Plant Genetics

RESEARCH HIGHLIGHTS Nature Reviews Genetics | AOP, published online 9 October 2014; doi:10.1038/nrg3843 PLANT GENETICS Notably, in accordance with the team’s earlier hypothesis on the specific stepwise progression of gene loss in Following the early root heterotrophic flowering plants (NAD(P)H genes followed by photosynthesis-related genes, of plastome degradation plastid-encoded RNA polymerase genes, ATP synthase genes and Parasitic organisms are often for high-throughput sequencing. housekeeping genes), coralroots genomically and morphologically De novo assembly was then used to are in the early stages of genome reduced. For example, some parasitic piece together the genomes and degradation. This study indicates that coralroots plants contain degraded plastid to define genes and pseudogenes. a specific path does seem to exist for are in the genomes owing to a reduced need In addition, chlorophyll content was plastid genome degradation (with to photosynthesize. A new study measured for several plants from some exceptions) and that coralroots initial steps now shows that coralroot orchids are each species. Some coralroot orchids are in the initial steps towards towards the in the early stages of the transition have green tissues, and it is thought the transition to a highly reduced transition to a to a parasitic lifestyle, and that this that these species are still able to chloroplast genome, which is typical condition has arisen independently at photosynthesize, whereas those of obligate parasites. “This condition highly reduced least twice in the genus. without any green tissues cannot. has arisen independently at least chloroplast Most coralroot orchids do not have The authors report a correlation twice in the genus, making coralroots genome leaves, produce little chlorophyll between the number of functional a powerful system in which to study and depend on fungi to obtain chloroplast genes and chlorophyll the transition to parasitism,” explains nutrients. This lifestyle is a form content in the coralroot species. Barrett. of myco-heterotrophy, which has Notably, non-green coralroots still The team now plan to explore the evolved independently many times. produce chlorophyll, but at 10‑fold nuclear genomes of heterotrophic “All orchids associate with fungi in the lower concentrations. Analyses of plants, such as coralroot orchids. earliest stages of their life cycles, but whole chloroplast genomes for the “Future genomic studies in these coralroots take this to a new level,” coralroots showed that fewer functional unique plants will provide a novel explains Craig Barrett, lead author of loci were found in the non-green perspective on parasitism and should the study. “Coralroots associate with species than green coralroots. In lead to the development of tools that ectomycorrhizal soil fungi, which are addition, most of the plastid genomes can be used in conservation of these in turn mutualists with surrounding were lacking functional genes from the often rare or threatened plants,” he trees and thus coralroots ‘embezzle’ NAD(P)H complex, which is involved concludes. Bryony Jones sugar, water and nutrients from this in chlororespiration. There have been pre-existing mutualism between fungi at least two independent losses of and trees.” photosynthesis in the genus — as ORIGINAL RESEARCH PAPER Barrett, C. F. et al. Investigating the path of plastid genome To gain insights into the early stages inferred from the gene deletions and degradation in an early-transitional clade of BRANDX of the transition to parasitism, the the presence of pseudogenes in both heterotrophic orchids, and implications for researchers isolated chloroplast DNA Corallorhiza Striata and Corallorhiza heterotrophic angiosperms. Mol. Biol. Evol. http:// dx.doi.org/10.1093/molbev/msu252 (2014) from nine species of coralroot orchids maculata. NATURE REVIEWS | GENETICS VOLUME 15 | NOVEMBER 2014 © 2014 Macmillan Publishers Limited. All rights reserved.

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