Autophagy-Related 2 Regulates Chlorophyll Degradation under Abiotic Stress Conditions in Arabidopsis

Abstract

Chloroplasts are extraordinary organelles for photosynthesis and nutrient storage in plants. During leaf senescence or under stress conditions, damaged chloroplasts are degraded and provide nutrients for developing organs. Autophagy is a high-throughput degradation pathway for intracellular material turnover in eukaryotes. Along with chloroplast degradation, chlorophyll, an important component of the photosynthetic machine, is also degraded. However, the chlorophyll degradation pathways under high light intensity and high temperature stress are not well known. Here, we identified and characterized a novel Arabidopsis mutant, sl2 (seedling lethal 2), showing defective chloroplast development and accelerated chlorophyll degradation. Map-based cloning combined with high-throughput sequencing analysis revealed that a 118.6 kb deletion region was associated with the phenotype of the mutant. Complementary experiments confirmed that the loss of function of ATG2 was responsible for accelerating chlorophyll degradation in sl2 mutants. Furthermore, we analyzed chlorophyll degradation under abiotic stress conditions and found that both chloroplast vesiculation and autophagy take part in chlorophyll degradation under high light intensity and high temperature stress. These results enhanced our understanding of chlorophyll degradation under high light intensity and high temperature stress.

Autophagy in Plant Abiotic Stress Management

“…Arabidopsis has a single ATG2 gene that is expressed ubiquitously throughout the plant. The atg2 knockout mutants display typical autophagy-defective phenotypes during senescence and stress conditions [ 73 , 74 ]. The T-DNA insertion Arabidopsis mutant atg2-5 shows impaired low-CO 2 -induced stomatal opening.…”

“…The fresh weight and chlorophyll retention of atg2-1 mutants are dramatically decreased compared with those of wild-type (WT) plants under high-temperature conditions. The mRNA levels of ATG5, ATG6, ATG12A and ATG18A also significantly increase in WT plants under high-temperature conditions [ 74 ]. In addition, autophagy plays a role in resetting the cellular memory of heat stress (HS).…”

How Lipids Contribute to Autophagosome Biogenesis, a Critical Process in Plant Responses to Stresses

“…In fact, the atg5-1 , atg7-2, and nbr1-1 mutants are all hypersensitive to heat stress (37 °C or 45 °C) [ 53 , 59 ]. In comparison, the atg2-1 mutant shows a only slight phenotype at 30 °C with a lower fresh weight and chlorophyll amount compared to WT [ 60 ]. Nevertheless, the atg2-1 mutant and the atg5-1 , atg7-2, and atg10-1 mutants all show an alteration in pollen development and anthers dehiscence after treatment at 30 °C [ 61 ].…”

“…Recently it was shown that the atg2 , atg5 , atg7 , atg10 , atg12a, and atg18a mutants were hypersensitive to a treatment with UV-B leading to a half reduction in the activity of PSII in comparison with the WT [ 66 , 67 ]. A high visible light treatment also induced a necrosis phenotype in the atg2-1 mutant [ 60 ]. In addition, several autophagy mutants, including atg7-1 [ 42 ], atg5-1 [ 44 ], and atg13a/atg13b [ 29 ], exhibit accelerated senescence of their rosette leaves when cultivated in short days suggesting the importance of autophagy in the management of cell homeostasis and plant development upon different photoperiods.…”

“…Moreover, in plants direct implication of LTPs in autophagy has not been described thus far. Plant ATG2 has been identified and is strictly required for autophagy; furthermore, ATG2 was implicated in maintaining ROS homeostasis [ 136 ], and chlorophyll degradation upon abiotic stress [ 60 ]. The molecular function of ATG2 in plant autophagy remains elusive, but, interestingly, plant atg2 mutants are capable of forming small-size ATG8-positive structures which presumably correspond to phagophores or immature autophagic structures unable to fuse with the vacuole and thus accumulate in the cytosol [ 137 ].…”