Arabidopsis Plastidial Folylpolyglutamate Synthetase Is Required for Seed Reserve Accumulation and Seedling Establishment in Darkness

Abstract

Interactions among metabolic pathways are important in plant biology. At present, not much is known about how folate metabolism affects other metabolic pathways in plants. Here we report a T-DNA insertion mutant (atdfb-3) of the plastidial folylpolyglutamate synthetase gene (AtDFB) was defective in seed reserves and skotomorphogenesis. Lower carbon (C) and higher nitrogen (N) content in the mutant seeds than that of the wild type were indicative of an altered C and N partitioning capacity. Higher levels of organic acids and sugars were detected in the mutant seeds compared with the wild type. Further analysis revealed that atdfb-3 seeds contained less total amino acids and individual Asn and Glu as well as NO3 −. These results indicate significant changes in seed storage in the mutant. Defects in hypocotyl elongation were observed in atdfb-3 in darkness under sufficient NO3 − conditions, and further enhanced under NO3 − limited conditions. The strong expression of AtDFB in cotyledons and hypocotyl during early developmental stage was consistent with the mutant sensitivity to limited NO3 − during a narrow developmental window. Exogenous 5-formyl-tetrahydrofolate completely restored the hypocotyl length in atdfb-3 seedlings with NO3 − as the sole N source. Further study demonstrated that folate profiling and N metabolism were perturbed in atdfb-3 etiolated seedlings. The activity of enzymes involved in N reduction and assimilation was altered in atdfb-3. Taken together, these results indicate that AtDFB is required for seed reserves, hypocotyl elongation and N metabolism in darkness, providing novel insights into potential associations of folate metabolism with seed reserve accumulation, N metabolism and hypocotyl development in Arabidopsis.

Folates in Plants: Research Advances and Progress in Crop Biofortification

“…Mammalian cells were also reported to take up MTX using reduced folate carrier (RFC)-mediated transport (Goldman et al, 1968; Dixon et al, 1994). The notion that folates can be taken up by a plant cell is supported by numerous studies, demonstrating that exogenously supplied folate, 5-formyl-THF, can rescue mutants deficient in folate production (Mehrshahi et al, 2010; Meng et al, 2014; Reyes-Hernandez et al, 2014). Moreover, the notion is substantiated by the known ability of plant cells to take-up antifolate drugs such as MTX (Cella et al, 1983).…”

“…Moreover, a crosstalk between folates and sucrose was previously observed in a study where seedling lethality of fpgs2 fpgs3 double mutants could be partially rescued by supplementation with sucrose (Mehrshahi et al, 2010). Another example of the involvement of folate metabolism in hypocotyl elongation is presented in a study revealing compromised hypocotyl elongation in atdfb mutants during skotomorphogenesis (Meng et al, 2014). …”

“…Insufficient nitrogen supplementation results in perturbations of plant metabolism and severe growth defects (Martin et al, 2002; Diaz et al, 2008; Lemaître et al, 2008). Thus, Arabidopsis seedlings mutant for either plastidial or mitochondrial FPGS demonstrate erroneous folate species distribution and display a disturbed nitrogen metabolism within a specific developmental window (Jiang et al, 2013; Meng et al, 2014). Mutants for the plastidial form of FPGS were shown to be defective in seed reserves exhibiting a defective C and N partitioning capacity (Meng et al, 2014).…”

“…Thus, Arabidopsis seedlings mutant for either plastidial or mitochondrial FPGS demonstrate erroneous folate species distribution and display a disturbed nitrogen metabolism within a specific developmental window (Jiang et al, 2013; Meng et al, 2014). Mutants for the plastidial form of FPGS were shown to be defective in seed reserves exhibiting a defective C and N partitioning capacity (Meng et al, 2014). Plants lacking the mitochondrial FPGS isoform demonstrated root shortening upon nitrogen-limited conditions (Jiang et al, 2013).…”

“…Thus, proper functioning of folate metabolism was demonstrated to be indispensable for plant development (Gambonnet et al, 2001; Ishikawa et al, 2003; Jabrin et al, 2003; Mehrshahi et al, 2010). Folates were reported to play important roles in signaling cascades (Stokes et al, 2013), as well as in nitrogen and carbon metabolism (Jiang et al, 2013; Meng et al, 2014). Folate supplementation was demonstrated to improve plant biotic stress resistance (Wittek et al, 2015).…”