Rajinikanth Mohan, Harding Scott A, Tsai Chung-Jui
Biotechnology Research Center, School of Forest Resources and Environmental Science, Michigan Technological University, Houghton, MI 49931, USA.
J Exp Bot. 2007;58(7):1761-70. doi: 10.1093/jxb/erm034. Epub 2007 Mar 12.
In plants, the glycine decarboxylase complex (GDC) cooperates with serine hydroxymethyltransferase (SHMT) to mediate photorespiratory glycine-serine interconversion. GDC is also postulated to be an integral component of one-carbon (C1) metabolism in heterotrophic tissues, although molecular evidence in plants is scarce. An initial report of a xylem-specific isoform of GDC component H-protein, PtgdcH1, in aspen (Populus tremuloides Michx.) provided molecular evidence consistent with an important role for GDC in plant C1 metabolism. PtgdcH1 is phylogenetically distinct from the leaf-abundant photorespiratory PtgdcH3, but both isoforms restored GDC activity in a yeast H-protein knockout mutant, suggesting their functional equivalence. The Populus genome contains eight transcriptionally active GDC genes, encoding four H-proteins, two T-proteins, and single P- and L-proteins. The two Populus T-protein isoforms, PtgdcT1 and PtgdcT2, exhibited differential expression in leaves and xylem, similar to PtgdcH3 and PtgdcH1. In silico identification of AC elements in the promoters of xylem-abundant PtgdcH1 and PtgdcT2, as well as many lignin biosynthetic genes of Populus is consistent with a prominent role for GDC in methyl-intensive lignification during wood formation. The AC element is absent from Arabidopsis GDC promoters, and GDC expression has not been linked to secondary growth in this herbaceous annual. Taken together, the results suggest that the association of distinct H-protein and T-protein isoforms with photorespiration and C1 metabolism is a distinguishing feature of Populus, and may signify molecular adaptation of GDC to cope with the C1 demands of lignification in woody perennials.
在植物中，甘氨酸脱羧酶复合体（GDC）与丝氨酸羟甲基转移酶（SHMT）协同作用，介导光呼吸过程中甘氨酸与丝氨酸的相互转化。尽管植物中关于GDC作为异养组织中一碳（C1）代谢不可或缺组分的分子证据很少，但也有此推测。最初有报道称在白杨（美洲山杨）中发现了一种木质部特异性的GDC组分H蛋白异构体PtgdcH1，这提供了分子证据，表明GDC在植物C1代谢中起重要作用。PtgdcH1在系统发育上与叶片中大量存在的光呼吸PtgdcH3不同，但这两种异构体都能在酵母H蛋白敲除突变体中恢复GDC活性，表明它们功能等效。杨树基因组包含8个转录活性GDC基因，分别编码4个H蛋白、2个T蛋白以及单个P蛋白和L蛋白。杨树的两种T蛋白异构体PtgdcT1和PtgdcT2在叶片和木质部中表现出差异表达，与PtgdcH3和PtgdcH1类似。通过计算机分析木质部中大量存在的PtgdcH1和PtgdcT2以及杨树许多木质素生物合成基因的启动子中的AC元件，结果表明GDC在木材形成过程中甲基密集型木质化中起重要作用。拟南芥GDC启动子中不存在AC元件，并且在这种一年生草本植物中，GDC表达与次生生长没有关联。综合来看，这些结果表明不同的H蛋白和T蛋白异构体与光呼吸和C1代谢的关联是杨树的一个显著特征，可能意味着GDC在分子水平上适应了木本多年生植物木质化对C1的需求。
