Feedback Inhibition of Chlorophyll Synthesis in the Phytochrome Chromophore-Deficientaureaandyellow-green-2Mutants of Tomato

Abstract

The aurea (au) and yellow-green-2 (yg-2) mutants of tomato (Solanum lycopersicum L.) are unable to synthesize the linear tetrapyrrole chromophore of phytochrome, resulting in plants with a yellow-green phenotype. To understand the basis of this phenotype, we investigated the consequences of the au and yg-2 mutations on tetrapyrrole metabolism. Dark-grown seedlings of both mutants have reduced levels of protochlorophyllide (Pchlide) due to an inhibition of Pchlide synthesis. Feeding experiments with the tetrapyrrole precursor 5-aminolevulinic acid (ALA) demonstrate that the pathway between ALA and Pchlide is intact in au and yg-2 and suggest that the reduction in Pchlide is a result of the inhibition of ALA synthesis. This inhibition was independent of any deficiency in seed phytochrome, and experiments using an iron chelator to block heme synthesis demonstrated that both mutations inhibited the degradation of the physiologically active heme pool, suggesting that the reduction in Pchlide synthesis is a consequence of feedback inhibition by heme. We discuss the significance of these results in understanding the chlorophyll-deficient phenotype of the au and yg-2 mutants.

elongated mesocotyl1, a Phytochrome-Deficient Mutant of Maize

“…Inconsistencies between the degree of Chl deficiency and the level of functional phytochrome have led to the proposal that feedback inhibition of Chl synthesis results from the block in plastidic heme degradation (Terry and Kendrick, 1999). Consistent with this hypothesis, darkgrown au and yg-2 mutants have a reduced level of the Chl precursor protochlorophyllide (Terry and Kendrick, 1999) and this phenomenon has also been observed in chromophore-deficient mutants of pea and Arabidopsis (Terry et al, 2001). The pale-green phenotype and reduced Chl accumulation in elm1 plants, therefore, may result in part from a similar negative feedback of Chl biosynthesis.…”

GLK Transcription Factors Coordinate Expression of the Photosynthetic Apparatus in Arabidopsis

“…The elevated accumulation of ProtoIX and Mg-ProtoIX is consistent with the reduced levels of GUN4 and CHLH transcripts and those of CHLM (encoding Mg-ProtoIX methyltransferase), as implied by the transcriptome analysis below. Additionally, decreased levels of heme and/or Pchlide in mutant seedlings may depress negative feedback of GluTR, either directly or via FLU, a negative regulator of chlorophyll biosynthesis (Terry and Kendrick, 1999;Meskauskiene et al, 2001;Goslings et al, 2004). This effect would further enhance levels of ProtoIX and Mg-ProtoIX relative to the wild type when treated with DP.…”

Disruption of phytoene desaturase gene results in albino and dwarf phenotypes in Arabidopsis by impairing chlorophyll, carotenoid, and gibberellin biosynthesis

“…It was reported that the expression of GA5 and GA4 in leaves, stems and flower buds was noticeably inhibited by GA treatment in Arabidopsis, suggesting the existence of a negative feedback regulation in GA biosynthesis [36]. Similarly, blocking the haem branch could feedback-regulate upstream genes in the tetrapyrrole biosynthesis pathway, which in turn inhibits the chlorophyll branch, another branch in the same pathway [37]. It is interesting to note that biosyntheses of both tetrapyrrole and GAs represent branches of the carotenoid biosynthesis pathway.…”