Spassieva Stefka D, Markham Jonathan E, Hille Jacques
Department of Molecular Biology of Plants, Research School GBB, University of Groningen, Kerklaan 30, 9751NN Haren, the Netherlands.
Plant J. 2002 Nov;32(4):561-72. doi: 10.1046/j.1365-313x.2002.01444.x.
The nectrotrophic fungus Alternaria alternata f.sp. lycopersici infects tomato plants of the genotype asc/asc by utilizing a host-selective toxin, AAL-toxin, that kills the host cells by inducing programmed cell death. Asc-1 is homologous to genes found in most eukaryotes from yeast to humans, suggesting a conserved function. A yeast strain with deletions in the homologous genes LAG1 and LAC1 was functionally complemented by Asc-1, indicating that Asc-1 functions in an analogous manner to the yeast homologues. Examination of the yeast sphingolipids, which are almost absent in the lag1Deltalac1Delta mutant, showed that Asc-1 was able to restore the synthesis of sphingolipids. We therefore examined the biosynthesis of sphingolipids in tomato by labeling leaf discs with l-[3-3H]serine. In the absence of AAL-toxin, there was no detectable difference in sphingolipid labeling between leaf discs from Asc/Asc or asc/asc leaves. In the presence of pathologically significant concentrations of AAL-toxin however, asc/asc leaf discs showed severely reduced labeling of sphingolipids and increased label in dihydrosphingosine (DHS) and 3-ketodihydrosphingosine (3-KDHS). Leaf discs from Asc/Asc leaves responded to AAL-toxin treatment by incorporating label into different sphingolipid species. The effects of AAL-toxin on asc/asc leaflets could be partially blocked by the simultaneous application of AAL-toxin and myriocin. Leaf discs simultaneously treated with AAL-toxin and myriocin showed no incorporation of label into sphingolipids or long-chain bases as expected. These results indicate that the presence of Asc-1 is able to relieve an AAL-toxin-induced block on sphingolipid synthesis that would otherwise lead to programmed cell death.
坏死营养型真菌链格孢菌番茄专化型（Alternaria alternata f.sp. lycopersici）通过利用宿主选择性毒素AAL毒素感染asc/asc基因型的番茄植株，该毒素通过诱导程序性细胞死亡来杀死宿主细胞。Asc-1与从酵母到人类的大多数真核生物中发现的基因同源，表明其功能保守。同源基因LAG1和LAC1缺失的酵母菌株可被Asc-1功能性互补，这表明Asc-1的功能与酵母同源物类似。对lag1Delta lac1Delta突变体中几乎不存在的酵母鞘脂进行检测，结果表明Asc-1能够恢复鞘脂的合成。因此，我们通过用l-[3-3H]丝氨酸标记叶圆片来研究番茄中鞘脂的生物合成。在没有AAL毒素的情况下，Asc/Asc或asc/asc叶片的叶圆片中鞘脂标记没有可检测到的差异。然而，在存在具有病理意义浓度的AAL毒素时，asc/asc叶圆片显示鞘脂标记严重减少，而二氢鞘氨醇（DHS）和3-酮二氢鞘氨醇（3-KDHS）中的标记增加。Asc/Asc叶片的叶圆片对AAL毒素处理的反应是将标记掺入不同的鞘脂种类中。同时施用AAL毒素和myriocin可部分阻断AAL毒素对asc/asc小叶的影响。同时用AAL毒素和myriocin处理的叶圆片未如预期那样将标记掺入鞘脂或长链碱基中。这些结果表明，Asc-1的存在能够缓解AAL毒素诱导的鞘脂合成阻断，否则该阻断会导致程序性细胞死亡。
