Lu W, Wang Y, Jiang Y, Li J, Liu H, Duan X, Song L
College of Horticultural Science, South China Agricultural University, Guangzhou 510642, China.
Plant Physiol Biochem. 2006 Nov-Dec;44(11-12):707-13. doi: 10.1016/j.plaphy.2006.09.020. Epub 2006 Oct 12.
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, alpha-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.
木葡聚糖内转糖基酶（XET）催化木葡聚糖的转糖基化反应，木葡聚糖是主要的半纤维素聚合物，一直被认为介导细胞壁中纤维素微纤丝的交联，并被认为参与细胞壁松弛的调控。为了了解荔枝果实裂果与基因表达模式之间的关系，从荔枝果实中鉴定出三个XET基因，然后使用抗裂品种‘淮枝’和易裂品种‘糯米糍’，检测它们在不同发育阶段果皮和果肉组织中的表达谱。采用RT-PCR和RACE-PCR（cDNA末端快速扩增）方法，从膨大果实中分离出三个分别编码XET的全长cDNA，长度分别为1267、1095和1156 bp，分别命名为LcXET1、LcXET2和LcXET3。Northern杂交分析表明，在‘糯米糍’中，LcXET1 mRNA在花后59天的果肉组织中积累比在花后73天的果皮组织中早得多。然而，在‘淮枝’中，它在果皮和果肉组织中几乎同时出现（花后66天），这表明‘糯米糍’和‘淮枝’中LcXET1在果皮和果肉组织中的差异积累与果实裂果密切相关。在整个果实发育过程中，LcXET2 mRNA在果皮和果肉组织中均能检测到，但在果皮和果肉组织中呈现出差异积累模式。在‘糯米糍’的果肉中，在‘糯米糍’快速膨大果实的花后59 - 73天可检测到强信号带，但在果皮组织中只能发现弱信号带。相反，在‘淮枝’果实的果皮和果肉组织中均能检测到中等信号带。此外，LcXET3在发育中的‘糯米糍’和‘淮枝’荔枝果实的果皮和果肉组织中均呈现组成型表达。此外，在荔枝的不同组织中观察到三个XET基因的差异表达模式，只有LcXET1是果实特异性的。为了进一步探讨LcXET在果实裂果中的作用，使用α-萘乙酸（NAA）处理‘糯米糍’以减少果实裂果。在NAA处理的果实中，LcXET1 mRNA在果皮中的积累增强，而LcXET2和LcXET3 mRNA在果肉组织中的积累增强。因此，与LcXET2和LcXET3相比，LcXET1更有可能在减少荔枝果实裂果中发挥作用。
