Dancs Gábor, Kondrák Mihály, Bánfalvi Zsófia
Agricultural Biotechnology Center, P,O, Box 411, H-2101 Gödöllõ, Hungary.
BMC Plant Biol. 2008 Jun 12;8:65. doi: 10.1186/1471-2229-8-65.
Potato is a staple food in the diet of the world's population and also being used as animal feed. Compared to other crops, however, potato tubers are relatively poor in the essential amino acid, methionine. Our aim was to increase the methionine content of tubers by co-expressing a gene involved in methionine synthesis with a gene encoding a methionine-rich storage protein in potato plants.
In higher plants, cystathionine gamma-synthase (CgS) is the first enzyme specific to methionine biosynthesis. We attempted to increase the methionine content of tubers by expressing the deleted form of the Arabidopsis CgS (CgSDelta90), which is not regulated by methionine, in potato plants. To increase the incorporation of free methionine into a storage protein the CgSDelta90 was co-transformed with the methionine-rich 15-kD beta-zein. Results demonstrated a 2- to 6-fold increase in the free methionine content and in the methionine content of the zein-containing protein fraction of the transgenic tubers. In addition, in line with higher methionine content, the amounts of soluble isoleucine and serine were also increased. However, all of the lines with high level of CgSDelta90 expression were phenotypically abnormal showing severe growth retardation, changes in leaf architecture and 40- to 60% reduction in tuber yield. Furthermore, the colour of the transgenic tubers was altered due to the reduced amounts of anthocyanin pigments. The mRNA levels of phenylalanine ammonia-lyase (PAL), the enzyme catalysing the first step of anthocyanin synthesis, were decreased.
Ectopic expression of CgSDelta90 increases the methionine content of tubers, however, results in phenotypic aberrations in potato. Co-expression of the 15-kD beta-zein with CgSDelta90 results in elevation of protein-bound methionine content of tubers, but can not overcome the phenotypical changes caused by CgSDelta90 and can not significantly improve the nutritional value of tubers. The level of PAL mRNA and consequently the amount of anthocyanin pigments are reduced in the CgSDelta90 transgenic tubers suggesting that methionine synthesis and production of anthocyanins is linked.
马铃薯是世界人口饮食中的主食，也被用作动物饲料。然而，与其他作物相比，马铃薯块茎中的必需氨基酸蛋氨酸含量相对较低。我们的目标是通过在马铃薯植株中共表达一个参与蛋氨酸合成的基因和一个编码富含蛋氨酸的贮藏蛋白的基因，来提高块茎中的蛋氨酸含量。
在高等植物中，胱硫醚γ-合酶（CgS）是蛋氨酸生物合成的第一个特异性酶。我们试图通过在马铃薯植株中表达不受蛋氨酸调控的拟南芥CgS缺失形式（CgSDelta90）来提高块茎中的蛋氨酸含量。为了增加游离蛋氨酸掺入贮藏蛋白的量，将CgSDelta90与富含蛋氨酸的15-kDβ-玉米醇溶蛋白共转化。结果表明，转基因块茎中游离蛋氨酸含量以及含玉米醇溶蛋白部分的蛋氨酸含量增加了2至6倍。此外，与较高的蛋氨酸含量一致，可溶性异亮氨酸和丝氨酸的含量也增加了。然而，所有高表达CgSDelta90的株系在表型上均异常，表现出严重的生长迟缓、叶片结构改变以及块茎产量降低40%至60%。此外，由于花青素色素含量减少，转基因块茎的颜色发生了改变。催化花青素合成第一步的苯丙氨酸解氨酶（PAL）的mRNA水平降低。
CgSDelta90的异位表达增加了块茎中的蛋氨酸含量，但导致马铃薯出现表型畸变。15-kDβ-玉米醇溶蛋白与CgSDelta90共表达导致块茎中与蛋白结合的蛋氨酸含量升高，但不能克服CgSDelta90引起的表型变化，也不能显著提高块茎的营养价值。CgSDelta90转基因块茎中PAL mRNA水平以及因此花青素色素的量降低，这表明蛋氨酸合成与花青素的产生有关。
