Chloroplasts mediate genetically controlled cell death via chloroplast-to-nucleus retrograde signaling. To decipher the mechanism, we examined chloroplast-linked lesion-mimic mutants of Arabidopsis (Arabidopsis thaliana) deficient in plastid division, thereby developing gigantic chloroplasts (GCs). These GC mutants, including crumpled leaf (crl), constitutively express immune-related genes and show light-dependent localized cell death (LCD), mirroring typical autoimmune responses. Our reverse genetic approach excludes any potential role of immune/stress hormones in triggering LCD. Instead, transcriptome and in silico analyses suggest that reactive electrophile species (RES) generated via oxidation of polyunsaturated fatty acids (PUFAs) or lipid peroxidation-driven signaling may induce LCD. Consistent with these results, the one of the suppressors of crl, dubbed spcrl4, contains a causative mutation in the nuclear gene encoding chloroplast-localized FATTY ACID DESATURASE5 (FAD5) that catalyzes the conversion of palmitic acid (16:0) to palmitoleic acid (16:1). The loss of FAD5 in the crl mutant might attenuate the levels of RES and/or lipid peroxidation due to the reduced levels of palmitic acid–driven PUFAs, which are prime targets of reactive oxygen species. The fact that fad5 also compromises the expression of immune-related genes and the development of LCD in other GC mutants substantiates the presence of an intrinsic retrograde signaling pathway, priming the autoimmune responses in a FAD5-dependent manner.

叶绿体通过叶绿体到细胞核的逆行信号传导介导基因控制的细胞死亡。为了破译这一机制，我们检查了拟南芥（ Arabidopsis thaliana ）叶绿体相关的病变模拟突变体，这些突变体缺乏质体分裂，从而发育出巨大的叶绿体（GC）。这些GC突变体，包括皱叶（ crl ），组成型表达免疫相关基因，并表现出光依赖性局部细胞死亡（LCD），反映了典型的自身免疫反应。我们的反向遗传方法排除了免疫/应激激素在触发 LCD 中的任何潜在作用。相反，转录组和计算机分析表明，通过多不饱和脂肪酸 (PUFA) 氧化或脂质过氧化驱动的信号传导产生的反应性亲电子物质 (RES) 可能会诱导 LCD。与这些结果一致， crl的抑制因子之一，称为 spcrl4 ，在编码叶绿体定位的脂肪酸去饱和酶 5 (FAD5) 的核基因中含有致病性突变，该突变催化棕榈酸 (16:0) 转化为棕榈油酸 ( 16:1）。 crl突变体中 FAD5 的缺失可能会减弱 RES 和/或脂质过氧化水平，因为棕榈酸驱动的 PUFA 水平降低，而棕榈酸驱动的 PUFA 是活性氧的主要目标。事实上， fad5还会损害免疫相关基因的表达以及其他 GC 突变体中 LCD 的发展，这一事实证实了内在逆行信号通路的存在，以 FAD5 依赖性方式启动自身免疫反应。