Root gravitropism requires lateral root cap and epidermal cells for transport and response to a mobile auxin signal

Re-orientation of Arabidopsis seedlings induces a rapid, asymmetric release of the growth regulator auxin from gravity-sensing columella cells at the root apex. The resulting lateral auxin gradient is hypothesized to drive differential cell expansion in elongation-zone tissues. We mapped those root tissues that function to transport or respond to auxin during a gravitropic response. Targeted expression of the auxin influx facilitator AUX1 demonstrated that root gravitropism requires auxin to be transported via the lateral root cap to all elongating epidermal cells. A three-dimensional model of the root elongation zone predicted that AUX1 causes the majority of auxin to accumulate in the epidermis. Selectively disrupting the auxin responsiveness of expanding epidermal cells by expressing a mutant form of the AUX/IAA17 protein, axr3-1, abolished root gravitropism. We conclude that gravitropic curvature in Arabidopsis roots is primarily driven by the differential expansion of epidermal cells in response to an influx-carrier-dependent auxin gradient.     

Long-term functional plasticity in plant hydraulic architecture in response to supplemental moisture

Background and Aims

Plasticity in structural and functional traits related to water balance may determine plant performance and survival in ecosystems characterized by water limitation or high levels of rainfall variability, particularly in perennial herbaceous species with long generation cycles. This paper addresses whether and the extent to which several such seasonal to long-term traits respond to changes in moisture availability.

Methods

Using a novel approach that integrates ecology, physiology and anatomy, a comparison was made of lifetime functional traits in the root xylem of a long-lived perennial herb (Potentilla diversifolia, Rosaceae) growing in dry habitats with those of nearby individuals growing where soil moisture had been supplemented for 14 years. Traditional parameters such as specific leaf area (SLA) and above-ground growth were also assessed.

Key Results

Individuals from the site receiving supplemental moisture consistently showed significant responses in all considered traits related to water balance: SLA was greater by 24 %; roots developed 19 % less starch storing tissue, an indicator for drought-stress tolerance; and vessel size distributions shifted towards wider elements that collectively conducted water 54 % more efficiently – but only during the years for which moisture was supplemented. In contrast, above-ground growth parameters showed insignificant or inconsistent responses.

Conclusions

The phenotypic changes documented represent consistent, dynamic responses to increased moisture availability that should increase plant competitive ability. The functional plasticity of xylem anatomy quantified in this study constitutes a mechanistic basis for anticipating the differential success of plant species in response to climate variability and change, particularly where water limitation occurs.     

Structural elucidation of the O-antigenic polysaccharide from enterohemorrhagic (EHEC) Escherichia coli O48:H21

The structure of the antigenic O-polysaccharide (O-PS) of the lipopolysaccharide (LPS) produced by the enterohemorrhagic strain of Escherichia coli O48:H21 (EHEC) has been elucidated. The O-PS obtained by mild acid hydrolysis of the LPS had [alpha]D +95 (water) and was composed of L-rhamnose (L-Rha), D-galactose (D-Gal), 2-amino-2-deoxy-D-glucose (D-GlcN), 2-amino-2-deoxy-D-galactose (D-GalN), and D-galacturonic acid (D-GalA) (1:1:1:1:1). From the results of methylation analysis, mass spectrometry, 2D NMR, and DOC-PAGE, the O-PS was shown to be a high molecular mass polymer of a repeating pentasaccharide unit having the structure: [structure: see text]. The D-Gal pA non-reducing end groups in the O-PS were partially O-acetylated (approximately 30%) at the O-2 and O-3 positions and the degree of acetylation was variable from batch to batch cell production.     

Barrier activity in Candida albicans mediates pheromone degradation and promotes mating

Mating in Candida albicans and Saccharomyces cerevisiae is regulated by the secretion of peptide pheromones that initiate the mating process. An important regulator of pheromone activity in S. cerevisiae is barrier activity, involving an extracellular aspartyl protease encoded by the BAR1 gene that degrades the alpha pheromone. We have characterized an equivalent barrier activity in C. albicans and demonstrate that the loss of C. albicans BAR1 activity results in opaque a cells exhibiting hypersensitivity to alpha pheromone. Hypersensitivity to pheromone is clearly seen in halo assays; in response to alpha pheromone, a lawn of C. albicans Deltabar1 mutant cells produces a marked zone in which cell growth is inhibited, whereas wild-type strains fail to show halo formation. C. albicans mutants lacking BAR1 also exhibit a striking mating defect in a cells, but not in alpha cells, due to overstimulation of the response to alpha pheromone. The block to mating occurs prior to cell fusion, as very few mating zygotes were observed in mixes of Deltabar1 a and alpha cells. Finally, in a barrier assay using a highly pheromone-sensitive strain, we were able to demonstrate that barrier activity in C. albicans is dependent on Bar1p. These studies reveal that a barrier activity to alpha pheromone exists in C. albicans and that the activity is analogous to that caused by Bar1p in S. cerevisiae.     

Conformational analysis of protein structures derived from NMR data

A study is presented of the conformational characteristics of NMR-derived protein structures in the Protein Data Bank compared to X-ray structures. Both ensemble and energy-minimized average structures are analyzed. We have addressed the problem using the methods developed for crystal structures by examining the distribution of ?, Ψ, and χ angles as indicators of global conformational irregularity. All these features in NMR structures occur to varying degrees in multiple conformational states. Some measures of local geometry are very tightly constrained by the methods used to generate the structure, e.g., proline ? angles, α-helix ?, Ψ angles, ω angles, and C_α chirality. The more lightly restrained torsion angles do show increasead clustering as the number of overall experimental observations increases. ?, Ψ, and χ₁ angle conformational heterogeneity is strongly correlated with accessibility but shows additional differences which reflect the differing number of observations possible in NMR for the various side chains (e.g., many for Trp, few for Ser). In general, we find that the core is defined to a notional resolution of 2.0 to 2.3 Å. Of real interest is the behavior of surface residues and in particular the side chains where multiple rotameric states in different structures can vary from 10% to 88%. Later generation structures show a much tighter definition which correlates with increasing use of J-coupling information, stereospecific assignments, and heteronumclear techniques. A suite of programs is being developed to address the special needs of NMR-derived structures which will take into account the existence of increased mobility in solution. © 1993 Wiley-Liss, Inc.     

Gene-Enzyme Relationships in Neurospora Invertase

A spontaneous, single-gene mutation responsible for a total lack of invertase activity in Neurospora crassa is described. The mutation is believed to lie in the structural gene for invertase, since an immunologically cross-reacting protein is made by the mutant strain. In addition, there was no evidence for a defect in regulation of invertase activity or synthesis by the following criteria. (i) The invertaseless condition was recessive in heterokaryons; (ii) no invertase inhibitor was found in mutant extracts by mixing experiments; and (iii) none of the several sugars able to induce activity in wild-type strains was able to induce activity in the mutant strain. It was also discovered that most of the wild-type enzyme (55 to 75%) cannot be washed free from the rapidly sedimenting cell debris. This finding provided additional support for the hypothesis that Neurospora invertase is located within or about the cell wall.     

Low and annually variable migratory connectivity in a long‐distance migrant: Whinchats Saxicola rubetra may show a bet‐hedging strategy

The spatial scale of non‐breeding areas used by long‐distance migrant animals can vary from specific, relatively small non‐breeding areas for each independent breeding population (high connectivity) to a distribution over a large non‐breeding area with mixing of breeding populations (low connectivity). Measuring variation in the degree of connectivity and how it arises is crucial to predict how migratory animals can respond to global habitat and climate change because low connectivity is likely to be an adaptation to environmental uncertainty. Here, we assess whether use of non‐breeding areas in a long‐distance migrant may be stochastic by measuring the degree of connectivity, and whether it is annually variable. Twenty‐nine wintering Whinchats tagged with geolocators over 2 years within 40 km² in central Nigeria were found to be breeding over 2.55 million km² (26% of the land area of Europe), without an asymptote being approached in the relationship between area and sample size. Ranges differed in size between years by 1.51 million km² and only 15% of the total breeding range across both years overlapped (8% overlap between years when only first‐year birds were considered), well above the range size difference and below the proportion of overlap that would be predicted from two equivalent groups breeding at random locations within the observed range. Mean distance between breeding locations (i.e. migratory spread) differed significantly between years (604 ± 18 km in 2013 and 869 ± 33 km in 2014). The results showed very low and variable connectivity that was reasonably robust to the errors and assumptions inherent in the use of geolocators, but with the caveat of having only ranges of 2 years to compare, and the sensitivity of range to the breeding locations of a small number of individuals. However, if representative, the results suggest the scope for between‐year variation (cohort effects) to determine migrant distribution on a large scale. Furthermore, for species with similarly low connectivity, we would predict breeding population trends to reflect average conditions across large non‐breeding areas: thus, as large areas of Africa become subject to habitat loss, migrant populations throughout Europe will decline.