Following (1) the large-scale molecular phylogeny of seed plants based on plastid rbcL gene sequences (published in 1993 by Chase et al., Ann. Missouri Bot. Gard. 80:528-580) and (2) the 18S nuclear phylogeny of flowering plants (published in 1997 by Soltis et al., Ann. Missouri Bot. Gard. 84:1-49), we present a phylogenetic analysis of flowering plants based on a second plastid gene, atpB, analyzed separately and in combination with rbcL sequences for 357 taxa. Despite some discrepancies, the atpB-based phylogenetic trees were highly congruent with those derived from the analysis of rbcL and 18S rDNA, and the combination of atpB and rbcL DNA sequences (comprising approximately 3000 base pairs) produced increased bootstrap support for many major sets of taxa. The angiosperms are divided into two major groups: noneudicots with inaperturate or uniaperturate pollen (monocots plus Laurales, Magnoliales, Piperales, Ceratophyllales, and Amborellaceae-Nymphaeaceae-Illiciaceae) and the eudicots with triaperturate pollen (particularly asterids and rosids). Based on rbcL alone and atpB/rbcL combined, the noneudicots (excluding Ceratophyllum) are monophyletic, whereas in the atpB trees they form a grade. Ceratophyllum is sister to the rest of angiosperms with rbcL alone and in the combined atpB/rbcL analysis, whereas with atpB alone, Amborellaceae, Nymphaeaceae, and Illiciaceae/Schisandraceae form a grade at the base of the angiosperms. The phylogenetic information at each codon position and the different types of substitutions (observed transitions and transversions in the trees vs. pairwise comparisons) were examined; taking into account their respective consistency and retention indices, we demonstrate that third-codon positions and transitions are the most useful characters in these phylogenetic reconstructions. This study further demonstrates that phylogenetic analysis of large matrices is feasible.

It is well accepted that lateral redistribution of the phytohormone auxin underlies the bending of plant organs towards light. In monocots, photoreception occurs at the shoot tip above the region of differential growth. Despite more than a century of research, it is still unresolved how light regulates auxin distribution and where this occurs in dicots. Here, we establish a system in Arabidopsis thaliana to study hypocotyl phototropism in the absence of developmental events associated with seedling photomorphogenesis. We show that auxin redistribution to the epidermal sites of action occurs at and above the hypocotyl apex, not at the elongation zone. Within this region, we identify the auxin efflux transporter ATP-BINDING CASSETTE B19 (ABCB19) as a substrate target for the photoreceptor kinase PHOTOTROPIN 1 (phot1). Heterologous expression and physiological analyses indicate that phosphorylation of ABCB19 by phot1 inhibits its efflux activity, thereby increasing auxin levels in and above the hypocotyl apex to halt vertical growth and prime lateral fluxes that are subsequently channeled to the elongation zone by PIN-FORMED 3 (PIN3). Together, these results provide new insights into the roles of ABCB19 and PIN3 in establishing phototropic curvatures and demonstrate that the proximity of light perception and differential phototropic growth is conserved in angiosperms.

Plants depend on the surrounding light environment to direct their growth. Blue light (300–500 nm) in particular acts to promote a wide variety of photomorphogenic responses including seedling establishment, phototropism and circadian clock regulation. Several different classes of flavin-based photoreceptors have been identified that mediate the effects of blue light in the dicotyledonous genetic model Arabidopsis thaliana. These include the cryptochromes, the phototropins and members of the Zeitlupe family. In this review, we discuss recent advances, which contribute to our understanding of how these photosensory systems are activated by blue light and how they initiate signaling to regulate diverse aspects of plant development.

Imbibed Arabidopsis (Arabidopsis thaliana) seeds are encapsulated by mucilage that is formed of hydrated polysaccharides released from seed coat epidermal cells. The mucilage is structured with water-soluble and adherent layers, with cellulose present uniquely in an inner domain of the latter. Using a reverse-genetic approach to identify the cellulose synthases (CESAs) that produce mucilage cellulose, cesa5 mutants were shown to be required for the correct formation of these layers. Expression of CESA5 in the seed coat was specific to epidermal cells and coincided with the accumulation of mucilage polysaccharides in their apoplast. Analysis of sugar composition showed that although total sugar composition or amounts were unchanged, their partition between layers was different in the mutant, with redistribution from adherent to water-soluble mucilage. The macromolecular characteristics of the water-soluble mucilage were also modified. In accordance with a role for CESA5 in mucilage cellulose synthesis, crystalline cellulose contents were reduced in mutant seeds and birefringent microfibrils were absent from adherent mucilage. Although the mucilage-modified5 mutant showed similar defects to cesa5 in the distribution of sugar components between water-soluble and adherent mucilage, labeling of residual adherent mucilage indicated that cesa5 contained less cellulose and less pectin methyl esterification. Together, the results demonstrate that CESA5 plays a major and essential role in cellulose production in seed mucilage, which is critical for the establishment of mucilage structured in layers and domains.