The evolution of land flora transformed the terrestrial environment. Land plants evolved from an ancestral charophycean alga from which they inherited developmental, biochemical, and cell biological attributes. Additional biochemical and physiological adaptations to land, and a life cycle with an alternation between multicellular haploid and diploid generations that facilitated efficient dispersal of desiccation tolerant spores, evolved in the ancestral land plant. We analyzed the genome of the liverwort Marchantia polymorpha, a member of a basal land plant lineage. Relative to charophycean algae, land plant genomes are characterized by genes encoding novel biochemical pathways, new phytohormone signaling pathways (notably auxin), expanded repertoires of signaling pathways, and increased diversity in some transcription factor families. Compared with other sequenced land plants, M. polymorpha exhibits low genetic redundancy in most regulatory pathways, with this portion of its genome resembling that predicted for the ancestral land plant. PAPERCLIP.

The green f luorescent protein (GFP) from the jellyfish Aequorea victoria is finding wide use as a genetic marker that can be directly visualized in the living cells of many heterologous organisms. We have sought to express GFP in the model plant Arabidopsis thaliana, but have found that proper expression of GFP is curtailed due to aberrant mRNA processing. An 84-nt cryptic intron is efficiently recognized and excised from transcripts of the GFP coding sequence. The cryptic intron contains sequences similar to those required for recognition of normal plant introns. We have modified the codon usage of the gfp gene to mutate the intron and to restore proper expression in Arabidopsis. GFP is mainly localized within the nucleoplasm and cytoplasm of transformed Arabidopsis cells and can give rise to high levels of f luorescence, but it proved difficult to efficiently regenerate transgenic plants from such highly f luorescent cells. However, when GFP is targeted to the endoplasmic reticulum, transformed cells regenerate routinely to give highly f luorescent plants. These modified forms of the gfp gene are useful for directly monitoring gene expression and protein localization and dynamics at high resolution, and as a simply scored genetic marker in living plants.

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In vitro mutagenesis of sequences required for the self-catalysed cleavage of a plant virus satellite RNA has allowed definition of an RNA segment with endoribonuclease activity. General rules have been deduced for the design of new RNA enzymes capable of highly specific RNA cleavage, and have been successfully tested against a new target sequence.