Biological Role Of EXP In Postharvest Fruit Softening And Lignification

Kiwifruit (Actinidia deliciosa cv. Bruno) and loquat (Eriobotrya japonica cv. Luoyangqing and Yingtiaobaisha) were used to investigate the postharvest texture changes and the EXP genes expression patterns during the storage life of fruits. The regulations of EXP expression under ethylene, ASA, 1 -MCP, LTC treatments were also studied in the experiment. Results were as following:Two cDNA fragments encoding EXP genes, i.e Ad-EXP1 and Ad-EXP2, were isolated from ripening fruit of kiwifruit by RT-PCR amplification using a pair of degenerate primers. The homology between these two members was 50.2% in nucleotide sequence and 73.9% in amino acid sequence, and it was revealed that Ad-EXP1 and Ad-EXP2 belong to subgroup A and subgroup B of EXP genes, respectively. However, gene expression of both members shared similar fruit ripening specific patterns, which was up-regulated by ethylene treatment and down-regulated by acetyl salicylic acid (ASA) treatment. The study suggests that members of both subgroup A and subgroup B of EXP family were involved in fruit ripening of kiwifruit, with higher expression of Ad-EXP1 than that of Ad-EXP2 in the ripening kiwifruit.Four cDNA fragments encoding EXP genes, i.e. Ej-EXP1, Ej-EXP2, Ej-EXP3 and Ej-EXP4, were isolated from loquat fruit. Phylogenetic analysis showed that Ej-EXP1 and Ej-EXP2 belong to subgroup C while Ej-EXP3 and Ej-EXP4 belong to subgroup B of EXP gene family. In 'Luoyangqing' loquat, fruit firmness increased during the postharvest storage at 0℃, which was the chilling injury temperture of loquat. Fruit lignification occurred in both occations. during the postharvest storage at 0℃. LTC and 1-MCP treatment allevated the chilling injury induced lignification. Our results showed that Ej-EXP1 mRNA accumulated during chilling injury temperature and the increasing mRNA level of Ej-EXP1 was consistent with the accumulation of lignin.While chilling injury symptoms could be alleviated by LTC and 1-MCP treatments, the accumulation of Ej-EXP1 mRNA expression was also inhibited. The expression pattern of Ej-EXP1 in 0℃indicated its possible role in chilling injury induced lignification of loquat fruit. Ej-EXP2, Ej-EXPS and Ej-EXP4 showed very low expression level in 'Luoyangqing' loquat fruit. In 'Yingtiaobaisha' loquat, fruit firmness decreased during the postharvest storage life at 20℃and fruit softening occurred, while 0℃storage delayed such firmness decrease and fruit softening. The expression of Ej-EXP1 and Ej-EXP4 increase during the first 2 d and decreased afterwards under both 20℃and 0℃conditions, only that the expression was inhibited by the low temperature. Both Ej-EXP2 and Ej-EXP3 showed very low expression level in 'Yingtiaobaisha' loquat fruit.In a conlcuison, Ad-EXP1 and Ad-EXP2 from subgroup A and B, respectively, and Ej-EXP1 and Ej-EXP4 from subgroup C and B, respectively, were involved in fruit ripening and softening in kiwifruit or 'Yingtiaobaisha' loquat fruit, respectively. In addition Ej-EXP1 was also involed in the chilling induced lignification in 'Luoyangqing' loquat fruit, while Ej-EXP1 and Ej-EXP1 might play minor role during the postharvest textrure changes of loquat fruit. Our study showed the diversity and overlapping expression pattern of different members of EXP gene family.