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

Abstract

Autophagy is an evolutionarily conserved system that degrades intracellular components including proteins and organelles, and is important in the adaptive response to starvation in various eukaryotic organisms. Plant chloroplasts convert light energy into chemical energy and assimilate atmospheric carbon dioxide (CO 2 ) for carbohydrate production through photosynthesis reactions. We previously described an autophagy process for chloroplast degradation, during which a portion of chloroplasts are mobilized into the vacuole via autophagic vesicles termed Rubisco-containing bodies. Our recent study demonstrated that the activation of autophagy in photoassimilate-limited leaves is required for the production of free amino acids (AAs) as an alternative energy source. The catabolism of free branchedchain amino acids (BCAAs) is particularly important for survival under starvation conditions. These recent findings suggest an additional role for chloroplasts as a reservoir of AA when photosynthetic energy production is limited. ARTICLE HISTORY

Pyrroline‐5‐carboxylate dehydrogenase is an essential enzyme for proline dehydrogenase function during dark‐induced senescence in Arabidopsis thaliana

“…Leaf senescence is the final stage of leaf development when valuable nutrients are remobilized to other sink organs (reviewed in Lim et al, 2007; Woo et al, 2019), thus supporting reproductive growth or adaptation to adverse environmental conditions. A highly organized and active physiological process, leaf senescence integrates various internal and external signals, including developmental age, nutrition status, hormone signal, water status, light, hypoxia and temperature change (Izumi & Ishida, 2019; Woo et al, 2019), to control dramatic structural and metabolic changes in leaf cells.…”

“…Recently the major cysteine protease senescence‐associated gene 12 ( SAG12 ) was shown to play a central role in ribulose‐1,5‐bisphosphate carboxylase‐oxygenase (RuBisCO) degradation during leaf senescence, and to participate in nitrogen remobilization that sustains seed production (James et al, 2018). Some free amino acids resulting from chloroplast degradation are used as an alternative energy source by mitochondria (Chen et al, 2017; Izumi & Ishida, 2019). Indeed, even though the number of mitochondria may decrease by about a third, the mitochondrial integrity and energy status are maintained until the final stage of senescence (Chrobok et al, 2016; Keech et al, 2007).…”

Proline, Cysteine and Branched-Chain Amino Acids in Abiotic Stress Response of Land Plants and Microalgae

“…Animals, including humans, are not able to synthesize BCAAs, thus they need to be absorbed through the diet. In plants, the breakdown of BCAAs is relevant in case of sugar deficiency, a condition often experienced by plants under drought conditions, to produce the energy necessary to survive stress [14].…”