No abstract

Fresh fruit of longan (Dimocarpus longan Lour.) are susceptible to pericarp browning and aril breakdown. Aril breakdown in longan fruit is regarded as one of the most important factors reducing quality and shortening storage life of the fruit. To better understand the molecular mechanism of aril breakdown, the expression patterns of three expansin (EXP) and three xyloglucan endotransglucosylase (XET) genes in relation to the aril breakdown of longan fruit stored at room temperature (25 °C) or low temperature (4 °C) were investigated. The results showed that aril breakdown index increased progressively during storage at 25 and at 4 °C. Northern blotting analysis revealed that the accumulations of three EXP and three XET genes exhibited differential characteristics with the occurrence of aril breakdown. During storage at 25 °C, the accumulations of Dl-XET3 increased after 1 day, suggesting that Dl-XET3 correlated well with the early aril breakdown, while Dl-EXP3 together with Dl-XET1 and Dl-XET2 was involved in later aril breakdown. However, expression of Dl-XET1 and Dl-XET2 could be mainly involved in aril breakdown of longan fruit stored at 4 °C. In addition, Dl-EXP2, whose accumulation increased sharply when longan fruit were transferred from low temperature to room temperature within 12 hours, was related to the aril breakdown in this storage period. These data indicated that Dl-EXPs and Dl-XETs were closely related to aril breakdown in longan fruit.

In this work, five expansin cDNAs (DlExp1-5) from 'Shijia' longan fruit were isolated and characterized. Moreover, the expression profiles of five expansin genes and the effect of naphthalene acetic acid (NAA) and thidiazuron (TDZ) on their expressions were investigated. The results showed that five expansins exhibited different expression patterns during fruit growth and development. DlExp1 was constitutively expressed in the pericarp while the levels of DlExp1 mRNA in the aril were very high at early stage of fruit development, and decreased gradually from 28 to 77 days after anthesis (DAA). DlExp2 and DlExp4 were related to the growth of pericarp, whereas the expression of DlExp2 and DlExp5 in the aril decreased from 28 to 77 DAA. In addition, NAA and TDZ applied at the stage of rapid pericarp (21 DAA) or aril growth (56 DAA) increased the accumulations of DlExp1 and DlExp2 mRNA in the pericarp and aril, while NAA and TDZ had no or little effect on the accumulations of DlExp3, DlExp4 and DlExp5. DlExp1 and DlExp2 also accumulated highly in rapidly growing tissues, such as young stems and leaves. These findings indicated that Exp genes played a different role in longan fruit growth and showed different response to plant growth substances.