Leaf photosynthetic rate is correlated with biomass and grain production in grain sorghum lines

Significant genetic variation in leaf photosynthetic rate has been reported in grain sorghum [Sorghum biocolor (L.) Moench]. The relationships between leaf photosynthetic rates and total biomass production and grain yield remain to be established and formed the purpose of this experiment. Twenty two grain sorghum parent lines were tested in the field during the 1988 growing season under well-watered and water-limited conditions. Net carbon assimilation rates were measured at mid-day during the 30 day period from panicle initiation to head exertion on upper-most fully expanded leaves using a portable photosynthesis system (LI-6200). Total biomass and grain production were determined at physiological maturity. The lines exhibited significant genetic variation in leaf photosynthetic rate, total biomass production and grain yield. Significant positive correlations existed between leaf photosynthesis and total biomass and grain production under both well-watered and water-limited conditions. The results suggest that leaf photosynthetic rate measured prior to flowering is a good indicator of productivity in grain sorghum.     

Human von Willebrand factor: a multivalent protein composed of identical subunits

A large-scale method for the isolation of von Willebrand factor (vWF) from human factor VIII concentrates was developed in order to study the structure of this protein and its platelet binding activity. vWF is composed of a number of glycoprotein subunits that are linked together by disulfide bonds to form a series of multimers. These multimers appear to contain an even number of subunits of 270K. Two minor components of Mr 140K and 120K were also identified, but these chains appear to result from minor proteolysis. The smallest multimer of vWF contained nearly equimolar amounts of the 270K, 140K, and 120K subunits, while the largest multimers contained less than 20% of the two minor components. Amino acid sequence analysis, amino acid composition, and cleavage by cyanogen bromide indicate that the 270K subunits are identical and each is a single polypeptide chain with an amino-terminal sequence of Ser-Leu-Ser-Cys-Arg-Pro-Pro-Met-Val-Lys and a carboxyl-terminal sequence of Glu-Cys-Lys-Cys-Ser-Pro-Arg-Lys-Cys-Ser-Lys. Platelet binding in the presence of ristocetin was 8-fold greater with multimers larger than five (i.e., containing more than 10 subunits of 270K) as compared to multimers less than three (containing less than six subunits of 270K). However, partially reduced vWF (Mr 500K), regardless of whether it was prepared from large or small molecular weight multimers, gave platelet binding similar to that of the smallest multimers. Likewise, partial proteolysis by elastase, thermolysin, trypsin, or chymotrypsin produced small "multimer-like" proteins with platelet binding properties similar to either partially reduced vWF or to the smallest multimers. We conclude that human vWF contains identical 270K subunits assembled into a multivalent structure. Disassembly by either partial reduction or partial proteolysis produces essentially monovalent protein with platelet binding properties similar to that of the smallest multimers. Multivalency is likely the primary factor responsible for the increase in biological activity with multimer size.     

The northern corn leaf blight resistance gene Ht1 encodes an nucleotide-binding,leucine-rich repeat immune receptor

Northern corn leaf blight, caused by the fungal pathogen Exserohilum turcicum, is a major disease of maize. The first major locus conferring resistance to E. turcicum race 0, Ht1, was identified over 50 years ago, but the underlying gene has remained unknown. We employed a map-based cloning strategy to identify the Ht1 causal gene, which was found to be a coiled-coil nucleotide-binding, leucine-rich repeat (NLR) gene, which we named PH4GP-Ht1. Transgenic testing confirmed that introducing the native PH4GP-Ht1 sequence to a susceptible maize variety resulted in resistance to E. turcicum race 0. A survey of the maize nested association mapping genomes revealed that susceptible Ht1 alleles had very low to no expression of the gene. Overexpression of the susceptible B73 allele, however, did not result in resistant plants, indicating that sequence variations may underlie the difference between resistant and susceptible phenotypes. Modelling of the PH4GP-Ht1 protein indicated that it has structural homology to the Arabidopsis NLR resistance gene ZAR1, and probably forms a similar homopentamer structure following activation. RNA sequencing data from an infection time course revealed that 1 week after inoculation there was a threefold reduction in fungal biomass in the PH4GP-Ht1 transgenic plants compared to wild-type plants. Furthermore, PH4GP-Ht1 transgenics had significantly more inoculation-responsive differentially expressed genes than wild-type plants, with enrichment seen in genes associated with both defence and photosynthesis. These results demonstrate that the NLR PH4GP-Ht1 is the causal gene underlying Ht1, which represents a different mode of action compared to the previously reported wall-associated kinase northern corn leaf blight resistance gene Htn1/Ht2/Ht3.     

Osmotic adjustment in sorghum: I. Mechanisms of diurnal osmotic potential changes

Osmotic adjustment, defined as a lowering of osmotic potential (ψ_π) due to net solute accumulation in response to water stress, has been considered to be a beneficial drought tolerance mechanism in some crop species. The objective of this experiment was to determine the relative contribution of passive versus active mechanisms involved in diurnal ψ_π changes in sorghum (Sorghum bicolor L. Moench) leaf tissue in response to water stress. A single sorghum hybrid (cv AT×623 × RT×430) was grown in the field under variable water supplies. Water potential, ψ_π, and relative water content were measured diurnally on expanding and the uppermost fully expanded leaves before flowering and on fully expanded leaves during the grain-filling period. Diurnal changes in total osmotic potential (Δψ_π) in response to water stress was 1.1 megapascals before flowering and 1.4 megapascals during grain filling in comparison with 0.53 megapascal under well-watered conditions. Under water-stressed conditions, passive concentration of solutes associated with dehydration accounted for 50% (0.55 megapascal) of the diurnal Δψ_π before flowering and 47% (0.66 megapascal) of the change during grain filling. Net solute accumulation accounted for 42% (0.46 megapascal) of the diurnal Δψ_π before flowering and 45% (0.63 megapascal) of the change during grain filling in water-stressed leaves. The relative contribution of changes in nonosmotic volume (decreased turgid weight/dry weight) to diurnal Δψ_π was less than 8% at either growth stages. Water stress did not affect leaf tissue elasticity or partitioning of water between the symplasm and apoplasm.     

Generation of von Willebrand factor epitope libraries expressed in E. coli

The von Willebrand factor (VWF) subunit is composed of several domains, often coinciding with structural regions, characterized through their specific interaction with a ligand. Since several monoclonal antibodies (MoAbs) have been shown to functionally interfere with one of the specific interactions, we have created libraries of bacterial clones expressing peptidic sequences of VWF to map antibodies directed against this protein. Randomly cleaved fragments of VWF cDNA have been cloned in a plasmid designed for the expression of small peptides as part of larger fusion proteins. The NovaTope system is a useful procedure for protein analysis, allowing screening of epitopes composed of contiguous amino acid residues. To map MoAbs with conformational discontinuous epitopes displayed on small as well as large peptidic domains, this technique had to be widely modified to obtain two VWF peptide libraries expressing two ranges of peptide length (15-70 and 100-300 amino acids). Screening with six MoAbs with an epitope in a known region was performed to control both libraries. Four MoAbs were mapped through the characterization of overlapping sequences for 5-10 different positively expressed clones respectively. Two of these mapped MoAbs had no known inhibitory effect and bind reduced VWF only. The fact that the two other MoAbs mapped VWF functional interactions with ligands, platelet GPIIb/IIIa and Factor VIII, respectively, demonstrate that our libraries are valuable tools to determine conformational epitopes.     

Triterpenoidal lupin saponins from the Chilean legume Lupinus oreophilus Phil

Two lupin saponins, 3beta,21alpha,22beta,24-tetrahydroxyolean-12-en-3-O-alpha-L-rhamnopyranosyl-(1-->2)-beta-D-galactopyranosyl-(1-->2)-beta-D-glucuronopyranosyl-21-O-alpha-L-arabinopyranoside and 3beta,21alpha,22beta,24-tetrahydroxyolean-12-en-3-O-beta-D-galactopyranosyl-(1-->2)-beta-D-glucuronopyranosyl-21-O-alpha-L-rhamnopyranoside, along with eight other saponins and one triterpene previously reported from other legumes, were isolated from the aerial parts of Lupinus oreophilus collected in northern Chile. The structures of the isolated compounds were established with the help of extensive spectroscopic techniques.     

Linking Aβ42-Induced Hyperexcitability to Neurodegeneration,Learning and Motor Deficits,and a Shorter Lifespan in an Alzheimer’s Model

Alzheimer’s disease (AD) is the most prevalent form of dementia in the elderly. β-amyloid (Aβ) accumulation in the brain is thought to be a primary event leading to eventual cognitive and motor dysfunction in AD. Aβ has been shown to promote neuronal hyperactivity, which is consistent with enhanced seizure activity in mouse models and AD patients. Little, however, is known about whether, and how, increased excitability contributes to downstream pathologies of AD. Here, we show that overexpression of human Aβ42 in a Drosophila model indeed induces increased neuronal activity. We found that the underlying mechanism involves the selective degradation of the A-type K+ channel, Kv4. An age-dependent loss of Kv4 leads to an increased probability of AP firing. Interestingly, we find that loss of Kv4 alone results in learning and locomotion defects, as well as a shortened lifespan. To test whether the Aβ42-induced increase in neuronal excitability contributes to, or exacerbates, downstream pathologies, we transgenically over-expressed Kv4 to near wild-type levels in Aβ42-expressing animals. We show that restoration of Kv4 attenuated age-dependent learning and locomotor deficits, slowed the onset of neurodegeneration, and partially rescued premature death seen in Aβ42-expressing animals. We conclude that Aβ42-induced hyperactivity plays a critical role in the age-dependent cognitive and motor decline of this Aβ42-Drosophila model, and possibly in AD.