Plants acquired mitochondrial linear plasmids horizontally from fungi likely during the conquest of land

Mitochondrial linear plasmids have been sporadically reported in fungi and plants. Yet, much remains obscure about the diversity, distribution, and evolution of mitochondrial linear plasmids. Here, through phylogenomic analyses across 7,163 cellular organisms (including 991 plants), we find that mitochondrial linear plasmids are widely present in land plants and fungi. Phylogenetic analyses indicate that plants are likely to have acquired mitochondrial linear plasmids horizontally from fungi before or during the conquest of terrestrial environments by plants. Gene content analyses show that mitochondrial linear plasmids harbor a highly dynamic and promiscuous repertoire of genes. Our study refines the understanding of the origin and evolution of mitochondrial linear plasmids.

Evolution of ethylene as an abiotic stress hormone in streptophytes

All land plants modulate their growth and physiology through intricate signaling cascades. The majority of these are at least modulated-and often triggered-by phytohormones. Over the past decade, it has become apparent that some phytohormones have an evolutionary origin that runs deeper than plant terrestrialization-many emerged in the streptophyte algal progenitors of land plants. Ethylene is such a case. Here we synthesize the current knowledge on the evolution of the phytohormone ethylene and speculate about its deeply conserved role in adjusting stress responses of streptophytes for more than half a billion years of evolution.

Crossroads in the evolution of plant specialized metabolism

The monophyletic group of embryophytes (land plants) stands out among photosynthetic eukaryotes: they are the sole constituents of the macroscopic flora on land. In their entirety, embryophytes account for the majority of the biomass on land and constitute an astounding biodiversity. What allowed for the massive radiation of this particular lineage? One of the defining features of all land plants is the production of an array of specialized metabolites. The compounds that the specialized metabolic pathways of embryophytes produce have diverse functions, ranging from superabundant structural polymers and compounds that ward off abiotic and biotic challenges, to signaling molecules whose abundance is measured at the nanomolar scale. These specialized metabolites govern the growth, development, and physiology of land plants-including their response to the environment. Hence, specialized metabolites define the biology of land plants as we know it. And they were likely a foundation for their success. It is thus intriguing to find that the closest algal relatives of land plants, freshwater organisms from the grade of streptophyte algae, possess homologs for key enzymes of specialized metabolic pathways known from land plants. Indeed, some studies suggest that signature metabolites emerging from these pathways can be found in streptophyte algae. Here we synthesize the current understanding of which routes of the specialized metabolism of embryophytes can be traced to a time before plants had conquered land.

Tracking the evolutionary innovations of plant terrestrialization

The gradual transition of the algal ancestor from the freshwater to land has always attracted evolutionary biologists. The recent report of high-quality reference genomes of five Charophyta algae (Spirogloea muscicola, Mesotaenium endlicherianum, Mesostigma viride, Chlorokybus atmophyticus and Penium margaritaceum) and one hornwort (Anthoceros angustus) species sheds light on this fascinating transition. These early diverging plants and algae could have gained new genes from soil bacteria and fungi through horizontal gene transfer (HGT), which was so common during plant terrestrialization and may outrun our expectations. Through reviewing and critical thinking about the advancements on these plant genomes, here, I propose three prospective research directions that need to address in the future: (i) due to the ubiquitous nature of viruses that is similar to soil bacteria and fungi, there is less attention to viruses that probably also play an important role in the genome evolution of plants via HGT; (ii) multicellularity has occurred many times independently, but we still know a little about the biological and ecological mechanisms leading to multi-cellularity in Streptophyta; (iii) and most importantly, the quantitative relationships between genetic innovations and environmental variables such as temperature, precipitation and solar radiation, need pioneering research collaborated by biological evolutionists, computer scientists, and ecologists, which are crucial for understanding the macroevolution of plants and could also be used to simulate the evolution of plants under future climate change.