Phenolic Biosynthesis And Accumulation In Fresh-cut Pitaya Fruit And The Possible Regulatory Mechanisms

Fresh-cut fruits and vegetables have been one of the important part in the urban market attributed to its freshness,convenience,health and nutrition.During the fresh-cut processing,the tissure is inevitably subjected to wounding stress,which will cause a series of adverse physiological reactions including nutrient and water loss,tissue browning,microbial growth.However,fruits and vegetables have strong adaptability to unfavorable environmental stresses as living organisms.In addition to accelerating the quality deterioration,wounding stress can induce defense reaponses of fruits and vegetables to produce more secondary metabolites to defend and heal the wounding damage.Phenolic compounds are the most common secondary metabolites and one of the most important antioxidants in fruits and vegetables.The accumulation of phenolic compounds enhances the nutritional quality of fruits and vegetables.For a long time,people have done many studies to investigate the adverse effect of wounding stress,little research was concern about the wound-induced secondary metabolism in fresh-cut fruits and vegetables during earlier storage.Consequently,on the base of maintaining the sensory quality and safety of fresh-cut fruits and vegetables,abiotic stresses including wounding stress can be applied to induce the accumulation of phenolic antioxidants and enhance the nutritional quality of certain fresh-cut fruits and vegetables.This paper took pitaya fruit as experimental materials,effect of different cutting styles on phenolic accumulation and antioxidant activity in pitaya fruit was investigated.Fresh-cut pitaya fruit which has the highest phenolic accumulation was selected for further r4esearch.Different storage temperature,reactive oxygen species(ROS)production inhibitor(diphenyliodonium iodide,DPI)and elicitor(glucose/glucose oxidase,G/GO)were utilized respectively to evaluate the role of ROS in wound-induced phenolic accumulation in fresh-cut pitaya fruit Moreover,safe and effective chamical(methyl jasmonate,MeJA)and physical(cold plasma,CP)method were selected respectively to investigate the effect of these abiotic stresses on the accumulation of phenolic compounds in fresh-cut pitaya fruit.Further,the molecular mechanisms of the wound-induced phenolic accumulation in fresh-cut pitaya fruit was elucidated by transcriptome method.The obtained information in this paper may provide foundations for the process and scientific diet of fresh-cut pitaya fruit.The results were as follows:1.The effect of different cutting styles(with wounding intensities:Whole,0 cm2 g-1;Slice,2.0 cm2 g-1;Half-slice,2.9 cm2 g-1;Quarter-slice,3.7 cm2 g-1)on phenolic content and antioxidant capacity of pitaya fruit during 4 d of storage at 15℃ was investigated.Results showed that cutting process somewhat caused quality deterioration,but no significant difference was observed among three cutting styles(p>0.05).On the other hand,wounding stress significantly activated phenylalnine ammonialyase(PAL)activity,induced phenolic accumulation and enhanced antioxidant activity of pitaya fruit.These phenomena were more obvious in the cutting style with higher degree of wounding intensity(p<0.05).2.Different storage temperature(5℃,10℃,15℃),ROS production inhibitor(DPI)and elicitor(G/GO)were utilized respectively to evaluate the role of ROS in wound-induced phenolic accumulation in fresh-cut pitaya fruit.Results showed that higher storage temperature promoted the production of ROS,activated higher PAL activity,enhanced the content of total phenolics and improved antioxidant activity of fresh-cut pitaya fruit.Moreover,treatment with DPI dramatically inhibited ROS generation,PAL activity and total phenolic content,while G/GO treatment significantly increased ROS level,PAL activity and phenolic accumulation(p<0.05).These results demonstrate that ROS play an important role in wound-induced phenolic accumulation in fresh-cut pitaya fruit.3.The regulating role of MeJA pretreatment on the wound-induced phenolic biosynthesis and accumulation in pitaya fruit and its possible mechanism were investigated.Results showed that MeJA pretreatment effectively inhibited the wound-induced decrease of organic acids,accelerated the consumption of sugars,stimulated higher energy status and further enhanced the wound-induced phenolic accumulation in fresh-cut pitaya,indicating that MeJA together with wounding stress can synergistically induce the utilization and transformation of carbohydrates to provide essential precursors and energy for the wound-induced phenolic accumulation in fresh-cut pitaya fruit.On the other hand,MeJA treatment alone did not show any significant physiological response in intact fruit,however,pitaya fruit pretreated with MeJA and followed by subsequently cutting process showed an augmented capacity to enhance defense responses against wounding stress.These enhanced responses included ROS burst,increased gene expression and activity of key enzymes in phenylpropanoid pathway,improved phenolic accumulation and antioxidant activity.These results demonstrate that MeJA pretreatment can effectively enhance the wound-induced phenolic accumulation of pitaya fruit through priming of defense responses instead of direct induction.4.The effect of CP treatment on the safety and phenolic accumulation in fresh-cut pitaya fruit was investigated.Results showed that CP not only efficiently inhibited the increase of total bacterial count,maintained well safety of fresh-cut pitaya fruit,but also promoted the consumption of fructose,enhanced energy status,accelerated ROS production at earlier storage,stimulated higher gene expression and activity of key enzymes in phenylpropanoid pathway,and further enhanced the wound-induced accumulation of phenolic compounds in fresh-cut pitaya fruit.These results demonstrate that CP can induce the accumulation of phenolic compounds by amplifying the ROS signal in fresh-cut pitaya fruit.5.The possible molecular regulatory metabolism in wound-induced phenolic accumulation of fresh-cut pitaya fruit was investigated by transcriptome sequencing project.Results showed that wounding stress induced up-regultion of most genes associated with signal transduction,substance metabolism,energy metabolism,ROS production and phenolic biosynthesis,enhanced their transcription and expression level and activated many transcription factors responsible for defense responses in fresh-cut pitaya fruit.These results demonstrate that wounding stress induces transcription and expression of genes involved in primary and secondary metabolism by regulating its signal transduction system.These results validate previous physiological results of wound-induced material consumption,energy metabolism,ROS production and phenolic accumulaition,elucidating the molecular mechanism of wound-induced defense responses in fresh-cut pitaya fruit in transcriptional level,and provide molecular biological information for further functional verification of transcription factors in pitaya fruit.