The autophagy-related genes AtATG5 and AtATG7 influence reserve mobilisation and responses to ABA during seed germination in Arabidopsis thaliana

Autophagy is an intracellular recycling mechanism that generally degrades cytoplasmic components non-selectively, but it can also target specific substrates under certain conditions. Here, we investigate the impact of autophagy on Arabidopsis seed biology through autophagy-related (ATG) genes AtATG5 and AtATG7, and their role in ABA responses. Seeds of atg5 and atg7 mutants germinate significantly slower than Col-0, especially under ABA, and show transcriptomic differences and histochemical alterations in the organization of lipid droplets and protein storage vacuoles. Notably, immunolocalization of ATG8 is observed in PSV of Col-0, but not in atg mutants. Differentially expressed genes in atg7 compared to Col-0 in response to ABA include reported targets of the transcription factor (TF) ABI5, a master regulator of ABA signalling in the seed. Interestingly, the decrease in ABI5 normally observed in Col-0 seeds after imbibition is delayed in atg mutants, which also show altered accumulation in developing seeds of the bZIP67, an ABI5 homolog that regulates reserve biosynthesis. Yeast two-hybrid and co-immunoprecipitation assays confirmed a direct interaction with the autophagy machinery in vitro and in vivo, mediated through ATG8. Our data highlight the relevance of autophagy in seed reserve mobilization, and seed germination through ABA responses by a TF decay mechanism.