We report the draft genome sequence of the model moss Physcomitrella patens and compare its features with those of flowering plants, from which it is separated by more than 400 million years, and unicellular aquatic algae. This comparison reveals genomic changes concomitant with the evolutionary movement to land, including a general increase in gene family complexity; loss of genes associated with aquatic environments (e.g., flagellar arms); acquisition of genes for tolerating terrestrial stresses (e.g., variation in temperature and water availability); and the development of the auxin and abscisic acid signaling pathways for coordinating multicellular growth and dehydration response. The Physcomitrella genome provides a resource for phylogenetic inferences about gene function and for experimental analysis of plant processes through this plant's unique facility for reverse genetics.

Plants are hosts to a wide array of pathogens from all kingdoms of life. In the absence of an active immune system or combinatorial diversifications that lead to recombination-driven somatic gene flexibility, plants have evolved different strategies to combat both individual pathogen strains and changing pathogen populations. The receptor-like kinase (RLK) gene-family expansion in plants was hypothesized to have allowed accelerated evolution among domains implicated in signal reception, typically a leucine-rich repeat (LRR). Under that model, the gene-family expansion represents a plant-specific adaptation that leads to the production of numerous and variable cell surface and cytoplasmic receptors. More recently, it has emerged that the LRR domains of RLK interact with a diverse group of proteins leading to combinatorial variations in signal response specificity. Therefore, the RLK appear to play a central role in signaling during pathogen recognition, the subsequent activation of plant defense mechanisms, and developmental control. The future challenges will include determinations of RLK modes of action, the basis of recognition and specificity, which cellular responses each receptor mediates, and how both receptor and kinase domain interactions fit into the defense signaling cascades. These challenges will be complicated by the limited information that may be derived from the primary sequence of the LRR domain. The review focuses upon implications derived from recent studies of the secondary and tertiary structures of several plant RLK that change understanding of plant receptor function and signaling. In addition, the biological functions of plant and animal RLK-containing receptors were reviewed and commonalities among their signaling mechanisms identified. Further elucidated were the genomic and structural organizations of RLK gene families, with special emphasis on RLK implicated in resistance to disease and development.

Desiccation-tolerance (DT), the ability to lose virtually all free intracellular water and then recover normal function upon rehydration, is one of the most remarkable features of bryophytes. The physiology of bryophytes differs in major respects from that of vascular plants by virtue of their smaller size; unlike vascular plants, the leafy shoots of bryophytes equilibrate rapidly with the water potential in their surroundings and tend to be either fully hydrated or desiccated and metabolically inactive. The time required to recover from desiccation increases and degree of recovery decreases with length of desiccation; both also depend upon temperature and intensity of desiccation. Tolerance in at least some species shows phenotypic plasticity. Recovery of respiration, photosynthesis and protein synthesis takes place within minutes or an hour or two; recovery of the cell cycle, food transport and the cytoskeleton may take 24 hours or more. Positive carbon balance is essential to survival of repea...

Desiccation-tolerance is the unique ability to revive from the air-dried state. In this review, I summarize the distribution of vegetative desiccation-tolerance within the land plants emphasizing bryophytes, and provide a current checklist of those moss, liverwort and hornwort species with documented and experimentally determined tolerance. Desiccation-tolerant species can survive equilibration with either modestly dry air (i.e., 70–80% RH or −30 to −48 MPa) or extremely dry air (i.e., 0–30% RH or less than −162 MPa), and have been identified within seven classes of bryophytes, the Andreaeopsida, Bryopsida, Polytrichopsida and Tetraphidopsida (mosses), Jungermanniopsida and Marchantiopsida (liverworts) and the Anthocerotopsida (hornworts); 210 out of 21,000 bryophyte species (ca. 1.0%) have been experimentally determined to possess vegetative desiccation tolerance—158 species of mosses, 51 species of liverworts and one species of hornwort. Finally, I propose a comprehensive survey of mosses, live...