Autophagy Controls Sulphur Metabolism in the Rosette Leaves of Arabidopsis and Facilitates S Remobilization to the Seeds

Sulphur deficiency in crops became an agricultural concern several decades ago, due to the decrease of S deposition and the atmospheric sulphur dioxide emissions released by industrial plants. Autophagy, which is a conserved mechanism for nutrient recycling in eukaryotes, is involved in nitrogen, iron, zinc and manganese remobilizations from the rosette to the seeds in Arabidopsis thaliana. Here, we have compared the role of autophagy in sulphur and nitrogen management at the whole plant level, performing concurrent labelling with S-34 and N-15 isotopes on atg5 mutants and control lines. We show that both S-34 and N-15 remobilizations from the rosette to the seeds are impaired in the atg5 mutants irrespective of salicylic acid accumulation and of sulphur nutrition. The comparison in each genotype of the partitions of N-15 and S-34 in the seeds (as % of the whole plant) indicates that the remobilization of S-34 to the seeds was twice more efficient than that of N-15 in both autophagy mutants and control lines under high S conditions, and also in control lines under low S conditions. This was different in the autophagy mutants grown under low S conditions. Under low S, the partition of S-34 to their seeds was indeed not twice as high but similar to that of N-15. Such discrepancy shows that when sulphate availability is scarce, autophagy mutants display stronger defects for S-34 remobilization relative to N-15 remobilization than under high S conditions. It suggests, moreover, that autophagy mainly affects the transport of N-poor S-containing molecules and possibly sulphate.