Response Of Two Apricot Cultivars Pistils With Different Cold Tolerance To Late Frost And Simulated Late Frost

Apricot（Prunus armeniaca L.）is one of the important cultivated tree in China.However,late frost is a major factor adversely influencing apricot growth and productivity.Therefore,it is important for apricot to clarify the mechanism of cold tolerance.To fully elucidate apricot responses to late frost and simulation late frost,we evaluated the cold tolerance of pistils in two apricot cultivars（cold-tolerant‘Weixuan 1’and cold-sensitive‘Longwangmao’）via morphological,anatomical,and physiological analyses.Additionally,the pistil transcriptomes of the two apricot cultivars were sequenced with RNA-seq technology,after which the detected unigenes were annotated and the differentially expressed genes were screened.Through the comparison of late frost and simulated late frost in apricot,the difference of cold-responsive genes in late frost and simulated late frost was preliminarily revealed.The results of this study provide new insights into the mechanisms underlying apricot responses to frost under natural conditions.The main results are following:（1）Under late frost,‘Longwangmao’and‘Weixuan 1’showed different cold tolerance.The cavities detected in two apricot pistils are likely due to the destruction of some cells or the loss of water from cells.The MDA contents,POD,CAT and SOD activities markedly increased at 6:00,but subsequently decreased at 16:00 in both apricot cultivars.Furthermore,the MDA content was lower in the‘Weixuan 1’pistils than in the‘Longwangmao’pistils;The CAT,POD and SOD activities were higher in the‘Weixuan 1’pistils than in the‘Longwangmao’,which to some extent reflected the stronger antioxidant capacity of‘Weixuan 1’pistils.（2）Three sampling time-points（0:00,6:00 and 16:00）were identified for transcriptome sequencing by observing the temperatures at sampling time-points as well as the morphological changes and anatomical characteristics of the pistils.There are 998 and 503 differentially expressed genes（DEGs）in‘Longwangmao’and‘Weixuan 1’pistils,respectively,which were significantly enriched in metabolic process,catalytic activity and cell part.The DEGs related to Ca²⁺signaling,MAPK cascade and metabolic pathway were also induced by late frost.Transcription factors,such as AP2/ERF,MYB,and WRKY,play key roles in cold tolerance of apricot.Notably,b HLH75,GDSL esterase/lipase EXL3,GDSL esterase/lipase LTL1,and Peroxidase 66 were more highly expressed in‘Weixuan 1’than in‘Longwangmao’under 6:00,suggesting these genes may contribute to the cold tolerance of‘Weixuan 1’.（3）Under simulated late frost,based on the relative conductivity,the semi-lethal temperatures in‘Longwangmao’and‘Weixuan 1’pistils were-2.30℃and-3.44℃,respectively.The MDA contents of two apricot cultivars increased gradually,and the POD and CAT activities increased gradually and subsequently decreased in low temperature stress and recovery period.Compared with the low temperature stress,the MDA contents in recovery period were lower,and the POD and CAT activities were higher.Additionally,the lower MDA contents as well as the higher CAT and POD activities in‘Weixuan 1’than in‘Longwangmao’imply that‘Weixuan 1’pistils are more tolerant to cold stress than‘Longwangmao’pistils.（4）For both analyzed apricot cultivars,RNA was extracted from three biological replicates of pistil samples collected at seven treatments（20℃,-2℃,-3℃,-4℃,-2℃-3h,-3℃-3h,and-4℃-3h）for the subsequent transcriptome sequencing under simulated late frost.Totally 6,306 and 3,573 differentially expressed genes（DEGs）were identified in‘Longwangmao’and‘Weixuan 1’,respectively.Cell,catalytic activity,and metabolic process were significantly enriched.A large number of DEGs associated with metabolic pathways responded to low temperature.Numerous DEGs associated with Ca²⁺siginaling,MAPK cascades,and transcription factors are induced by low temperature.Notably,the expression of Calcium-transporting ATPase 11 and CBF3/DREB1A-like were significant higher in‘Weixuan1’than in‘Longwangmao’,suggesting these genes may contribute to the cold tolerance of‘Weixuan 1’.（5）Under late frost and simulated late frost,the dissected structures of‘Longwangmao’and‘Weixuan 1’pistils were caused by the loss of water and death of the cells.The membrane lipid peroxidation was observed in‘Longwangmao’and‘Weixuan 1’pistils.To resist the damage of cold to plants,the activity of antioxidant enzymes also increased,and then the excessive active oxygen in cells was eliminated.The differentially expressed genes related to Ca²⁺signaling,MAPK cascades and transcription factors were induced by late frost.Through the analysis of KEGG enrichment,the main enriched pathways were related to metabolism.However,there are some differences in the patterns of DEGs under late frost and simulated late frost.These differences may be caused by stress time,cooling rate and other environmental factors（i.e.,wind,humidity,and light）.