Colonization of land by plants was a major transition on Earth, but the developmental and genetic innovations required for this transition remain unknown. Physiological studies and the fossil record strongly suggest that the ability of the first land plants to form symbiotic associations with beneficial fungi was one of these critical innovations. In angiosperms, genes required for the perception and transduction of diffusible fungal signals for root colonization and for nutrient exchange have been characterized. However, the origin of these genes and their potential correlation with land colonization remain elusive. A comprehensive phylogenetic analysis of 259 transcriptomes and 10 green algal and basal land plant genomes, coupled with the characterization of the evolutionary path leading to the appearance of a key regulator, a calcium-and calmodulin-dependent protein kinase, showed that the symbiotic signaling pathway predated the first land plants. In contrast, downstream genes required for root colonization and their specific expression pattern probably appeared subsequent to the colonization of land. We conclude that the most recent common ancestor of extant land plants and green algae was preadapted for symbiotic associations. Subsequent improvement of this precursor stage in early land plants through rounds of gene duplication led to the acquisition of additional pathways and the ability to form a fully functional arbuscular mycorrhizal symbiosis.symbiosis | plant evolution | algae | plant-microbe interactions | phylogeny T he colonization of land by plants 450 Mya created a major transition on Earth, causing the burial of large amounts of carbon, with resultant decreases in atmospheric CO 2 leading to a dramatically altered climate. This transition provided the foundation for the majority of extant terrestrial ecosystems (1, 2). The terrestrial environment that these early plants colonized must have presented many challenges, primary among them being the acquisition of mineral nutrients. It has been suggested that the appearance of the arbuscular mycorrhizal (AM) symbiosis and other beneficial associations with fungi such as Mucoromycotina facilitated this colonization of land by improving plants' ability to capture nutrients.Based on recent phylogenetic analyses, Zygnematales, one of the paraphyletic "advanced charophytes" (i.e., Coleochaetales, Charales, and Zygnematales), has been identified as the closest green algal relative to land plants, whereas the chlorophytes diverged much earlier (Fig. 1A) (3, 4). On the other side of this transition, bryophyte lineages (i.e., liverworts, mosses, and hornworts) are considered to be the earliest diverging land plants, although their branching order remains debated (5-7). Key innovations present in bryophytes but not in advanced charophytes thus are good candidates for understanding the basis of land colonization by plants. In support of previous suppositions, one innovation that discriminates bryophytes from charophytes is the ability to develop beneficial ass...

SummaryGibberellins (GAs) are essential for the development of fertile flowers in tomato, and may also be required immediately after fertilization. In the GA-biosynthetic pathway, the reactions catalyzed by GA 20-oxidases have been implicated as site of regulation. To study the regulation of GA biosynthesis in flower and early fruit development, we isolated three tomato GA 20-oxidase cDNA clones, Le20ox-1, -2 and -3. The three genes showed different organ-specific patterns of mRNA accumulation. Analysis of the transcript levels of the three GA 20-oxidase genes, as well as those of copalyl diphosphate synthase (LeCPS) and GA 3β-hydroxylase (Le3OH-2) during flower bud and early fruit development, revealed temporally distinct patterns of mRNA accumulation. Up until anthesis, transcripts were observed for LeCPS, Le20ox-1, -2 and Le3OH-2, with an accumulation of Le20ox-1 mRNA. In contrast to the high level of Le3OH-2 transcripts in the fully open flower, mRNA levels of Le20ox-1, -2 and LeCPS were reduced at this stage. After anthesis, LeCPS and Le20ox-1 transcripts increased again. In addition, Le20ox-3 transcripts increased whereas the transcripts of Le3OH-2 decreased to an undetectable level. In situ hybridization results demonstrated that during early stages of bud development, Le20ox-2 transcripts were localized in the tapetum and placenta. The presented results supply novel data about localization of GA biosynthesis gene transcripts, and indicate that transcript levels of GA biosynthesis genes are all highly regulated during flower bud development.

The MADS-box genes of land plants are extensively diverged to form a superfamily and are important in various aspects of development including the specification of floral organs as homeotic selector genes. The closest relatives of land plants are the freshwater green algae charophyceans. To study the origin and evolution of land plant MADS-box genes, we characterized these genes in three charophycean green algae: the stonewort Chara globularis, the coleochaete Coleochaete scutata, and the desmid Closterium peracerosum-strigosum-littorale complex. Phylogenetic analyses suggested that MADS-box genes diverged extensively in the land plant lineage after the separation of charophyceans from land plants. The stonewort C. globularis mRNA was specifically detected in the oogonium and antheridium together with the egg and spermatozoid during their differentiation. The expression of the C. peracerosum-strigosum-littorale-complex gene increased when vegetative cells began to differentiate into gametangial cells and decreased after fertilization. These expression patterns suggest that the precursors of land plant MADS-box genes originally functioned in haploid reproductive cell differentiation and that the haploid MADS-box genes were recruited into a diploid generation during the evolution of land plants.charophytes ͉ land plants ͉ Chara ͉ Coleochaete ͉ Closterium D iversity in form, a hallmark of extant species, is probably caused by modifications of ancestral gene networks regulating development and by the generation of novel developmental processes (1). The evolution of transcription factors, which have critical functions in development, through gene duplication and subsequent functional divergence has been hypothesized to be a major force in developmental evolution (reviewed in ref.2). The adaptation of green plants to a terrestrial environment and their subsequent diversification are tightly linked to the evolution of the body plan of land plants (3, 4). Members of the MADS-box gene family regulate various aspects of development in flowering plants and therefore were probably involved in the evolution of the morphology of land plants.MADS-box genes are characterized by the conserved MADS domain and are found in a wide range of eukaryotes including metazoans, fungi, slime mold, and green plants (5). These genes have been classified into several groups (6). MIKC C -and MIKC*-type MADS-box genes contain intervening (I), keratinlike (K), and C-terminal (C) domains (7,8). They are present in all major land plant taxa including seed plants, pteridophytes, and bryophytes but have not been found in other organisms such as green algae (5, 6, 9). The Arabidopsis genome (10) contains Ϸ38 MIKC C -type and 5 MIKC*-type genes (6, 9). The MIKC*-type genes tend to form a monophyletic group, including several subgroups of genes with unknown functions (9), but the MIKC Ctype MADS-box genes have been classified into about a dozen subfamilies with diverse functions and expression patterns (reviewed in ref. 6). The floral homeotic genes ar...

Using degenerate primers designed by deduced amino acid sequences of known aldehyde oxidases (AO) from maize and bovine, two independent cDNA fragments were amplified by reverse transcription-polymerase chain reaction (PCR). The two corresponding full-length cDNAs (atAO-1 and atAO-2; 4,484 and 4,228 bp long, respectively) were cloned by screening the Arabidopsis cDNA library followed by rapid amplification of cDNA end-PCR. These cDNAs are highly homologous at both the nucleotide and amino acid sequence levels, and the deduced amino acid sequences showed high similarity with those of maize and tomato AOs. They contain consensus sequences for two iron-sulfur centers and a molybdenum cofactor (MoCo)-binding domain. In addition, another cDNA having a sequence similar to that of the cDNAs was screened (atAO-3; 3,049 bp), and a putative AO gene (AC002376) was reported on chromosome 1, which (atAO-4) was distinct from, but very similar to, the above three AOs. atAO-1, 2, 3, and 4 were physically mapped on chromosomes 5, 3, 2 and 1, respectively. These data indicate that there is an AO multigene family in Arabidopsis. atAO-1 protein was shown to be highly similar to one of the maize AOs in respect to a region thought to be involved in determination of substrate specificity, suggesting that they might encode a similar type of AO, which could efficiently oxidize indole-3-acetaldehyde to indole-3-acetic acid (IAA). atAO-1 and atAO-2 genes were expressed at higher levels in lower hypocotyls and roots of the wild-type seedlings, while atAO-3 was slightly higher in cotyledons and upper hypocotyls. The expression of atAO-1 was more abundant in the seedlings of an IAA overproducing mutant (superroot1; sur1) than in those of wild type. atAO-2 and atAO-3 transcripts were rather evenly distributed in these seedlings. A possible involvement of atAO genes in phytohormone biosynthesis in Arabidopsis is discussed.

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