Differential expression and regulation of longan XET genes in relation to fruit growth

Xyloglucan endotransglycosylase (XET) catalyses the transglycosylation of xyloglucan, the major hemicellulose polymer, which has been thought to mediate the cross-linkining of cellulose microfibrils in cellular walls and proposed to be involved in the control of cell wall relaxation. To understand the relationship between longan fruit growth and XET gene expression patterns, three XET genes from fruit were identified and then their expression profiles in pericarp and aril tissues of fruit at different development stages were investigated. Three full-length cDNAs of 1077, 1093 and I IS I bp encoding XETs, named DIXET1, DIXET2 and DIXET3, respectively, were isolated from expanding fruit using RT-PCR and RACE-PCR (rapid amplification of cDNA ends) methods. Northern blotting analysis showed that three DIXET mRNAs exhibited different patterns during fruit growth and development. Accumulation of MET] kept changeable during the fruit growth and development, DI-XET2 mRNA accumulations in the pericarp, and in the aril increased gradually following fruit growth during the whole fruit developmental stage. In addition, DIXET3 did not accumulate in the pericarp during the whole fruit development stage, but it could be detected in the aril of fruit during the rapid aril growth stage (63-74 DAA). These results indicated that DIXET2 was associated with growth of pericarp and aril in longan fruit, while DIXET1 was related to pericarp growth and DIXET3 was primarily responsible for aril growth. To further characterize the expressions of three XETs regulated by plant growth substances, alpha-naphthalene acetic acid (NAA) and thidiazuron (TDZ) were used to treat longan fruit at 21 DAA and 56 DAA when pericarp, and aril grew rapidly respectively, it was found that treatment at 21 DAA, only TDZ increased the accumulation of DIXET2 mRNA at 12 h after treatment. However, both NAA and TDZ increased the accumulation of DIXET3 after 36 h treatment, although DIXET-3 did not accumulate in the whole fruit. While, treatment at 56 DAA, both NAA and TDZ increased the accumulations of DIXETs in aril except DIXET3 in aril by NAA treatment. In addition. the expression patterns of the three XETs showed different tissue specificity. These results suggested that XET genes played a different role in longan fruit growth and showed different response to plant growth substances which existed difference at different growth stage. (C) 2007 Published by Elsevier Ireland Ltd.