图1 苹果酸调控植物程序性细胞死亡模式图(改自Zhao et al., 2018; Heng et al., 2018)(A) 在拟南芥中, MOD1编码烯脂酰还原酶, 突变体mod1中底物NADH在叶绿体大量积累, NADH与草酰乙酸(OAA)在plNAD-MDH的催化作用下生成苹果酸, 通过DiT1运输至细胞质, 继而被某个/某些转运蛋白转运至线粒体。苹果酸在线粒体中被mMDH1催化形成草酰乙酸, 并伴随着NADH的积累, 为线粒体膜上的mETC复合体提供电子, 产生ROS, 进而导致mod1突变体产生程序性细胞死亡(PCD)。(B) 水稻OsALMT7基因编码1个定位于质膜的苹果酸转运蛋白, 可以将苹果酸转运至细胞内。苹果酸可以维持核DNA稳定, 减少H2O2和丙二醛的积累, 从而抑制PCD。在其突变体paab1-1中, 苹果酸积累减少, 伴随着核DNA片段化, 同时积累H2O2和丙二醛, 促进PCD。

Figure 1 A proposed model of malate-regulated programmed cell death in plants (modified from Zhao et al., 2018; Heng et al., 2018)(A) In Arabidopsis thaliana, MOD1 encodes an enoyl-acyl carrier protein reductase, the deficiency of MOD1 causes an increased level of NADH in the chloroplasts, which drives oxaloacetate (OAA) to be converted to malate by plNAD-MDH. Malate is transported out of the chloroplast into the cytosol by DiT1, and then be transported into the mitochondrion by an unidentified transporter or transporters. In the mitochondrion, malate is converted to OAA by mMDH1, and simultaneously NADH is generated to provide electrons for mETC to induce ROS formation and initiate programmed cell death (PCD) process in the mod1 cells. (B) In Oryza sativa, OsALMT7 encodes a putative aluminum-activated plasma membrane localized malate transporter, which could transport malate into cells. Malate is involved in nuclear DNA stability and inhibits excessive accumulation of H2O2 and malondialdehyde, which protects cells from PCD. The paab1-1 mutant harbors a mutation in OsALMT7, and its panicle contained less malate than wild type, particularly at the apical portions. The apical spikelets in the paab1-1 mutant undergo PCD accompanied by nuclear DNA fragmentation and accumulation of higher levels of H2O2 and malondialdehyde.