The plastid-localized methylerythritol phosphate (MEP) pathway synthesizes the isoprenoid precursors for the production of essential photosynthesis-related compounds and hormones. We have identified an Arabidopsis thaliana mutant, rif1, in which posttranscriptional upregulation of MEP pathway enzyme levels is caused by the loss of function of At3g47450, a gene originally reported to encode a mitochondrial protein related to nitric oxide synthesis. However, we show that nitric oxide is not involved in the regulation of the MEP pathway and that the encoded protein is a plastid-targeted homolog of the Bacillus subtilis YqeH protein, a GTPase required for proper ribosome assembly. Consistently, in rif1 seedlings, decreased levels of plastome-encoded proteins were observed, with the exception of ClpP1, a catalytic subunit of the plastidial Clp protease complex. The unexpected accumulation of ClpP1 in plastids with reduced protein synthesis suggested a compensatory mechanism in response to decreased Clp activity levels. In agreement, a negative correlation was found between Clp protease activity and MEP pathway enzyme levels in different experiments, suggesting that Clp-mediated degradation of MEP pathway enzymes might be a mechanism used by individual plastids to finely adjust plastidial isoprenoid biosynthesis to their functional and physiological states.

SUMMARYCarotenoids are plastidial isoprenoids essential for plant life. In Arabidopsis thaliana carotenoid biosynthesis is strongly upregulated when seedlings that germinate in the dark (etiolated) emerge from the soil and light derepresses photomorphogenesis, causing etioplasts to become chloroplasts. We found that carotenoid biosynthesis is also induced when deetiolation is derepressed in the absence of actual light, eventually resulting in improved greening (chlorophyll accumulation) upon illumination. The increased production of carotenoids in the dark correlates with an upregulated activity of phytoene synthase (PSY; the first committed enzyme of carotenogenesis) and the induction of PSY gene expression in cotyledons (where carotenoids accumulate in dark-grown seedlings). The metabolic precursors for carotenoid synthesis under these conditions are mostly supplied by the plastidial methylerythritol 4-phosphate (MEP) pathway. Accumulation of flux-controlling MEP pathway enzymes, such as deoxyxylulose 5-phosphate synthase (DXS), is posttranscriptionally increased when deetiolation is derepressed in the dark. Unlike the situation observed in light-grown plants, however, the sole overexpression of DXS in dark-grown seedlings does not increase carotenoid accumulation. By contrast, induced expression of a PSY-encoding transgene results in increased carotenoid levels and a concomitant post-transcriptional accumulation of DXS. These data provide evidence for a feedback mechanism by which PSY controls metabolic flux to the carotenoid pathway in plants.

Nitric oxide (NO) has emerged as a central signaling molecule in plants and animals. However, the long search for a plant NO synthase (NOS) enzyme has only encountered false leads. The first works describing a pathogen-induced NOS-like plant protein were soon retracted. New hope came from the identification of NOS1, an Arabidopsis thaliana protein with an atypical NOS activity that was found to be targeted to mitochondria in roots. Although concerns about the NO-producing activity of this protein were raised (causing the renaming of the protein to NO-associated 1), compelling data on its biological role were missing until recently. Strong evidence is now available that this protein functions as a GTPase that is actually targeted to plastids, where it might be required for ribosome function. These and other results support the argument that the defective NO production in loss-of-function mutants is an indirect effect of interfering with normal plastid functions and that plastids play an important role in regulating NO levels in plant cells.