Differential expression of litchi XET genes in relation to fruit growth¹

Abstract

Xyloglucan endotransglycosylase (XET) catalyses the transglycosylation of xyloglucan, the major hemicellulose polymer, which has been thought to mediate the cross-linking of cellulose microfibrils in cellular walls and proposed to be involved in the control of cell wall relaxation. To understand the relationship between litchi fruit cracking and gene expression patterns, three XET genes from litchi fruit were identified and then examined for their expression profiles in pericarp and aril tissues at different development stages, using a cracking-resistant cultivar, ‘Huaizhi’, and a cracking-susceptible cultivar, ‘Nuomici’. Three full-length cDNAs of 1267, 1095 and 1156 bp encoding XETs, named LcXET1, LcXET2 and LcXET3, respectively, were isolated from expanding fruit using RT-PCR and RACE-PCR (rapid amplification of cDNA ends) methods. Northern blotting analysis showed that LcXET1 mRNA accumulation occurred much earlier in aril tissues at 59 days after anthesis (DAA) than in pericarp tissues at 73 DAA in ‘Nuomici’. However, it appeared at almost the same time (66 DAA) in pericarp and aril tissues in ‘Huaizhi’, which suggested that differential accumulation of LcXET1 in pericarp and aril tissues in ‘Nuomici’ and ‘Huaizhi’ was closely associated with fruit cracking. LcXET2 mRNA accumulation could be detected in pericarp and aril tissues throughout fruit development but exhibited a differential accumulation pattern between pericarp and aril tissues. In the aril of ‘Nuomici’, intensive signal bands were detectable at 59−73 DAA in rapidly expanding fruits of ‘Nuomici’ but only weak bands could be found in the pericarp tissues. In contrast, moderate signal bands were detectable both in pericarp and aril tissues of ‘Huaizhi’ fruits. Furthermore, LcXET3 showed constitutive expression in both pericarp and aril tissues of developing ‘Nuomici’ and ‘Huaizhi’ litchi fruit. In addition, differential expression patterns of three XETs genes were observed in different tissues of litchi, with only LcXET1 being fruit-specific. To further address the role of LcXET in fruit cracking, α-naphthalene acetic acid (NAA) was used to treat ‘Nuomoci’ to reduce fruit cracking. Enhanced LcXET1 mRNA accumulation appeared in pericarp while LcXET2 and LcXET3 mRNA accumulation enhanced in aril tissues in the NAA-treated fruits. Thus, LcXET1 is more likely to play a role in reducing litchi fruit cracking than LcXET2 and LcXET3.

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.