Darkened Leaves Use Different Metabolic Strategies for Senescence and Survival

Abstract

In plants, an individually darkened leaf initiates senescence much more rapidly than a leaf from a whole darkened plant. Combining transcriptomic and metabolomic approaches in Arabidopsis (), we present an overview of the metabolic strategies that are employed in response to different darkening treatments. Under darkened plant conditions, the perception of carbon starvation drove a profound metabolic readjustment in which branched-chain amino acids and potentially monosaccharides released from cell wall loosening became important substrates for maintaining minimal ATP production. Concomitantly, the increased accumulation of amino acids with a high nitrogen-carbon ratio may provide a safety mechanism for the storage of metabolically derived cytotoxic ammonium and a pool of nitrogen for use upon returning to typical growth conditions. Conversely, in individually darkened leaf, the metabolic profiling that followed our C-enrichment assays revealed a temporal and differential exchange of metabolites, including sugars and amino acids, between the darkened leaf and the rest of the plant. This active transport could be the basis for a progressive metabolic shift in the substrates fueling mitochondrial activities, which are central to the catabolic reactions facilitating the retrieval of nutrients from the senescing leaf. We propose a model illustrating the specific metabolic strategies employed by leaves in response to these two darkening treatments, which support either rapid senescence or a strong capacity for survival.

BrphyBis critical for rapid recovery to darkness in matureBrassica rapaleaves

“…Three days of dark resulted in a 23% reduction of 339 chlorophyll levels in wild type leaves while levels remained low in the mutant. This is 340 consistent with similar experiments performed on individually darkened A.thaliana 341 leaves where total chlorophyll levels and protein decline was observed after two 342 days of darkness (Weaver and Amasino, 2001 (Guo et al, 2004;Keech et al, 2007;Law et al, 2018). The (Table S5).…”

“…2A). This 611 senescence response and redistribution of cell resources is typical of the dark 612 response observed in various plant systems (Guo et al, 2004;Brouwer et al, 2012; 613 Song et al, 2014;Law et al, 2018;Sobieszczuk-Nowicka et al, 2018). Upon return 614 to light upregulated and downregulated functional groups are essentially reversed.…”

“…To find differences between the wild-type and phyB response to darkness and re-208 illumination we looked at sets of genes that were significantly changed in these 209 contrasts (darkened vs. pre (Mi et al, 2019). For the data in Table S3, we created a list by matching the A. thaliana gene IDs from 234 the (Law et al, 2018) analysis up to the B. rapa genes they were found to be the 235 nearest homolog of (see GO analysis section above). We considered the genes that 236 were differentially expressed after 3 days (D3) of darkness applied on an 237 individualized leaf (IDL) or a whole plant (DP).…”

Vacuolar sucrose homeostasis is critical for development, seed properties and survival of dark phases of Arabidopsis

“…Latter 565 mutants exhibit, similar to amiR vi1-2 plants (Fig. 5A,B prime metabolic rearrangements (Law et al, 2018). Thus, the observation of an 578 increase of VI activity upon onset of an extended dark period concurs with these dark 579 induced metabolic modifications (Fig.…”

An additional role for chloroplast proteins—an amino acid reservoir for energy production during sugar starvation

“…The amount of free basic AAs, BCAAs and aromatic AAs are extremely low in control plants grown without any stress treatment; 16 however numerous metabolomic analyses report an increase in free basic AAs, BCAAs and aromatic AAs during energy-starved conditions. [17][18][19][20] During these conditions, free amino acids are degraded through catabolic enzyme cascades into simple carbon skeletons that can be integrated into the mitochondrial TCA cycle for respiratory energy production. 16 Enzymes involved in BCAA catabolic reactions are essential for survival during extended darkness, [18][19][20][21] leading us to hypothesize that autophagy could be a major route to supply free BCAAs to this catabolic cascade.…”