Liu Pei, Goh Chong-Jin, Loh Chiang-Shiong, Pua Eng-Chong
Department of Biological Sciences, Faculty of Science, National University of Singapore, Kent Ridge, Singapore 119260, Republic of Singapore Present address: Department of Plant Biology, Cornell University, Ithaca, NY 14853, USA.
Physiol Plant. 2002 Feb;114(2):241-250. doi: 10.1034/j.1399-3054.2002.1140210.x.
Metallothioneins (MTs) are cysteine-rich polypeptides that are involved in metal detoxification and homeostasis in both prokaryotes and eukaryotes. In this study, we report the isolation and characterization of three members (MT2A, MT2B and MT3) of the MT-like gene family from ripening banana fruit and their differential expression in various banana organs and during fruit development and ripening. All members of the MT-like gene encode small cysteine-rich polypeptides of 65-79 amino acid residues. MT2A shared a high sequence similarity (54-77%) with several type-2 MTs in plants, while MT3 was highly homologous (51-61%) with type-3 MTs. The three members expressed differentially in various organs but transcripts were generally more abundant in reproductive than vegetative organs. During fruit development, the MT2A transcript was barely detectable in ovary but increased to a high level in young fruit at 20 days after shooting (DAS) and declined gradually thereafter as fruit developed. In contrast, both MT2B and MT3 expressed poorly in young fruits (20-60 DAS) and transcripts were detected only in fruits at later stages of development. As ripening progressed, expression of MT2A decreased but that of MT3 increased. Expression of MT members during ripening appeared to be differentially regulated by ethylene, whose levels were low in FG and TY fruit but surged climacteristically in MG and declined sharply as ripening advanced further. Exogenous application of ethylene at 5 ppm or higher concentrations down-regulated MT2A expression and the inhibitory effect of ethylene could be partially suppressed by the presence of norbornadiene, an inhibitor of ethylene action. Ethylene had no effect on transcript accumulation of MT2B and MT3. However, MT3 expression was greatly enhanced in response to metals such as CdSO4, CuSO4 and ZnSO4. These results suggest that increased MT3 expression may be associated with excess metal ions present in ripening fruit tissues. This study also provided evidence, for the first time, that ethylene and metals play a regulatory role in expression of MT-like genes in banana.
金属硫蛋白（MTs）是富含半胱氨酸的多肽，在原核生物和真核生物中都参与金属解毒和体内平衡。在本研究中，我们报告了从成熟香蕉果实中分离和鉴定MT样基因家族的三个成员（MT2A、MT2B和MT3），以及它们在香蕉不同器官、果实发育和成熟过程中的差异表达。MT样基因的所有成员都编码富含半胱氨酸的小多肽，含有65 - 79个氨基酸残基。MT2A与植物中的几种2型MT具有高度的序列相似性（54 - 77%），而MT3与3型MT高度同源（51 - 61%）。这三个成员在不同器官中差异表达，但转录本通常在生殖器官中比营养器官中更丰富。在果实发育过程中，MT2A转录本在子房几乎检测不到，但在抽穗后20天（DAS）的幼果中增加到高水平，此后随着果实发育逐渐下降。相反，MT2B和MT3在幼果（20 - 60 DAS）中表达较弱，转录本仅在发育后期的果实中检测到。随着成熟进程，MT2A的表达下降，而MT3的表达增加。MT成员在成熟过程中的表达似乎受到乙烯的差异调节，乙烯在FG和TY果实中的水平较低，但在MG中呈跃变上升，随着成熟进一步推进急剧下降。外源施加5 ppm或更高浓度的乙烯下调了MT2A的表达，乙烯的抑制作用可被乙烯作用抑制剂降冰片二烯部分抑制。乙烯对MT2B和MT3的转录本积累没有影响。然而，MT3的表达在响应诸如硫酸镉、硫酸铜和硫酸锌等金属时显著增强。这些结果表明，MT3表达增加可能与成熟果实组织中存在的过量金属离子有关。本研究还首次提供了证据，表明乙烯和金属在香蕉MT样基因的表达中起调节作用。
