PLETHORA-autophagy axis in plant regeneration

Abstract

All life-forms in the world exhibit growth and respond to stimuli, where this response plays an extrinsic role in shaping organismal longevity beyond the default genetic blueprint. Both plants and animals are higher-order eukaryotes with complex organ systems and specialized stem cell niches. However, plants are especially vulnerable to injury and cellular stress due to their non-motile physiology and continual dependence on external developmental cues. Despite these limitations, the plant kingdom displays remarkable cellular and organ-level regenerative capacities, ranging from simple wound healing to local cell proliferation and even de novo organogenesis. This raises an essential question–what pathways determine these different cell-fates in damaged plant tissue? In this thesis, I leveraged the regenerative plasticity of detached plant leaves to explore the mechanisms regulating wound healing and organ regeneration in Arabidopsis thaliana. Here I discovered that autophagy, an ancient cellular recycling process, is essential for de novo root regeneration but completely dispensable for wound-callus formation at the cut end of an excised leaf. Interestingly, this phenotype hinges on the transcriptional upregulation of specific genes in the ATG8 (autophagy-related gene 8) family, specifically ATG8F and ATG8H. Using a suite of molecular and genetic tools, I established that the plant-specific developmental regulators PLETHORAs (PLT) activate these ATG8 genes during de novo root regeneration. A striking observation from this work is that the PLTs operate in distinct, non-overlapping roles during this particular regeneration process, which was not seen across developmental or regenerative phases in previous studies. Herein, PLT3 transcriptionally upregulates PLT7, and both PLT3 and PLT7 directly activate ATG8H and ATG8F during de novo root regeneration. Perturbing the PLT-ATG8 genetic axis severely compromises organelle turnover in cells post injury, and this compromised cellular homeostasis leads to the ectopic accumulation of ROS in cells undergoing active reprogramming during de novo root regeneration. However, external ROS quenching successfully boosts rooting, and restoring this redox balance is essential for the activation of stem cell regulators for de novo root regeneration. Taken together, my findings reveal the role of plant-specific developmental regulators in the focused activation of a cellular quality-control mechanism conserved across kingdoms to manage cellular stress and facilitate de novo root regeneration in plants. This thesis provides genetic, molecular, and cellular insights into how plants selectively channel autophagy and ROS to facilitate the specific cell-fate transitions required for de novo root regeneration.