulated ABA catabolic genes,CYP707A1,thereby reducing ABA contents in drought-stressed plants.The drought stress led to reduced stomatal width and stomatal apertures,while application EBR alleviated reduction of stomata index and further maintained photosynthetic capacity.However,Sl BRI1 overexpression accelerated stomatal closure,and further decreased photosynthetic capacity and antioxidant enzymes activity.Sl BRI1 overexpression also increased H₂O₂ and O₂^-accumulation which led to enhanced electrolyte leakage and malondialdehyde?MDA?content.In addition,Sl BRI1 overexpression reduced expression levels of stress-related and antioxidant-related genes.All these cumulative effects of SlBRI1 overexpression ultimately decreased the drought tolerance of tomato seedlings.SlBRI1 weak mutant abs reduced endogenous BR signaling intensity,and significantly increased the drought tolerance of tomato seedlings.3.The effect of different time application EBR on tomato seedlings under drought stress.This research was about the relationship of different time application EBR and drought tolerance of tomato.Expression levels of DWARF and CPD were significantly decreased in different time application EBR,and expression were the least in 23d application EBR berfore drought stress plants.23d application EBR berfore drought stress reduced water conservation and more electrolyte leakage in leaves,and drought index,recovery index were higher than other plants.Furthermore,23d application EBR berfore drought stress decreased expression of drought-related genes and antioxidant enzymes activity which resulted in enhanced H₂O₂,electrolyte leakage and MDA contents.Additionally,23d application EBR berfore drought stress reduced proline contents which decreased osmoregulation abilities.In contrast,3d application EBR berfore drought stress alleviated plants wilting degree and damage,and finally increased drought tolerance of tomato seedling.4.Atrd29A promoter induced Sl BRI1 expression,but decreased drought tolerance of tomato.This research was about the regulation mechanism of induced SlBRI1 expression during drought stress and drought tolerance of tomato.With the prolonged drought stress,adversity-induced Atrd29A promoter improved SlBRI1 expression which reduced wilting degree,drought index,recovery index in transgenic plants.Increase SlBRI1 expression further decreased photosynthetic capacity,and inhibited antioxidant enzymes activity and drought-related genes expression.Furthermore,increase Sl BRI1 expression led to enhanced H₂O₂ content and electrolyte leakage,and finally decreased drought resistance of tomato seedling.5.SlBRI1 overexpression increased plant growth and development,quality and yield of fruit.Our results indicated that SlBRI1 expression was very low in roots,higher in flowers and red fruits,and highest in stems and leaves.SlBRI1 overexpression enhanced the endogenous BR signaling intensity thereby increasing the seed germination rate,main root length,hypocotyl length,CO₂ assimilation rate,and fresh weight of aerial part.The transgenic plants exhibited flatter and longer leaves,reduced leaf thickness and leaf mass per area,as well as larger flowers with longer petals and style.SlBRI1 overexpression promoted ethylene production and accelerated fruit ripening by upregulating the expression of ethylene biosynthetic genes ACO1,ACS2 and ACS4.Lycopene biosynthesis related genes?SlDXS,Sl GGPS,SlPSY1 and CYC-B?were also all markedly up-regulated,and finally resulted in significantly increased total carotenoid accumulation,lycopene and lutein contents in the transgenic lines.SlBRI1 overexpression also caused an increase in levels of ascorbic acid,soluble solids and soluble sugars during fruit ripening.SlBRI1 overexpression exhibited a slight increase in fruit yield and number per plant compared with WT,but a decrease in individual fruit weight.Therefore,application EBR could increase drought tolerance of plant before drought stress,while early application EBR and SlBRI1 overexpression improved BR signaling intensity and plant growth potential,but decreased drought tolerance of plant.