Evolution, on its millennial course, gave rise to eukaryotes from prokaryotes when bilayer organelles, mitochondria, and plastids, evolved from free-living prokaryotes engulfed via endosymbiosis. These organelles retained a fraction of their ancient genome by moving large parts to the nucleus. Hence, they rely heavily on importing nuclear-encoded, cytosolically-synthesized preproteins for >95% of their proteome[1]. Most organelle targeted preproteins have a short N-terminus sequence, named transit peptide(TP). TPs are essential for targeting and recognition of multi-protein translocon machineries present in both outer(TOC) & inner membrane(TIC), which select, sort, and translocate preproteins into the plastid[2]. During evolution, with increasing cellular compartmentalization, new regulatory subunits emerged to control the translocation process[3]. However, a fundamental question remains; how and where did the translocons came from?
In plants, plastids are named for their plasticity in form and function. Different plastids are found in various tissues and are often responsible for coloration of leaf, flowers, and fruits[4]. In plants plastid biogenesis is an essential developmental step and mostly occurs from proplastid (premature) that has very few or no pre-existing translocons[5],[6]. Such as, in rapid greening upon germination or leaf re-emergence in spring. First proplastids replicate and later light exposure transforms them into chloroplasts[7]. Based on environmental conditions (dark/light), developmental state, or tissue type; proplastids can develop into non-green plastids or chloroplasts[6],[8]. Thus maturation of plastid is multidirectional, transitioning into other sub-types, such as from chloroplast to chromoplast in fruit ripening[11] or vice versa upon regreening of citrus fruits[12] or green plastid turning in gerontoplasts upon aging[13]. Given this complexity of plastid development, it is obvious that the composition and regulation of translocons must change during a plant’s life cycle, to support and regulate the high diversity of preproteins translocation. Although accepted that different translocon structures exist with different regulatory subunits, only limited research efforts have focused on developing plastids and hence their evolution remains elusive[14]. One limiting factor is the inability to make genetically “knocked out” viable mutants. Hence, understanding the evolution of core translocon “design” and assembly steps remains a major challenge.
To alleviate these, range of model systems, that are simple to complex in tissue structure and photoheterotrophic (permitting nonlethal translocon mutations) to autotrophic in growth needs to be studied. Hence, I propose a comparative study of translocon biogenesis in complex plant(pea), simple plant(duckweed) and a lower non-flowering moss to disentangle the evolutionary ontology of how early eukaryotes transported proteins across plastid membranes with limited translocons.