Occurrence Of Chilling Injury And Regulation Of ROP Gene Expression In Postharvest Loquat And Peach Fruits

Plant ROP is regarded as a switch in responses to environmental changes and important roles ROP proteins played in plant defense against various biotic or abiotic stresses have been reported,with the exception of low temperature stress.In the present study,physiological behaviors with respect to different postharvest temperature treatments and the involvement of ROP gene expression were investigated in loquat(Eriobotrya japonica cv.Luoyangqing) and peach[Prunus persica(L.) Batsch,cvs.Hujingmilu and Yulu]fruits.Effects of low temperature conditioning(LTC) treatment on regulation of ROP expression and the relationship between ROP and chilling stress were analysed.The main results were as followings:1.Lignification occurred in 'Luoyangqing' loquat fruit after 8 d storage at 20℃with flesh firmness changed from 4.15 N to 5.87 N,lignin content and EC increased by 53%and 19.7%as compared with those on the harvest day,respectively. Lignification also occurred as apparent symptom of chilling injury in 'Luoyangqing' loquat fruit after 8 d storage at 0℃with flesh firmness changed from 4.15 N to 5.92 N, lignin content and EC increased by 79%and 20.7%,respectively.LTC treatment(fruit conditioned at 5℃for 6 d before 0℃storage) alleviated postharvest chilling injury of loquat fruit.The flesh firmness,lignin content and EC of LTC-treated fruit was 82%, 70%and 74%of those of fruit stored at 0℃,respectively.Fruit ripening and senescence occurred rapidly for postharvest 'Hujingmilu' and 'Yulu' peach fruits stored at 8℃,and decay rate was extremely high during shelf life.Flesh browning occurred as a chilling injury symptom of 'Hujingmilu' and 'Yulu' fruits stored at 5℃and 0℃,and the ripening process was arrested by 0℃storage after the transfer of the fruits to the shelf(20℃).However,the LTC-treated fruit ripened normally,and in which both fruit chilling injury and decay were significantly inhibited.Both total sugar content and sucrose content declined in peach fruit under all temperature treatments tested,with a maximum decrease found in fruit stored at 5℃,and the second in fruit stored at 0℃.2.Four ROP cDNA fragments,i.e EjROP1.1(808 bp),EjROP1.2(756 bp), EjROP2(818 bp) and EjROP3(881 bp),were isolated from ripening loquat fruit,all of which shared over 80%nucleotide identity with known ROPs from other plants. Sequence analysis revealed that EjROP1.1,EjROP2 and EjROP3 might be functional while EjROP1.2 might be a dominant negative mutation resulted from a 65 bp deletion in the corresponding nucleotide sequence as compared with EjROP1.1.Two peach ROP genes,i.e.PpROP1 and PpROP2,555 bp and 861 bp in length, respectively,were identified from peach fruit with a combination of EST BLAST and RT-PCR amplification.Phylogenetic analysis showed that EjROP3 belong to typeⅡROP protein and the other five ROPs from loquat or peach clustered to typeⅠ.3.Expression of EjROP1.2 was constitutively low at 20℃in loquat fruit. However,such gene expression was induced dramatically in fruit stored under low temperatures,with 2.5-3 times increase within 6 h at 5℃and 0℃.Transcripts of PpROP1 and PpROP2 showed the highest level in chilling injured peach fruit stored at 5℃and it increased gradually with the development of fruit chilling injury symptoms.Expression of EjROP1.2,PpROP1 and PpROP2 were all inhibited by LTC treatment,which alleviated chilling injury of loquat or peach fruit as well,indicating the involvement of ROP genes in low temperature stress response in different fruits.4.EjROP1.1,EjROP2 and EjROP3 showed similar expression pattern in loquat fruit stored at 20℃and 0℃.All of them increased during the postharvest storage and peaked at 4 d or 6 d after harvest and then declined.Expression of EjROP1.1,EjROP2 and EjROP3 correlated well with the increse in flesh firmness and lignin content, suggesting that these three ROP genes might be involved in lignification of postharvest loquat fruit resulted from either senescence or chilling injury.EjROP1.2 might function as a dominant negative protein,and then regulate loquat lignification with feedback mechanism.