Understanding down-regulation of photosynthesis under water stress: future prospects and searching for physiological tools for irrigation management

Photosynthetic down-regulation and/or inhibition under water stress conditions are determinants for plant growth, survival and yield in drought-prone areas. Current knowledge about the sequence of metabolic events that leads to complete inhibition of photosynthesis under severe water stress is reviewed. An analysis of published data reveals that a key regulatory role for Rubisco in photosynthesis is improbable under water stress conditions. By contrast, the little data available for other Calvin cycle enzymes suggest the possibility of a key regulatory role for some enzymes involved in the regeneration of RuBP. There are insufficient data to determine the role of photophosphorylation. Several important gaps in our knowledge of this field are highlighted. The most important is the remarkable scarcity of data about the regulation/inhibition of photosynthetic enzymes other than Rubisco under water stress. Consequently, new experiments are urgently needed to improve our current understanding of photosynthetic down-regulation under water stress. A second gap is the lack of knowledge of photosynthetic recovery after irrigation of plants which have been subjected to different stages of water stress. This knowledge is necessary in order to match physiological down-regulation by water stress with controlled irrigation programmes.     

USING PHYLOGENETIC TREES TO STUDY SPECIATION AND EXTINCTION

One tool in the study of the forces that determine species diversity is the null, or simple, model. The fit of predictions to observations, good or bad, leads to a useful paradigm or to knowledge of forces not accounted for, respectively. It is shown how simple models of speciation and extinction lead directly to predictions of the structure of phylogenetic trees. These predictions include both essential attributes of phylogenetic trees: lengths, in the form of internode distances; and topology, in the form of internode links. These models also lead directly to statistical tests which can be used to compare predictions with phylogenetic trees that are estimated from data. Two different models and eight data sets are considered. A model without species extinction consistently yielded predictions closer to observations than did a model that included extinction. It is proposed that it may be useful to think of the diversification of recently formed monophyletic groups as a random speciation process without extinction.     

Neonatal vitamin K administration and childhood cancer in the north of England: retrospective case-control study.

OBJECTIVE: To explore the possible association between intramuscular vitamin K given to neonates and the subsequent development of childhood cancer. DESIGN: Retrospective case-control study on the basis of hospital records. SETTING: The former Northern Health region of England. SUBJECTS: 685 children who were born and lived in the region and who developed cancer before their 15th birthday, and 3442 controls also born between 1960 and 1991 and matched only for date and hospital of birth. The notes of a further 701 index cases were untraceable. MAIN EXPOSURE MEASURE: Administration of intramuscular vitamin K versus no exposure to vitamin K. RESULTS: There was no association between the administration of vitamin K and the development of all childhood cancers (unadjusted odds ratio 0.89; 95% confidence interval 0.69 to 1.15) or for all acute lymphoblastic leukaemia (1.20; 0.75 to 1.92), but there was a raised odds ratio for acute lymphoblastic leukaemia developing 1-6 years after birth (1.79; 1.02 to 3.15). No such association was seen in a separate cohort-based study not dependent on case note retrieval in which the rates of acute lymphoblastic leukaemia in children born in hospital units where all babies received vitamin K were compared with those born in units where less than a third received prophylaxis. CONCLUSIONS: It is not possible, on the basis of currently published evidence, to refute the suggestion that neonatal intramuscular vitamin K administration increases the risk of early childhood leukaemia. Any association may have been masked in earlier studies that did not use controls matched for time and locality by other unidentified factors affecting the spatiotemporal variations in incidence of leukaemia.     

The Effects of Mutation and Natural Selection on Codon Bias in the Genes of Drosophila

Codon bias varies widely among the loci of Drosophila melanogaster, and some of this diversity has been explained by variation in the strength of natural selection. A study of correlations between intron and coding region base composition shows that variation in mutation pattern also contributes to codon bias variation. This finding is corroborated by an analysis of variance (ANOVA), which shows a tendency for introns from the same gene to be similar in base composition. The strength of base composition correlations between introns and codon third positions is greater for genes with low codon bias than for genes with high codon bias. This pattern can be explained by an overwhelming effect of natural selection, relative to mutation, in highly biased loci. In particular, this correlation is absent when examining fourfold degenerate sites of highly biased genes. In general, it appears that selection acts more strongly in choosing among fourfold degenerate codons than among twofold degenerate codons. Although the results indicate regional variation in mutational bias, no evidence is found for large scale regions of compositional homogeneity.     

STUDY ON THE PROLIFERATIVE STATE OF HAEMOPOIETIC STEM CELLS (CFU)

Hydroxyurea was used to study the proliferation rate of haemopoietic stem cells (CFUJ in normal mice, after irradiation or transplantation into irradiated recipients. It was demonstrated that the proliferation rate of endogenous CFU_S (endo-CFU,) and exogenous CFU_S (exo-CFU_s) are identical. After irradiation (650 R) the surviving endo-CFU_s begin to proliferate immediately. By contrast exo-CFU, transplanted into the irradiated recipient mouse (850 R), begin to proliferate only after about 30 hr. However, injection of isoproterenol (which stimulates adenyl cyclase) or dibutyryl cyclic adenosine 3′,5′-monophosphate shortly after marrow cell graft, triggers the transplanted CFU_S into cell cycle as shown by an almost immediately increased sensitivity to hydroxyurea. Isoproterenol is capable of inducing DNA synthesis also in stem cells of normal mice but it takes about 20 hr before CFU, become to be increasingly sensitive to hydroxyurea.     

Erratum: Human Demography in the Pleistocene: Do Mitochondrial and Nuclear Genes Tell the Same Story? Evol. Anthropol. 8: 81–86.

Eugene E. Harris and Jody Hey (1999). Human Demography in the Pleistocene: Do Mitochondrial and Nuclear Genes Tell the Same Story? Evol. Anthropol. 8: 81–86. On page 84 at the end of 1st paragraph of the 2nd column should read “. . .intergenetic variation Xq 13.3 to about 535,000 years,³⁹. . .” On page 84 in the 2nd paragraph of the 3rd column should read “. . .and seem to indicate widespread or restricted gene flow among populations.”^19,48,49 On page 85 in the 2nd paragraph of the 1st column should read “. . .united by gene flow at zones of overlap.”⁵³     

Using nuclear haplotypes with microsatellites to study gene flow between recently separated Cichlid species

When populations or species have recently separated they often share genetic variation. However, it can be difficult to determine whether shared polymorphisms are the result of gene flow, the result of the persistence of variation in both populations since the time of common ancestry, or both of these factors. We have developed an empirical protocol for using loci that include unique nuclear DNA sequence haplotypes together with linked microsatellites or short tandem repeats (STRs). These 'HapSTRs' offer the potentially high resolution associated with the high mutation rate of STRs, together with the advantages of low homoplasy of unique sequence DNA. We also describe a new procedure for estimating the likelihood of HapSTR data under an Isolation with Migration model. An example using Cichlid fishes from Lake Malawi is described. The analysis suggests that the species have been exchanging genes since the time they began to diverge.     

Highly conserved protein kinases involved in the regulation of carbon and amino acid metabolism

It has been clear for over a decade and a half that ancient signalling pathways controlling fundamental cellular processes are highly conserved throughout the eukaryotes. Two plant protein kinases, sucrose non-fermenting 1 (SNF1)-related protein kinase (SnRK1) and general control non-derepressible 2 (GCN2)-related protein kinase are reviewed here. These protein kinases show an extraordinary level of conservation with their fungal and animal homologues given the span of time since they diverged from them. However, close examination of the signalling pathways in which they operate also reveals intriguing differences in activation and function.