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The Zygnematophyceae is the sister clade to the land plants, but their biology remains mysterious. In a new study, a resolved phylogeny and a scenario for the evolution of multicellularity in that clade are proposed.

Plant hormone signaling pathways have diversified during plant evolution. A new study reveals conservation of DELLA functions in growth and environmental stress responses across land plants.

Nitrogen-fixing symbioses between plants and bacteria are restricted to a few plant lineages. The plant partner benefits from these associations by gaining access to the pool of atmospheric nitrogen. By contrast, other plant species, including all cereals, rely only on the scarce nitrogen present in the soil and what they can glean from associative bacteria. Global cereal yields from conventional agriculture are dependent on the application of massive levels of chemical fertilisers. Engineering nitrogen-fixing symbioses into cereal crops could in part mitigate the economic and ecological impacts caused by the overuse of fertilisers and provide better global parity in crop yields. Comparative phylogenetics and phylogenomics are powerful tools to identify genetic and genomic innovations behind key plant traits. In this review we highlight recent discoveries made using such approaches and we discuss how these approaches could be used to help direct the engineering of nitrogen-fixing symbioses into cereals.

Summary Bacterial lipo-chitooligosaccharides (LCOs) are key mediators of the nitrogen-fixing root nodule symbiosis (RNS) in legumes. The isolation of LCOs from arbuscular mycorrhizal fungi suggested that LCOs are also signaling molecules in arbuscular mycorrhiza (AM). However, the corresponding plant receptors have remained uncharacterized. Here we show that petunia and tomato mutants in the LysM receptor-like kinases LYK10 are impaired in AM formation. Petunia and tomato LYK10 proteins have a high affinity for LCOs (Kd in the nM range) comparable to that previously reported for a legume LCO receptor essential for the RNS. Interestingly, the tomato and petunia LYK10 promoters, when introduced into a legume, were active in nodules similarly to the promoter of the legume orthologous gene. Moreover, tomato and petunia LYK10 coding sequences restored nodulation in legumes mutated in their orthologs. This combination of genetic and biochemical data clearly pinpoints Solanaceous LYK10 as part of an ancestral LCO perception system involved in AM establishment, which has been directly recruited during evolution of the RNS in legumes., Highlights • Mutants in Solanaceaous LysM receptors LYK10 are impaired in arbuscular mycorrhiza • LYK10 proteins have a high affinity for lipo-chitooligosaccharidic signal molecules • LYK10 promoter is expressed in arbuscule-containing cells in tomato roots • Solanaceaous LYK10 can restore nodulation in legumes mutated in their orthologs, Soil rhizobial bacteria and arbuscular mycorrhizal (AM) fungi produce lipo-chitooligosaccharidic (LCO) signal molecules. Girardin et al. show that plant LCO receptors are involved in establishment of the ancient AM symbiosis and have been recruited during evolution for establishment of the nitrogen-fixing root nodule symbiosis with rhizobia.

Beneficial associations between plants and arbuscular mycorrhizal fungi play a major role in terrestrial environments and in the sustainability of agroecosystems. Proteins, microRNAs, and small molecules have been identified in model angiosperms as required for the establishment of arbuscular mycorrhizal associations and define a symbiotic 'toolkit' used for other interactions such as the rhizobia-legume symbiosis. Based on recent studies, we propose an evolutionary framework for this toolkit. Some components appeared recently in angiosperms, whereas others are highly conserved even in land plants unable to form arbuscular mycorrhizal associations. The exciting finding that some components pre-date the appearance of arbuscular mycorrhizal fungi suggests the existence of unknown roles for this toolkit and even the possibility of symbiotic associations in charophyte green algae.

The transition from water to land was a major evolutionary step for the green lineage. Based on fossil data, this event probably occurred some 480–430 million years ago, during the Ordovician and the early Silurian and initiated the explosive evolution that led to the modern diversity of photosynthetic organisms living on Earth. The chronological steps are still puzzling, but the great advances in genetics have allowed some of them to be positioned on the time axis. Chloroplastic organisms evolving towards terrestrialization have had to solve many problems: limited water supply, scarcity of mineral and especially phosphorus, harmful effect of ultraviolet and cosmic rays, pronounced fluctuations of temperature and attacks from new and diversified microbes. Many adaptations, such as the modification of the life cycle (sporophytes, seeds), organ diversification (root and leaves), the appearance of complex phenolic compounds (lignin, flavonoids), vascularization, the accumulation of new compounds (cutin, suberin), the development of specialized cells and the establishment of symbiotic interactions, have all played major roles during the transition from water to land and have resulted in the rich plant biodiversity of today. Some molecular and biochemical aspects putatively associated with land plant emergence are summarized here.