Roles Of Controlled Atmosphere Storage On Tomato Fruit Volatile Compounds

Aroma directly reflects the freshness of horticultural products and is an important quality index.Fruit aroma is determined by volatiles that are derived from secondary metabolites.More and more studies have shown that postharvest storage is an important factor affecting the content of aroma volatiles in fruits.Excessive pursuit of shelf life leads to aroma loss resulting in reduction of flavor.Controlled atmosphere storage（CA）is widely used for long-term storage of horticultural products in developed countries.However,the regulatory effect and mechanism of CA on fruit aroma remains unclear.In this study,using model fruit tomato as material,the regulation effect of CA on content of aroma volatiles was analyzed by headspace solid phase microextraction（HS-SPME）combined with gas chromatography-mass spectrometry（GC-MS）.Moreover,using Micro-Tom tomato SlERF.E1 transgenic materials,the mechanism of CA regulating fruit aroma volatiles was studied.The main results are as follows:1.Postharvest CA treatment effectively reduced the loss of aroma volatiles in tomato fruits caused by low temperature storage.The content of aroma volatiles in red ripe tomato fruit（BR+7）stored at 4℃for 28 days（C28d）decreased significantly,which was about 57%of that at the day of harvest（Day 0）.Among them,the loss of aroma volatiles derived from fatty acids and branched chain amino acids pathway was about 73%and 58%,respectively.The aroma volatile content of tomato fruit could not be restored even if the tomato fruit after a 1 d storage at 20℃（C28d S1）.CA treatment（3%O₂+4%CO₂）could effectively reduce the loss of aroma volatiles in tomato fruits caused by low temperature storage.PLS-DA analysis with aroma volatiles as independent variables showed that the tomato fruit of CA and RA had different distributions.14 aroma volatiles with important contribution to the distinction between CA and RA fruits were identified by calculating the importance projection（VIP）values of variables,including linalool,6-methyl-5-hepten-2-one（MHO）,C6 aldehydes and C6 alcohols.The contents of carotenoid derivative MHO and terpene aroma linalool in tomato fruits treated with CA were higher than those in control fruits.2.SlERF.E1,a member of ERF-VII subfamily,whose transcript content was significantly accumulated in response to CA treatment.Previous studies have shown that ERF-VII subfamily is a key transcription factor for plants to sense hypoxia.The transcripts of 5 members of tomato ERF-VII subfamily during postharvest CA storage were detected by RT-q PCR.The results showed that the content of SlERF.E1 transcripts in tomato fruits increased with the extension of postharvest storage time.Compared with the control tomato fruit of RA,the content of SlERF.E1transcripts of tomato fruits stored in CA for 7 days and 28 days increased by about 3 times and 2times,respectively.There was no significant difference in transcript content of other 4 members of tomato ERF-VII subfamily between CA and RA fruits during storage.3.Overexpression of SlERF.E1 in transgenic tomato fruits significantly changed the content of aroma volatiles in tomato fruits.We constructed a transgenic line 35S::SlERF.E1（OE in short）with the full-length overexpression of SlERF.E1 sequence and another line 35S::Δ13::SlERF.E1（d13 in short）with the conserved motif at the N-terminus of the SlERF.E1 sequence deleted.The DFA results of electronic nose analysis showed that there were obvious differences among OE,d13 and WT fruits during ripening.GC-MS results showed that the total content of aroma volatiles in OE tomato fruits was significantly higher than that in WT,at the ripening stages of BR,BR+3and BR+7,mainly characterized by a significant increase in the contents of aroma volatiles derived from carotenoids and phenylpropanoids.The carotenoid-pathway volatiles accumulated in d13 fruits,but the contents of volatiles derived from fatty acids,branched chain amino acids and phenylalanine decreased,resulting in a total content lower than that of WT.4.Contents of volatiles from SlERF.E1 transgenic tomato fruit（OE）after CA storage were significantly higher than that of the control.Compared with WT,d13 and OE tomato fruits treated with CA,the aroma volatile content of OE fruit for C28d S1 was higher than that at the day of harvest（Day 0）,which achieved the effective maintenance of aroma volatiles after long-term low temperature storage.The contents of aroma volatiles those derived from fatty acids,carotenoids,branched chain amino acids,phenylpropanoids,phenylalanine and terpenes were 1.4,2.1,3.5,3.3,3.3,3.7,6.8 times higher in OE fruit for C28d S1 than those in WT fruit,respectively.Based on PLS-DA analysis,12 aroma volatiles with VIP value greater than 1 were identified.The content of MHO in OE tomato fruits treated with CA was significantly higher than that of RA.