The mitochondrion – an organelle commonly involved in programmed cell death in <i>Arabidopsis thaliana</i>

TLDR

Plant cells undergoing programmed cell death display chromatin condensation and DNA cleavage before membrane or organelle ultrastructural changes. The study aimed to detect early PCD events by measuring mitochondrial membrane potential changes in Arabidopsis protoplasts with microscopy and flow cytometry. Microscopy and flow cytometry were used to quantify mitochondrial membrane potential changes during PCD at single‑cell and population levels in Arabidopsis protoplasts. Early loss of mitochondrial membrane potential is a common, essential marker of plant PCD, occurring before nuclear changes, being reduced by cyclosporin A, and observed with multiple stimuli and mutants, though cytochrome c release is not required and chloroplast–mitochondria communication appears involved.

Abstract

Summary Plant cells undergoing programmed cell death (PCD) at late stages typically show chromatin condensation and endonucleolytic cleavage prior to obvious membrane or organelle ultrastructural changes. To investigate possible early PCD‐associated events, we used microscopic observations and flow cytometry to quantitate mitochondrial membrane potential (ΔΨ m ) changes during PCD at the single cell and population levels using Arabidopsis protoplasts. A ΔΨ m loss was commonly induced early during plant PCD and was important for PCD execution, as evidenced by the concomitant reduction of the change in ΔΨ m and PCD by cyclosporin A, which inhibits mitochondrial permeability transition pores in animal cells. ΔΨ m loss occurred prior to nuclear morphological changes and was only associated with mitochondrial cytochrome c release (an apoptotic trigger in animals) in response to one of three PCD elicitors. Three different stimuli in wild type implicated ΔΨ m changes in PCD: ceramide, protoporphyrin IX, and the hypersensitive response elicitor AvrRpt2. Additionally, the behavior of the conditional ectopic cell death mutant accelerated cell death2 and ACD2 ‐overproducing plants also implicated ΔΨ m alteration as key for PCD execution. Because ACD2 is largely a chloroplast component in mature plants, the observation that the cell death in acd2 mutants requires changes in mitochondrial functions implicates communication between chloroplasts and mitochondria in mediating PCD activation. We suggest that ΔΨ m loss is a common early marker in plant PCD, similar to what has been documented in animals. However, unlike in animal cells, in plant cells, mitochondrial cytochrome c release is not an obligatory step in PCD control.