| Mulberry has a long culture history and has been played important role in economy in China with varieties of utilization purpose and its foliage is the only food for the silkworm. Mulberry has plenty of germ-plasm resources and distributes in all over the country. It was subjected to research in many areas including planting, breeding, genetics, evolution, ecology, physiology, biochemistry, resistance, development, differentiation, tissue culture, transgene, molecular biology and a foundation research background was formed. So mulberry not only was an economic plant but also was a good research material.Ethylene functions as a hormone in plants and plays important roles in the regulation of developmental programs, and in responses to stresses and defense of pathogens. The final step of ethylene biosynthesis in higher plants is the conversion of 1-aminocyclopropane-l-carboxylate (ACC) to ethylene by ACC oxidase. The ACC oxidase, on biochemical grounds, was considered an evolutionarily crucial step for the development of seed plants where the pathway of ethylene biosynthesis could easily be regulated. ACC oxidase is a major molecular marker for both ethylene formation and ethylene responsiveness.To further our understanding of the function of ACC oxidase in woody plant development and adverse environmental stress responses, also to better utilization of the mulberry tree, we isolated a mulberry 1-aminocyclopropane-l-carboxylate oxidase gene in the form of cDNA from Morus alba leaf, named MaACO1, and the involvement of ACC oxidase in mulberry development and stress responses were investigated through RT-PCR analysis.A GenBank Blastn search revealed that the nucleotide sequence of MaACO1 share high similarity with that of other ACC oxidase genes and with the highest identity of 85-86% to that of Rosaceae Prunus plants in coding region. The deduced amino sequence has a motif similar to the putative Fe2+ binding sites (His 177, Asp 179, and His234) that exist among the ACC oxidases and other dioxygenases.In the green leaf tissue, the expression level of MaACO1 is highest in the cotyledons and a pair of first leaves, is less in the fully emerged leaf, and even less in the middle and lower expanded leaves as well as the newly emerged leaf. The accumulation of MaACO1 transcripts in a pair of first leaves and cotyledons is preceding obvious senescence symptom because the cotyledons are still green while the expression level of MaACO1 has increased.In reproductive organ flowers, MaACO1 expression exhibits a tissue-specific manner in pistil. Transcripts of MaACO1 in young flowers remain at the same level as those in leaves, but become more abundant in stigma/style especially after pollination. Ovaries accumulate fewer MaACO1 transcripts than leaves do, and ovaries of pollinated flowers have a higher ACC oxidase expression level than those of unpollinated flowers and still less than in leaves. MaACO1 expression level in stamen was found no change before anthesis and post-anthesis.The expression of MaACO1 has a temporal dependence manner in the wounded leaves. A higher expression level of ACC oxidase gene was detected one hour after mechanical wounding treatment at the wounded sites and the accumulation of MaACO1 transcripts in wounded leaves reaches peak four hours after treatment and maintains a high expression level until twelve hours. We suggest that the wounding induces the cell death in wounded site and promotes the expression of MaACO1, the ethylene synthesis is increased, initiates ethylene signal pathway to participate plant defense response.In the case of cold treatment, ACC oxidase gene expression, a change was not detected within twenty-four hours when plants were placed in a 4℃cabinet, but the expression level of mulberry house-keep gene actin gene was found to decease four hours after cold treatment. These results suggest that ACC oxidase gene remains at a high expression level while plants have an overall lowered metabolism level at low temperature. When comparing the expression level between MaACO1 and mulberry cold acclimation protein gene WAP27, we find that MaACO1 transcripts are still at a high expression level when WAP27 transcripts have disappeared under over-cold treatment for three weeks.In the condition of water shortage, MaACO1 expression level increased in water stress and the quantity was different among individuals. This indicated that mulberry ACC oxidase may be associated with drought tolerance. The expression level of ACC oxidase changed in NaCI salinity, this change may be associated with tissue death resulted by salinity but not response to salinity stress directly. |
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