The fraction of expanding to expanded leaves determines the biomass response of Populus to elevated CO2

We examined whether the effects of elevated CO₂ on growth of 1-year old Populus deltoides saplings was a function of the assimilation responses of particular leaf developmental stages. Saplings were grown for 100 days at ambient (approximately 350 ppm) and elevated (ambient + 200 ppm) CO₂ in forced-air greenhouses. Biomass, biomass distribution, growth rates, and leaf initiation and expansion rates were unaffected by elevated CO₂. Leaf nitrogen (N), the leaf C:N ratio, and leaf lignin concentrations were also unaffected. Carbon gain was significantly greater in expanding leaves of saplings grown at elevated compared to ambient CO₂. The Rubisco content in expanding leaves was not affected by CO₂ concentration. Carbon gain and Rubisco content were significantly lower in fully expanded leaves of saplings grown at elevated compared to ambient CO₂, indicating CO₂-induced down-regulation in fully expanded leaves. Elevated CO₂ likely had no overall effect on biomass accumulation due to the more rapid decline in carbon gain as leaves matured in saplings grown at elevated compared to ambient CO₂. This decline in carbon gain has been documented in other species and shown to be related to a balance between sink/source balance and acclimation. Our data suggest that variation in growth responses to elevated CO₂ can result from differences in leaf assimilation responses in expanding versus expanded leaves as they develop under elevated CO₂. Received: 28 September 1998 / Accepted: 23 June 1999     

Changes in gene expression in Arabidopsis shoots during phosphate starvation and the potential for developing smart plants

Our aim was to generate and prove the concept of "smart" plants to monitor plant phosphorus (P) status in Arabidopsis. Smart plants can be genetically engineered by transformation with a construct containing the promoter of a gene up-regulated specifically by P starvation in an accessible tissue upstream of a marker gene such as beta-glucuronidase (GUS). First, using microarrays, we identified genes whose expression changed more than 2.5-fold in shoots of plants growing hydroponically when P, but not N or K, was withheld from the nutrient solution. The transient changes in gene expression occurring immediately (4 h) after P withdrawal were highly variable, and many nonspecific, shock-induced genes were up-regulated during this period. However, two common putative cis-regulatory elements (a PHO-like element and a TATA box-like element) were present significantly more often in the promoters of genes whose expression increased 4 h after the withdrawal of P compared with their general occurrence in the promoters of all genes represented on the microarray. Surprisingly, the expression of only four genes differed between shoots of P-starved and -replete plants 28 h after P was withdrawn. This lull in differential gene expression preceded the differential expression of a new group of 61 genes 100 h after withdrawing P. A literature survey indicated that the expression of many of these "late" genes responded specifically to P starvation. Shoots had reduced P after 100 h, but growth was unaffected. The expression of SQD1, a gene involved in the synthesis of sulfolipids, responded specifically to P starvation and was increased 100 h after withdrawing P. Leaves of Arabidopsis bearing a SQD1::GUS construct showed increased GUS activity after P withdrawal, which was detectable before P starvation limited growth. Hence, smart plants can monitor plant P status. Transferring this technology to crops would allow precision management of P fertilization, thereby maintaining yields while reducing costs, conserving natural resources, and preventing pollution.     

Transplantation of human glial-restricted neural precursors into injured spinal cord promotes functional and sensory recovery without causing allodynia

Background aimsTraumatic injuries of the central nervous system cause damage and degeneration of specific cell populations with subsequent functional loss. Cell transplantation is a strategy to treat such injuries by replacing lost or damaged cell populations. Many kinds of cells are considered candidates for intraspinal transplantation. Human neural precursor cells (hNPC) derived from post-mortem fetal tissue are easy to isolate and expand, and are capable of producing large numbers of neuronal and glial cells. After transplantation into the nervous system, hNPC produce mature neural phenotypes and permit functional improvement in some models of neurodegenerative disease. In this study, we aimed to elucidate the therapeutic effect of different neuronal and glial progenitor populations of hNPC on locomotor and sensory functions of spinal cord-injured (SCI) ratsMethodsDifferent populations of progenitor cells were obtained from hNPC by cell sorting and neural induction, resulting in cell cultures that were NCAM⁺ A2B5⁺, NCAM⁺ A2B5^? or A2B5⁺ NG2⁺. These different cell populations were then tested for efficacy in repair of the injured spinal cord by transplantation into rats with SCIResultsThe A2B5⁺ NG2⁺ population of hNPC significantly improved locomotor and sensory (hindlimb) functional recovery of SCI rats. Importantly, no abnormal pain responses were observed in the forelimbs following transplantationConclusionsThis treatment approach can improve functional recovery after SCI without causing allodynia. Further studies will be conducted to investigate the ability of A2B5⁺ NG2⁺ cells to survive, differentiate and integrate in the injured spinal cord.     

Mechanism of the Ca-Dependent Interaction between S100A4 and Tail Fragments of Nonmuscle Myosin Heavy Chain IIA

The interaction between the calcium-binding protein S100A4 and the C-terminal fragments of nonmuscle myosin heavy chain IIA has been studied by equilibrium and kinetic methods. Using site-directed mutants, we conclude that Ca²⁺ binds to the EF2 domain of S100A4 with micromolar affinity and that the K_d value for Ca²⁺ is reduced by several orders of magnitude in the presence of myosin target fragments. The reduction in K_d results from a reduced dissociation rate constant (from 16 s^− 1 to 0.3 s^− 1 in the presence of coiled-coil fragments) and an increased association rate constant. Using peptide competition assays and NMR spectroscopy, we conclude that the minimal binding site on myosin heavy chain IIA corresponds to A1907-G1938; therefore, the site extends beyond the end of the coiled-coil region of myosin. Electron microscopy and turbidity assays were used to assess myosin fragment filament disassembly by S100A4. The latter assay demonstrated that S100A4 binds to the filaments and actively promotes disassembly rather than just binding to the myosin monomer and displacing the equilibrium. Quantitative modelling of these in vitro data suggests that S100A4 concentrations in the micromolar region could disassemble myosin filaments even at resting levels of cytoplasmic [Ca²⁺]. However, for Ca²⁺ transients to be effective in further promoting dissociation, the elevated Ca²⁺ signal must persist for tens of seconds. Fluorescence recovery after photobleaching of A431/SIP1 cells expressing green fluorescent protein-myosin IIA, immobilised on fibronectin micropatterns to control stress fibre location, yielded a recovery time constant of around 20 s, consistent with in vitro data.     

Phytophthora himalsilva sp. nov. an unusually phenotypically variable species from a remote forest in Nepal

The Himalaya have received little investigation for Phytophthora species. In a remote forest in Western Nepal ten isolates of an unknown Phytophthora were recovered from the rhizosphere of Quercus, Castanopsis, Carpinus and Cupressus spp. The Phytophthora, formally named here as a P. himalsilva sp. nov., is homothallic with either amphigynous or paragynous antheridia and papillate, highly variable sporangia which may also be facultatively caducous. Based on ITS, β-tubulin, and cox I sequences Phytophthora himalsilva falls within Phytophthora Clade 2c together with Phytophthora citrophthora, Phytophthora meadii, Phytophthora colocasiae, and Phytophthora botryosa. It is suggested that Clade 2c has radiated within Asia. Molecular and sporangial characters indicate that P. himalsilva and P. citrophthora may share a recent common ancestor although they have diverged in their breeding systems. Although highly local the P. himalsilva isolates exhibited significant variation in growth rates and optimum temperatures for growth. This may reflect adaptation to different niches within a heterogeneous sub-tropical to temperate forest environment. Their cox I polymorphisms were also rather variable, including possible clustering for subsite. The occurrence of a previously unknown Phytophthora in a remote forest in Nepal highlights once again the plant health risk associated with moving rooted plants and soil between different bio-geographical regions of the world and the need for rapid pathological screening of potential risk organisms.     

A role for nuclear localised proteasomes in mediating auxin action

A number of important cellular events in animals and yeast are regulated by protein degradation, and it is becoming apparent that such regulated proteolysis is involved in many facets of plant physiology and development. We have investigated the role of protein degradation by proteasomes in plants using NtPSA1, a tobacco gene that is predominantly expressed in young developing tobacco tissues and has extensive homology to yeast and human alpha-type proteasome subunit genes. The NtPSA1 cDNA was used to complement a lethal mutation of the yeast PRC1 alpha subunit gene indicating that NtPSA1 encodes a functional proteasome subunit, and transient expression of an NtPSA1::GUS protein fusion in onion cells confirmed that the nuclear localisation signal that is present in the NtPSA1 peptide sequence is active in plant cells. Plants transformed with an antisense NtPSA1 gene had reduced levels of NtPSA1 mRNA and exhibited reduced apical dominance. In addition, these low NtPSA1 plants displayed several morphological defects associated with auxin resistance such as reduced stamen length, and showed increased tolerance to high concentrations of auxin. These results support a role for nuclear localised proteasomes in floral development and auxin responses.     

Article Watch: September 2013

This column highlights recently published articles that are of interest to the readership of this publication. We encourage ABRF members to forward information on articles they feel are important and useful to Clive Slaughter, GRU-UGA Medical Partnership, 1425 Prince Ave., Athens, GA 30606; Phone: 706-713-2216; Fax: 706-713-2221; E-mail: ude.agu@thgualsc; or to any member of the editorial board. Article summaries reflect the reviewer''s opinions and not necessarily those of the association.