Anis M. Limami

Day respiration ( R d ) is the metabolic, nonphotorespiratory process by which illuminated leaves liberate CO 2 during photosynthesis. R d is used routinely in photosynthetic models and is thus critical for calculations. However, metabolic details associated with R d are poorly known, and this can be problematic to predict how R d changes with environmental conditions and relates to night respiration. It is often assumed that day respiratory CO 2 release just reflects ‘ordinary’ catabolism (glycolysis and Krebs ‘cycle’). Here, we carried out a pulse‐chase experiment, whereby a 13 CO 2 pulse in the light was followed by a chase period in darkness and then in the light. We took advantage of nontargeted, isotope‐assisted metabolomics to determine non‐‘ordinary’ metabolism, detect carbon remobilisation and compare light and dark 13 C utilisation. We found that several concurrent metabolic pathways (‘ordinary’ catabolism, oxidative pentose phosphates pathway, amino acid production, nucleotide biosynthesis and secondary metabolism) took place in the light and participated in net CO 2 efflux associated with day respiration. Flux reconstruction from metabolomics leads to an underestimation of R d , further suggesting the contribution of a variety of CO 2 ‐evolving processes. Also, the cornerstone of the Krebs ‘cycle’, citrate, is synthetised de novo from photosynthates mostly in darkness, and remobilised or synthesised from stored material in the light. Collectively, our data provides direct evidence that leaf day respiration (i) involves several CO 2 ‐producing reactions and (ii) is fed by different carbon sources, including stored carbon disconnected from current photosynthates.

Little is known about the effect of nitrogen nutrition on seedling susceptibility to seed-borne pathogens. We have previously shown that seedlings grown under high nitrate (5 mM) conditions are less susceptible than those grown under low nitrate (0.1 mM) and ammonium (5 mM) in the Arabidopsis-Alternaria brassicicola pathosystem. However, it is not known how seedling metabolism is modulated by nitrogen nutrition, nor what is its response to pathogen infection. Here, we addressed this question using the same pathosystem and nutritive conditions, examining germination kinetics, seedling development, but also shoot ion contents, metabolome, and selected gene expression. Nitrogen nutrition clearly altered the seedling metabolome. A similar metabolomic profile was observed in inoculated seedlings grown at high nitrate levels and in not inoculated-seedlings. High nitrate levels also led to specific gene expression patterns (e.g., polyamine metabolism), while other genes responded to inoculation regardless of nitrogen supply conditions. Furthermore, the metabolites best correlated with high disease symptoms were coumarate, tyrosine, hemicellulose sugars, and polyamines, and those associated with low symptoms were organic acids (tricarboxylic acid pathway, glycerate, shikimate), sugars derivatives and β-alanine. Overall, our results suggest that the beneficial effect of high nitrate nutrition on seedling susceptibility is likely due to nutritive and signaling mechanisms affecting developmental plant processes detrimental to the pathogen. In particular, it may be due to a constitutively high tryptophan metabolism, as well as down regulation of oxidative stress caused by polyamine catabolism.