NBR1-mediated selective autophagy of ARF7 modulates root branching

Abstract

Auxin is a critical hormone involved in growth and developmental regulation in plants. For instance, auxin dictates the roots architecture via the Auxin Response Factor (ARF) family of transcription factors, which control different aspects of lateral roots (LR) formation. In Arabidopsis, ARF7 is essential for the correct definition of LRs pre-branch sites (PBS) through an oscillatory mechanism controlling gene expression. Moreover, ARF7 also plays a pivotal role during the decision making necessary to initiate LR formation. Despite the relevance of ARF7 to this process, there is a surprising lack of knowledge about how proteostasis of ARF7 is regulated and how this impacts its function during root architecture patterning. Here, we show that ARF7 accumulates in autophagy mutants, localizes to autophagosomes and it is targeted for degradation through selective autophagy via the cargo adaptor NBR1. Moreover, we report that rhythmic changes to ARF7 abundance are a whole plant process and require functional autophagy to occur. Surprisingly, autophagy deficiency does not seem to impact PBS establishment but affects LR formation; this might be explained by the fact that autophagy mainly seems to regulate ARF7 abundance in mature root tissue but only play an accessory role in younger root tissue. In conclusion we report a novel role for autophagy during plant development, through the selective degradation of ARF7 to optimize root branching

AUXIN RESPONSE FACTOR protein accumulation and function

“…Moreover, ARF7 protein accumulation is also regulated by the autophagy pathway. [ 50 ] Finally, the deep evolutionary history of the ARFs, which have existed in three distinct clades early in the plant lineage, [ 6–8 ] raises the possibility that distinct mechanisms could have arisen to differentially regulate members of the three classes of ARF. Taken together, evidence suggests that multiple systems regulate ARF protein accumulation.…”