Research articleDifferential expression of litchi XET genes in relation to fruit growth1
Introduction
Litchi (Litchi chinensis Sonn.) is a subtropical to tropical fruit of high commercial value in international trade. Fruit cracking during growth and development is an on-going problem that results in a great loss of yield and commercial value [9], [10], [13], [14]. In particular, the high-priced cultivars, ‘Nuomici’ and ‘Guiwei’, suffer serious fruit cracking [9], [13], [33]. Much attention has been paid to biochemical and physiological mechanism of litchi fruit cracking [1], [10], [11], [13], [15], [28], [33], but the mechanisms are still not fully understood.
Fruit growth and development involves cell wall loosening. It is well known that the plant cell wall is a complex network of cellulose, hemicelluloses, pectins and structural proteins [3], [4], [12], [17], [24]. Xyloglucan is the principal hemicellulose component of primary cell walls of dicotyledonous plants [8], [21], [23]. Most probably, modification of cellulose-xyloglucan cross-links is a key step in controlling cell extensibility. Xyloglucan endotransglycosylase (XET, EC 2.4.1.207) is an enzyme mediating the reversible formation of xyloglucan cross-links and catalyzing molecular grafting of newly arriving xyloglucan molecules into the cell wall structure [6], [7], [12], [29]. Thus, XET emerged as an important candidate for a wall-loosening enzyme [7], [18], [20], [26]. However, the role of XET in litchi fruit growth and development in relation to fruit cracking is unclear. In practice, the auxin α-naphthalene acetic acid (NAA) is used to reduce fruit cracking of ‘Nuomoci’ [10], [13]. Unfortunately, there is little information on the auxin-regulated expression of modified-cell wall genes during fruit growth and development.
The objective of the work was to identify XET genes. The biotechnology interest of this investigation can be indicated further the mRNA accumulation profiles of XET genes in pericarp and aril tissues and the relationship between XET gene expression patterns and fruit growth, using cracking-resistant and cracking-susceptible cultivars.
Section snippets
Isolation and sequence analysis of cDNAs encoding for litchi XET
XETs are encoded by a multigene family [4], [10]. In this study, three full-length cDNAs, named LcXET1, LcXET2 and LcXET3, respectively, were cloned from aril tissues of mature litchi fruit by a combination of RT-PCR and 5′- and 3′-RACE (Fig. 1). The sequences of LcXET1, LcXET2 and LcXET3 of litchi were registered in the GenBank, with the corresponding accession numbers of DQ995515, DQ995514 and DQ995516. BLAST search of GenBank revealed that LcXET1 shared 83% identity with that of PaXET and
Plant materials
Three 10-year-old trees of ‘Nuomici’ (cracking-susceptible variety) and ‘Huaizhi’ (cracking-resistant variety) litchi (L. chinensis Sonn.) were selected in a commercial orchard near Guangzhou, Guangdong, China in 2004. Twenty panicles located in different directions of each tree were tagged. In addition, 10 fruits from each tree for the two varieties were used to measure fruit diameter on a 1- or 2-week basis for a period of 7 weeks, beginning at 31 DAA (April 30, 2004) and ending at 80 DAA
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The first two authors contributed equally to this work.