Identification Of Cold Resistance And Preliminary Study On Mechanism Of Intergeneric Hybrids Between Longan And Litchi

Longan?Limocarpus longan Lour.?and litchi?Litchi chinensis Sonn.?are the most important economic fruit trees in South China.In this study,litchi,longan and their true intergeneric hybrid populations were used as materials to establish a comprehensive evaluation method of cold resistance,and the cold resistance of intergeneric hybrid populations and photosynthetic characteristics were analyzed.Combining transcriptome data to perform functional classification and regulatory pathway analysis of persistently specifically expressed genes in cold resistanceand non-cold-resistant germplasm at different time periods.The cold resistance mechanism of intergeneric hybrids was initially investigated.The main results were list as follows:1.A total of 9 representative litchi,longan and intergeneric hybrids were selected from 3representative species.The cold-resistance comprehensive evaluation function F=-0.2733LT₅₀-0.2674MDA+0.2335SS-0.2256Pro was constructed by measuring its low temperature lethal temperature and relevant cold-resistance physiological indexes,combining with correlation analysis and principal component analysis.LT₅₀0 is the low temperature lethal temperature of plants;MDA is the rate of change of malondialdehyde content from-4^oC to-6 ^oC;SS is the change rate of soluble sugar content from-6 ^oC to-8 ^oC.Pro is the rate of change of proline content from 0 ^oC to-2 ^oC.As there are extremely negative correlations between the cold resistance comprehensive score F and LT₅₀0 of 9materials,the correlation coefficient is as high as 0.970,indicating that LT₅₀0 can represent these comprehensive indicators and evaluate the cold resistance of litchi and longan more accurately and objectively.2.49 SZ?‘Shixia'בZiniangxi'?intergeneric hybrids and their parents were used as materials,The cold tolerance of the population was determined by measuring their LT₅₀.The cold resistance of the population was normally distributed and widely separated.The average LT₅₀0 was-4.55 ^oC,the coefficient of variation was 55.71%,the proportion of super-high parents was 21.28%,the proportion of ultra-low parents was 57.45%,the super-dominant dominance rate is 12.24%,of which SZ71 had the strongest cold resistance,LT₅₀is-8.1 ^oC,SZ2 has weaker cold resistance,and LT₅₀0 is-1.38 ^oC.3.The net photosynthetic rate and chlorophyll fluorescence parameters of each SZ population in winter and summer were measured respectively.Correlation analysis showed that there was a significant negative correlation between the LT₅₀0 and the net photosynthetic rate in winter.The correlation coefficient was-0.240^*.Among the 12measured chlorophyll fluorescence parameters,there was a significant correlation between the measured values of q_N and Y?NO?in winter and the low lethal temperature of plants,and the correlation coefficients were-0.256^*and 0.320^*.These photosynthetic indicators can play a very helpful role in the evaluation of cold resistance.4.Through phenotypic observation,it was found that with the extension of the low temperature treatment,the area where plant leaves suffer from cold damage continues to grow as water-stained leaf spot.Ultrastructural observation revealed that the chloroplasts of the non-cold-resistant germplasm left the cell wall,the lamellar structure was disordered,the mitochondria became foamed,and the internal hemorrhoids disappeared at 0^oC treatment for 6 h.The cold-resistant germplasm had no structural abnormalities of the organelles.By observing the ultrastructure at this time point,it can be used to determine the cold-resistant germplasm and non-cold-resistant germplasm in the Shizi population.5.Using cold-resistant germplasm‘SZ71'and non-cold-resistant germplasm‘SZ2',through low temperature phenotype observation,chlorophyll fluorescence imaging system and ultrastructural observation to determine the key time points?0h,3h,6h?for low temperature processing for transcriptome Sequencing.The GO functional annotation and KEGG metabolic pathway analysis were performed on 402 DEGs of different cold resistant germplasm at different time periods.These genes mainly include metabolic processes and catalytic activities,and are mainly involved in flavonoid biosynthesis,starch and sucrose metabolism,galactose metabolism,and carbon fixation during photosynthesis.It is indicated that the molecular mechanism of the low temperature response of interspecific hybrids in litchi and longan is regulated by multiple gene networks.Among these 402differential genes,five candidate genes related to cold resistance were selected:ERF2,GATL8,CBF3,PBP1 and CAX.