Physiological Mechanism And Transcriptome Analysis Of Sweet Potato In Response To Low Light Stress

Southwest China is one of the main sweet potato production regions,and the intercropping mode of corn/sweet potato is often used to save land resources and improve the multiple cropping index in this area.As a low-level crop in intercropping,sweet potato has been exposed to low light stress in the symbiotic stage with high-level maize,which seriously affects the normal growth and development of sweet potato.Therefore,it is particularly important to study the effect of low light stress on sweet potato and its adaptive mechanism.So far,the previous researches on sweet potato under weak light stress are mostly related with agronomic traits and physiological and biochemical aspects,and there are few studies on the response of sweet potato at the molecular level caused by weak light stress.In this study,five popular sweet potato cultivars in this region were selected as materials and treated with low light stress through artificial shading,and studying the morphological,physiological and biochemical response and changes of different sweet potato cultivars.Transcriptome analysis was carried out on two sweet potato cultivars with different low light stress tolerance to explore the changes of gene expression level in response to low-light stress and the expression differences of photosynthesis-related gene expression.The main findings are as follows:（1）Effects of low light stress on the growth and development of sweet potatoLow light stress changed the growth morphology of sweet potato,significantly decreased the main vine diameter of sweet potato,and increased its length.The dry matter content of sweet potato vines decreased significantly under weak light,and the impact increases with the prolongation of weak light stress;after recovering to normal light,the dry matter content of vines increased rapidly.The total root length,root surface area and root volume of sweet potato in low light decreased,and among the five cultivars,Yushu 17 declined most in the compensation coefficient of root surface area and root volume.Low light stress significantly increased the shoot-root ratio of sweet potato,changed the biomass distribution ratio of sweet potato,and prolonged the period of biomass increasing,but the expansion of tuberous roots lagged behind.Low light stress caused the yield of sweet potato to decrease to varying degrees,and the yield of Wanshu 7 and Mianshu 6 decreased more.At the same time as the yield decreased,the quality of sweet potato roots also changed,the starch content decreased,the soluble protein content increased,and the soluble sugar content was different due to different sweet potato cultivars.Shading period correlation analysis showed that sweet potato yield was significantly positively correlated with main stem diameter,shoot dry matter content,total root length,root surface area and root volume.（2）Effects of low light stress on leaf structure and photosynthetic physiological characteristics of sweet potatoThe leaf of different sweetpotato cultivars changed greatly and all of them became thinner obviously in weak light,and returned to normal after recovering to normal light.The changes of leaf structure mainly showed that the thickness of palisade tissue and spongy tissue decreased,but the palisade tissue dropped more than the spongy tissue and resulted in a significant decrease in the palisade-to-sea ratio.Because of weak light,the net photosynthetic rate and stomatal conductance of sweet potato leaves decreased significantly,and the intercellular CO₂ concentration increased significantly;after returning to normal light,the photosynthetic parameters returned close to normal.Under the influence of low light,the chlorophyll content of sweet potato leaves increased significantly.Since the increase of chlorophyll b content was more obvious than that of chlorophyll a,the chlorophyll a/b value decreased significantly;the chlorophyll content and chlorophyll a/b of sweet potato leaves was not significantly different from the control after resuming normal light.Low light stress also reduced the activity of Rubisco in leaves,and the activity of the enzyme recovered after returning to normal light,but the difference was not significant compared with the control.Correlation analysis showed that the net photosynthetic rate was significant positively correlated with palisade tissue thickness,spongy tissue thickness and palisade-to-sea ratio;total chlorophyll content was significantly negative correlation with palisade tissue thickness and net photosynthetic rate,and significantly positively correlated with intercellular CO₂ concentration.（3）Transcriptomic analysis of sweet potato leaves in response to low light stressThe gene expression levels of the two tested sweetpotato cultivars changed under low light stress.There were 1173 genes differentially expressed in Yuhongxinshu-4 and7036 genes in Wanshu-7,which were significantly higher than those in Yuhongxinshu-4;The number of differentially expressed genes in the two cultivars decreased significantly after returning to normal light,with 245 and 1453 differentially expressed genes,respectively.In addition,the KEGG enrichment analysis found that the differentially expressed genes of two different low-light-stress-tolerant sweetpotato cultivars had the same enrichment pathway under low light,mainly glutathione metabolism and flavonoid biosynthesis.The differentially expressed genes were all enriched in the pathways of photosynthesis,photosynthetic pigment synthesis and carbon metabolism after returning to normal light.The differentially expressed genes related to weak light stress response in the two cultivars both overlapped and had specificity.Shade period,genes involved in redox chloroplast membrane quinone oxidoreductase homolog（Tai6.44472）and flavonol synthase/flavanone 3-hydroxylase（Tai6.31070）,in Yuhongxinshu-4 and Wanshu-7 was significantly down-regulated;ferredoxin-related genes involved in photosynthesis were significantly down-regulated in Yuhongxinshu-4（Tai6.21350）and significantly up-regulated in Wanshu-7（Tai6.42466）.Photosystem II 22 k Da protein-encoding genes（Tai6.18802,Tai6.19673,Tai6.28593）were up-regulated in Yuhongxinshu-4 after returning to normal light,while genes involved in photosynthesis and carbon fixation were significantly down-regulated in Wanshu-7.In conclusion,low-light stress lead to significant changes in the agronomic morphology,intrinsic physiological response and molecular level of sweet potato.These changes differences vary with different cultivars.This provides a theoretical reference to the cultivation of low-light-tolerance sweet potato varieties and intercropping sweet potato cultivation in the future.