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Mutations in the Arabidopsis Homolog of LST8/G L, a Partner of the Target of Rapamycin Kinase, Impair Plant Growth, Flowering, and Metabolic Adaptation to Long Days

Institut Jean-Pierre Bourgin, Unité Mixte de Recherche 1318, Institut National de la Recherche Agronomique AgroParisTech, 78026 Versailles cedex, France.
The Plant Cell (Impact Factor: 9.58). 02/2012; 24(2):463-81. DOI: 10.1105/tpc.111.091306
Source: PubMed

ABSTRACT The conserved Target of Rapamycin (TOR) kinase forms high molecular mass complexes and is a major regulator of cellular adaptations to environmental cues. The Lethal with Sec Thirteen 8/G protein β subunit-like (LST8/GβL) protein is a member of the TOR complexes, and two putative LST8 genes are present in Arabidopsis thaliana, of which only one (LST8-1) is significantly expressed. The Arabidopsis LST8-1 protein is able to complement yeast lst8 mutations and interacts with the TOR kinase. Mutations in the LST8-1 gene resulted in reduced vegetative growth and apical dominance with abnormal development of flowers. Mutant plants were also highly sensitive to long days and accumulated, like TOR RNA interference lines, higher amounts of starch and amino acids, including proline and glutamine, while showing reduced concentrations of inositol and raffinose. Accordingly, transcriptomic and enzymatic analyses revealed a higher expression of genes involved in nitrate assimilation when lst8-1 mutants were shifted to long days. The transcriptome of lst8-1 mutants in long days was found to share similarities with that of a myo-inositol 1 phosphate synthase mutant that is also sensitive to the extension of the light period. It thus appears that the LST8-1 protein has an important role in regulating amino acid accumulation and the synthesis of myo-inositol and raffinose during plant adaptation to long days.

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    • "The role of TOR signaling in the induction of biosynthesis and the repression of catabolic pathways was underlined by RNA sequencing and microarray analysis studying gene expression changes in response to TOR inactivation (Ren et al., 2012; Caldana et al., 2013). These transcriptional changes were shown to be accompanied by an increase in starch content (Moreau et al., 2012) and an accumulation of organic and amino acids (Ren et al., 2012; Caldana et al., 2013), as well as a decrease in galactinol and raffinose levels (Moreau et al., 2012). This led to the conclusion that TOR downregulation mimics starvation (Caldana et al., 2013) and strengthens the importance of the TOR pathway in starvation responses. "
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    • "Functionally, these findings suggest that a mitochondrial TOR target might participate in the responses to reactive oxygen species and in the modulation of cell walls (Leiber et al., 2010). In agreement, disruption of TOR signalling, in tor-and lst8-depleted lines, represses the expression of genes encoding regulators of cell-wall expansion such as expansins and extensins (Moreau et al., 2012; Ren et al., 2012). Therefore, TOR acts as a sensor of nutrient and energy levels to promote growth by favouring the increase in cytoplasmic volume and modulating cell-wall structure accordingly. "
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    • "Furthermore MIPS and galactinol synthases were found to be significantly repressed in TOR RNAi lines or in the lst8 mutants grown in long days. These results are in agreement with the observed lack of galactinol and raffinose accumulation in plants where the TORC1 activity is reduced (Moreau et al., 2012; Ren et al., 2012). The MIPS genes could therefore serve as a hub for adjusting the plant metabolism to changes in environmental conditions. "
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