Abstract
Upon illumination, etiolated seedlings experience a transition from heterotrophic to photoautotrophic growth. During this process, the tetrapyrrole biosynthesis pathway provides chlorophyll for photosynthesis. This pathway has to be tightly controlled to prevent the accumulation of photoreactive metabolites and to provide stoichiometric amounts of chlorophyll for its incorporation into photosynthetic protein complexes. Therefore, plants have evolved regulatory mechanisms to synchronize the biosynthesis of chlorophyll and chlorophyll-binding proteins. Two phytochrome-interacting factors (PIF1 and PIF3) and the DELLA proteins, which are controlled by the gibberellin pathway, are key regulators of this process. Here, we show that impairment of TARGET OF RAPAMYCIN (TOR) activity in Arabidopsis (Arabidopsis thaliana), either by mutation of the TOR complex component RAPTOR1B or by treatment with TOR inhibitors, leads to a significantly reduced accumulation of the photoreactive chlorophyll precursor protochlorophyllide in darkness but an increased greening rate of etiolated seedlings after exposure to light. Detailed profiling of metabolic, transcriptomic, and physiological parameters revealed that the TOR-repressed lines not only grow slower, they grow in a nutrient-saving mode, which allows them to resist longer periods of low nutrient availability. Our results also indicated that RAPTOR1B acts upstream of the gibberellin-DELLA pathway and its mutation complements the repressed greening phenotype of pif1 and pif3 after etiolation.
Highlights
Upon illumination, etiolated seedlings experience a transition from heterotrophic to photoautotrophic growth
We provide several lines of evidence suggesting that the impairment of TARGET OF RAPAMYCIN (TOR) complex activity, either by mutation of its component protein RAPTOR1B or by administering drugs that inhibit the TOR kinase, results in increased greening of seedlings after extended etiolation
TOR is crucial for the control of cell cycle activity within the root meristem during the heterotrophic-tophotoautotrophic transition of young light-grown Arabidopsis seedlings (Xiong et al, 2013)
Summary
Upon illumination, etiolated seedlings experience a transition from heterotrophic to photoautotrophic growth During this process, the tetrapyrrole biosynthesis pathway provides chlorophyll for photosynthesis. The transcriptional regulation of tetrapyrrole biosynthesis during greening is mediated by LONG HYPOCOTYL5, PHYTOCHOROME-INTERACTING FACTORS (PIFs), GOLDEN2-LIKE transcription factors, FARRED ELONGATED HYPOCOTYL3, REVEILLE1, and scarecrow-like transcription factors (Kobayashi and Masuda, 2016) Among these regulators, PIFs are important transcription factors that inhibit chlorophyll biosynthesis by controlling the expression of a number of biosynthetic and ROS-responsive genes (Leivar et al, 2009; Stephenson et al, 2009; Chen et al, 2013; Liu et al, 2013; Toledo-Ortiz et al, 2014). Ga mutants, which accumulate DELLAs, have more Pchlide under dark conditions, the mutants have high rates of greening after transfer to light, implying that they have elevated POR protein activity (Cheminant et al, 2011)
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