Unveiling abiotic stress-induced cell death modalities in the heterocytous cyanobacterium Anabaena sp. PCC 7120

Regulated cell death (RCD) in cyanobacteria is a post-stress phenomenon necessary for stress adaptability and eco-physiology. However, the sequential modulations in cellular processes conferring regulated forms of death remained elusive. In this study, we evaluated morpho-physiological and biochemical determinants of cell death in a time-dependent manner to understand the sequential cellular events during the process. The death was instigated by exposing heterocytous cyanobacterium Anabaena sp. PCC 7120–1 mM H₂O₂, 250 mM NaCl and sulfur-starvation. A significant changes in the sequences of morpho-physiological and biochemical events were observed in different stress conditions. Upon H₂O₂ treatment rapid oxidation of cytosol and disintegration of cell envelop was observed prior to DNA fragmentation. On the other hand, salt-stress and sulfur-deprivation induced phosphatidylserine (PS) externalisation, gradual rise in redox potential, and DNA fragmentation prior to membrane disintegration. Furthermore, we analysed the time-dependent expression of five orthocaspases, the putative executioner of cyanobacterial cell death. Out of five, only two orthocaspases anaOC2 and anaOC6 exhibit expression pattern synergistic and coherent with DNA fragmentation in salt treated and sulfur-deficient cells. However, no orthocaspases expression was evident during H₂O₂ treatment. Considering these parameters, H₂O₂ induced death was regarded as accidental cell death (ACD), whereas in salt and sulfur stresses mediated two different RCD subroutines, which were orthocaspases-dependent. Overall for the first time, we demonstrated the existence of three death modalities in cyanobacteria based on sequential cellular events and relative expression of orthocaspases.