Induced Deactivation of Genes Encoding Chlorophyll Biosynthesis Enzymes Disentangles Tetrapyrrole-Mediated Retrograde Signaling.

Short-term inducible gene inactivation for the first three enzymatic steps in the Mg branch towards chlorophyll synthesis does not modify retrograde signaling-mediated nuclear gene expression. Long-term gene silencing of the same genes involved in tetrapyrrole biosynthesis causes either a pale-green or a necrotic leaf phenotype and feedback-regulated ALA synthesis or 1O2-induced signaling affect nuclear gene expression.In photosynthetic organisms, tetrapyrrole-mediated retrograde signals are proposed to contribute to a balanced nuclear gene expression (NGE) in response to metabolic activity in chloroplasts. We followed an experimental short-term approach that allowed the assessment of modified NGE during the first hours of specifically modified enzymatic steps of the Mg branch of tetrapyrrole biosynthesis, when pleiotropic effects of other signals can be avoided. In response to 24-h-induced silencing of CHLH, CHLM, and CHL27 encoding the CHLH subunit of Mg chelatase, the Mg protoporphyrin methyltransferase and Mg protoporphyrin monomethylester cyclase, respectively, deactivated gene expression rapidly led to reduced activity of the corresponding enzymes and altered Mg porphyrin levels. But NGE was not substantially altered. When these three genes were continuously inactivated for up to 4 d, changes of transcript levels of nuclear genes were determined. CHL27 silencing for more than 24h results in necrotic leaf lesions and modulated transcript levels of oxidative stress-responsive and photosynthesis-associated nuclear genes (PhANGs). The prolonged deactivation of CHLH and CHLM results in slightly elevated transcript levels of PhANGs and tetrapyrrole-associated genes. These time-resolved studies indicate a complex scenario for the contribution of tetrapyrrole biosynthesis on NGE mediated by 1O2-induced signaling and feedback-regulated ALA synthesis. [ABSTRACT FROM PUBLISHER]