The anatomically simple plants that first colonized land must have acquired molecular and biochemical adaptations to drought stress. Abscisic acid (ABA) coordinates responses leading to desiccation tolerance in all land plants. We identified ABA nonresponsive mutants in the model bryophyte Physcomitrella patens and genotyped a segregating population to map and identify the ABA NON-RESPONSIVE (ANR) gene encoding a modular protein kinase comprising an N-terminal PAS domain, a central EDR domain, and a C-terminal MAPKKK-like domain. anr mutants fail to accumulate dehydration tolerance-associated gene products in response to drought, ABA, or osmotic stress and do not acquire ABA-dependent desiccation tolerance. The crystal structure of the PAS domain, determined to 1.7-Å resolution, shows a conserved PAS-fold that dimerizes through a weak dimerization interface. Targeted mutagenesis of a conserved tryptophan residue within the PAS domain generates plants with ABA nonresponsive growth and strongly attenuated ABA-responsive gene expression, whereas deleting this domain retains a fully ABA-responsive phenotype. ANR orthologs are found in early-diverging land plant lineages and aquatic algae but are absent from more recently diverged vascular plants. We propose that ANR genes represent an ancestral adaptation that enabled drought stress survival of the first terrestrial colonizers but were lost during land plant evolution.

If the highly efficient C photosynthesis pathway could be transferred to crops with the C pathway there could be yield gains of up to 50%. It has been proposed that the multiple metabolic and developmental modifications associated with C photosynthesis are underpinned by relatively few master regulators that have allowed the evolution of C photosynthesis more than 60 times in flowering plants. Here we identify a component of one such regulator that consists of a pair of -elements located in coding sequence of multiple genes that are preferentially expressed in bundle sheath cells of C leaves. These motifs represent duons as they play a dual role in coding for amino acids as well as controlling the spatial patterning of gene expression associated with the C leaf. They act to repress transcription of C photosynthesis genes in mesophyll cells. These duons are also present in the C model , and, in fact, are conserved in all land plants and even some algae that use C photosynthesis. C photosynthesis therefore appears to have coopted an ancient regulatory code to generate the spatial patterning of gene expression that is a hallmark of C photosynthesis. This intragenic transcriptional regulatory sequence could be exploited in the engineering of efficient photosynthesis of crops.

The bryophytes are a morphologically and ecologically diverse group of plants that have recently emerged as major model systems for a variety of biological processes. In particular, the genome sequence of the moss, Physcomitrella patens, has significantly enhanced our understanding of the evolution of developmental processes in land plants. However, to fully explore the diversity within bryophytes, we need additional genomic resources. Here, we describe analyses of the transcriptomes of a male and a female isolate of the moss, Ceratodon purpureus, generated using the 454 FLX technology. Comparative analyses between C. purpureus and P. patens indicated that this strategy generated nearly complete coverage of the protonemal transcriptome. An analysis of the overlap in gene sets between C. purpureus and P. patens provides new insights into the evolution of gene family composition across the land plants. In spite of the overall transcriptomic similarity between the two species, Ka /Ks analysis of P. patens and C. purpureus suggests considerable physiological and developmental divergence. Additionally, while the codon usage was very similar between these two mosses, C. purpureus genes showed a slightly greater codon usage bias than P. patens genes potentially because of the contrasting mating system of the two species. Finally, we found evidence of a genome doubling ~65-76 MYA that likely coincided with the contemporaneous polyploidy event inferred for P. patens but postdates the divergence of P. patens and C. purpureus. The powerful laboratory tools now available for C. purpureus will enable the research community to fully exploit these genomic resources.

The majority of plants use C3 photosynthesis, but over sixty independent lineages of angiosperms have evolved the C4 pathway. In most C4 species, photosynthesis gene expression is compartmented between mesophyll and bundle sheath cells. We performed DNaseI-SEQ to identify genome-wide profiles of transcription factor binding in leaves of the C4 grasses Zea mays, Sorghum bicolor and Setaria italica as well as C3Brachypodium distachyon. In C4 species, while bundle sheath strands and whole leaves shared similarity in the broad regions of DNA accessible to transcription factors, the short sequences bound varied. Transcription factor binding was prevalent in gene bodies as well as promoters, and many of these sites could represent duons that impact gene regulation in addition to amino acid sequence. Although globally there was little correlation between any individual DNaseI footprint and cell-specific gene expression, within individual species transcription factor binding to the same motifs in multiple genes provided evidence for shared mechanisms governing C4 photosynthesis gene expression. Furthermore, interspecific comparisons identified a small number of highly conserved transcription factor binding sites associated with leaves from species that diverged around 60 million years ago. These data therefore provide insight into the architecture associated with C4 photosynthesis gene expression in particular and characteristics of transcription factor binding in cereal crops in general.One sentence summaryGenome-wide patterns of transcription factor binding in vivo defined by DNaseI for leaves of C3 and C4 grasses

Leaves comprise multiple cell types but our knowledge of the patterns of gene expression that underpin their functional specialization is fragmentary. Our understanding and ability to undertake the rational redesign of these cells is therefore limited. We aimed to identify genes associated with the incompletely understood bundle sheath of C 3 plants, which represents a key target associated with engineering traits such as C 4 photosynthesis into Oryza sativa (rice). To better understand the veins, bundle sheath and mesophyll cells of rice, we used laser capture microdissection followed by deep sequencing. Gene expression of the mesophyll is conditioned to allow coenzyme metabolism and redox homeostasis, as well as photosynthesis. In contrast, the bundle sheath is specialized in water transport, sulphur assimilation and jasmonic acid biosynthesis. Despite the small chloroplast compartment of bundle sheath cells, substantial photosynthesis gene expression was detected. These patterns of gene expression were not associated with the presence or absence of specific transcription factors in each cell type, but were instead associated with gradients in expression across the leaf. Comparative analysis with C 3 Arabidopsis identified a small gene set preferentially expressed in the bundle sheath cells of both species. This gene set included genes encoding transcription factors from 14 orthogroups and proteins allowing water transport, sulphate assimilation and jasmonic acid synthesis. The most parsimonious explanation for our findings is that bundle sheath cells from the last common ancestor of rice and Arabidopsis were specialized in this manner, and as the species diverged these patterns of gene expression have been maintained.