Revealing the Sequence and Resulting Cellular Morphology of Receptor-Ligand Interactions during Plasmodium falciparum Invasion of Erythrocytes

During blood stage Plasmodium falciparum infection, merozoites invade uninfected erythrocytes via a complex, multistep process involving a series of distinct receptor-ligand binding events. Understanding each element in this process increases the potential to block the parasite’s life cycle via drugs or vaccines. To investigate specific receptor-ligand interactions, they were systematically blocked using a combination of genetic deletion, enzymatic receptor cleavage and inhibition of binding via antibodies, peptides and small molecules, and the resulting temporal changes in invasion and morphological effects on erythrocytes were filmed using live cell imaging. Analysis of the videos have shown receptor-ligand interactions occur in the following sequence with the following cellular morphologies; 1) an early heparin-blockable interaction which weakly deforms the erythrocyte, 2) EBA and PfRh ligands which strongly deform the erythrocyte, a process dependant on the merozoite’s actin-myosin motor, 3) a PfRh5-basigin binding step which results in a pore or opening between parasite and host through which it appears small molecules and possibly invasion components can flow and 4) an AMA1–RON2 interaction that mediates tight junction formation, which acts as an anchor point for internalization. In addition to enhancing general knowledge of apicomplexan biology, this work provides a rational basis to combine sequentially acting merozoite vaccine candidates in a single multi-receptor-blocking vaccine.     

Vegetative segregation of mitochondria in yeast: Estimating parameters using a random model

Summary Yeast zygotes which are heteroplasmic for mitochondrial genes reproduce vegetatively to form clones of diploid progeny which are homoplasmic. This vegetative segregation of mitochondrial genes has been interpreted in terms of a random distribution of mitochondria or mitochondrial genomes between mother and bud at cell division. We have developed equations which permit calculation of the number of segregating units in the zygote and the number of those units which enter the bud, assuming that segregation of the units is genetically random and numerically variable or equal. Use of the equations requires data from partial pedigree analyses: we isolate zygotes, separate the first bud, then determine the frequency of mitochondrial alleles among the progeny of mother cells whose first buds were homoplasmic. Application of this method to data from five crosses suggests that most zygotes have a small number of segregating units (usually less than a dozen) and only one or two enter the first bud. Analysis of the frequency of buds which are nearly but not quite homoplasmic indicates that the segregating units may be mitochondria or portions thereof which include many mitochondrial genomes, all the genomes in a unit being genetically identical in most but not all cases. These results are compatible with, but do not prove, the hypothesis of random vegetative segregation of mitochondria.     

Random partitioning of cytoplasmic organelles at cell division: The effect of organelle and cell volume

When a cell divides, some cytoplasmic organelles may be partitioned randomly between the daughters. The number of organelles in each daughter is usually calculated from the binomial distribution, which assumes that the organelles occupy zero volume. We developed equations to predict numerical partitioning taking the volume of the organelles and of the cell into account. The effect of large organelle volume is that daughter cells receive equal or nearly equal numbers of organelles more often than predicted by the binomial distribution. However, numerical solutions show that volume effects are very small unless the number of organelles is very small or they occupy more than about 50% of the available cell volume.     

Large seasonal variation of soil respiration in a secondary tropical moist forest in Puerto Rico

Tropical forests are the largest contributors to global emissions of carbon dioxide (CO₂) to the atmosphere via soil respiration (R_s). As such, identifying the main controls on R_s in tropical forests is essential for accurately projecting the consequences of ongoing and future global environmental changes to the global C cycle. We measured hourly R_s in a secondary tropical moist forest in Puerto Rico over a 3‐year period to (a) quantify the magnitude of R_s and (b) identify the role of climatic, substrate, and nutrient controls on the seasonality of R_s. Across 3 years of measurements, mean R_s was 7.16 ± 0.02 μmol CO₂ m^‐2 s^‐1 (or 2,710 g C m^‐2 year^‐1) and showed significant seasonal variation. Despite small month‐to‐month variation in temperature (~4°C), we found significant positive relationships between daily and monthly R_s with both air and soil temperature, highlighting the importance of temperature as a driver of R_s even in warm ecosystems, such as tropical forests. We also found a significant parabolic relationship between mean daily volumetric soil moisture and mean daily R_s, with an optimal moisture value of 0.34 m³ m^‐3. Given the relatively consistent climate at this site, the large range in mean monthly R_s (~7 μmol CO₂ m^‐2 s^‐1) was surprising and suggests that even small changes in climate can have large implications for ecosystem respiration. The strong positive relationship of R_s with temperature at monthly timescales particularly stands out, as moisture is usually considered a stronger control of R_s in tropical forests that already experience warm temperatures year‐round. Moreover, our results revealed the strong seasonality of R_s in tropical moist forests, which given its high magnitude, can represent a significant contribution to the seasonal patterns of atmospheric (CO₂) globally.     

Interplay Between Abscisic Acid and Jasmonic Acid and its Role in Water-oxidative Stress in Wild-type, ABA-deficient, JA-deficient, and Ascorbate-deficient Arabidopsis Plants

The interplay between jasmonic acid (JA) and abscisic acid (ABA) in plant responses to water stress and in water-stress-enhanced oxidative stress was investigated in Arabidopsis thaliana plants subjected to water stress by water deprivation. For this purpose a drought assay was conducted using Arabidopsis mutants impaired in ABA (aba2), JA (aos), and ascorbate (vtc1) biosynthesis. Our results show an interaction between ABA and JA during their biosynthesis. Moreover, the coordinated action of ABA and JA protected wild-type, aba2, and aos plants from the effects of stress. However, this effect was not observed in the vtc1 mutant, which showed a distinct decrease in the F _v/F _m ratio, concomitant with a marked fall in relative water content (RWC), despite high endogenous concentrations of JA and ABA. This finding indicates the relevance of ascorbate metabolism in plant acclimation to stress. Despite the interaction between the two phytohormones, drought-associated stomatal closure is regulated mainly by ABA and weakly by JA, whereas JA plays a role in the formation of antioxidants regulating ascorbate and glutathione metabolism. A time course analysis revealed the relevance of plant age and stress duration in the responses of the mutants compared to wild-type plants. Here we discuss the relationship between ABA, JA, ascorbate, and glutathione in plants under water stress.     

Tropical understory herbaceous community responds more strongly to hurricane disturbance than to experimental warming

The effects of climate change on tropical forests may have global consequences due to the forests’ high biodiversity and major role in the global carbon cycle. In this study, we document the effects of experimental warming on the abundance and composition of a tropical forest floor herbaceous plant community in the Luquillo Experimental Forest, Puerto Rico. This study was conducted within Tropical Responses to Altered Climate Experiment (TRACE) plots, which use infrared heaters under free‐air, open‐field conditions, to warm understory vegetation and soils + 4°C above nearby control plots. Hurricanes Irma and María damaged the heating infrastructure in the second year of warming, therefore, the study included one pretreatment year, one year of warming, and one year of hurricane response with no warming. We measured percent leaf cover of individual herbaceous species, fern population dynamics, and species richness and diversity within three warmed and three control plots. Results showed that one year of experimental warming did not significantly affect the cover of individual herbaceous species, fern population dynamics, species richness, or species diversity. In contrast, herbaceous cover increased from 20% to 70%, bare ground decreased from 70% to 6%, and species composition shifted pre to posthurricane. The negligible effects of warming may have been due to the short duration of the warming treatment or an understory that is somewhat resistant to higher temperatures. Our results suggest that climate extremes that are predicted to increase with climate change, such as hurricanes and droughts, may cause more abrupt changes in tropical forest understories than longer‐term sustained warming.