Proliferating cell nuclear antigen (PCNA) as a proliferative marker during embryonic and adult zebrafish hematopoiesis

We investigated the expression of proliferative cell nuclear antigen (PCNA) in zebrafish to delineate the proliferative hematopoietic component during adult and embryonic hematopoiesis. Immunostaining for PCNA and enhanced green fluorescence protein (eGFP) was performed in wild-type and fli1-eGFP (endothelial marker) and gata1-eGFP (erythroid cell marker) transgenic fish. Expression of PCNA mRNA was examined in wild-type and chordin morphant embryos. In adult zebrafish kidney, the renal tubules are surrounded by endothelial cells and it is separated into hematopoietic and excretory compartments. PCNA was expressed in hematopoietic progenitor cells but not in mature neutrophils, eosinophils or erythroid cells. Some PCNA+ cells are scattered in the hematopoietic compartment of the kidney while others are closely associated with renal tubular cells. PCNA was also expressed in spermatogonial stem cells and intestine crypts, consistent with its role in cell proliferation and DNA synthesis. In embryos, PCNA is expressed in the brain, spinal cord and intermediate cell mass (ICM) at 24 h-post fertilization. In chordin morphants, PCNA is significantly upregulated in the expanded ICM. Therefore, PCNA can be used to mark cell proliferation in zebrafish hematopoietic tissues and to identify a population of progenitor cells whose significance would have to be further investigated.     

Roughness effects of diatomaceous slime fouling on turbulent boundary layer hydrodynamics

Abstract

Biofilm fouling significantly impacts ship performance. Here, the impact of biofilm on boundary layer structure at a ship-relevant, low Reynolds number was investigated. Boundary layer measurements were performed over slime-fouled plates using high resolution particle image velocimetry (PIV). The velocity profile over the biofilm showed a downward shift in the log-law region (ΔU⁺), resulting in an effective roughness height (k_s) of 8.8?mm, significantly larger than the physical thickness of the biofilm (1.7?±?0.5?mm) and generating more than three times as much frictional drag as the smooth-wall. The skin-friction coefficient, C_f, of the biofilm was 9.0?×?10^?3 compared with 2.9?×?10^?3 for the smooth wall. The biofilm also enhances turbulent kinetic energy (tke) and Reynolds shear stress, which are more heterogeneous in the streamwise direction than smooth-wall flows. This suggests that biofilms increase drag due to high levels of momentum transport, likely resulting from protruding streamers and surface compliance.     

Whole-tree level water balance and its implications on stomatal oscillations in orange trees [Citrus sinensis (L.) Osbeck] under natural climatic conditions

Sustained cyclic oscillations in stomatal conductance, leaf water potential, and sap flow were observed in young orange trees growing under natural conditions. The oscillations had an average period of approximately 70 min. Water uptake by the roots and loss by the leaves was characterized by large time lags which led to imbalances between water supply and demand in the leaves. The bulk of the lag in response between stomatal movements and the upstream water balance resided downstream of the branch, with branch level sap flow lagging behind the stomatal conductance by approximately 20 min while the stem sap flow had a much shorter time lag of only 5 min behind the branch sap flow. This imbalance between water uptake and loss caused transient changes in internal water deficits which were closely correlated to the dynamics of the leaf water potential. The hydraulic resistance of the whole tree fluctuated throughout the day, suggesting transient changes in the efficiency of water supply to the leaves. A simple whole-tree water balance model was applied to describe the dynamics of water transport in the young orange trees, and typical values of the hydraulic parameters of the transpiration stream were estimated. In addition to the hydro-passive stomatal movements, whole-tree water balance appears to be an important factor in the generation of stomatal oscillations.     

Contrasting stomatal sensitivity to temperature and soil drought in mature alpine conifers

Conifers growing at high elevations need to optimize their stomatal conductance (g_s) for maximizing photosynthetic yield while minimizing water loss under less favourable thermal conditions. Yet the ability of high‐elevation conifers to adjust their g_s sensitivity to environmental drivers remains largely unexplored. We used 4 years of sap flow measurements to elucidate intraspecific and interspecific variability of g_s in Larix decidua Mill. and Picea abies (L.) Karst along an elevational gradient and contrasting soil moisture conditions. Site‐ and species‐specific g_s response to main environmental drivers were examined, including vapour pressure deficit, air temperature, solar irradiance, and soil water potential. Our results indicate that maximum g_s of L. decidua is >2 times higher, shows a more plastic response to temperature, and down‐regulates g_s stronger during atmospheric drought compared to P. abies. These differences allow L. decidua to exert more efficient water use, adjust to site‐specific thermal conditions, and reduce water loss during drought episodes. The stronger plasticity of g_s sensitivity to temperature and higher conductance of L. decidua compared to P. abies provide new insights into species‐specific water use strategies, which affect species' performance and should be considered when predicting terrestrial water dynamics under future climatic change.     

Hydraulic redistribution of foliar absorbed water causes turgor‐driven growth in mangrove seedlings

Although foliar water uptake (FWU) has been shown in mature Avicennia marina trees, the importance for its seedlings remains largely unknown. A series of experiments were therefore performed using artificial rainfall events in a greenhouse environment to assess the ecological implications of FWU in A. marina seedlings. One‐hour artificial rainfall events resulted in an increased leaf water potential, a reversed sap flow, and a rapid diameter increment signifying a turgor‐driven growth of up to 30.1 ± 5.4 μm. Furthermore, the application of an artificial rainfall event with deuterated water showed that the amount of water absorbed by the leaves and transported to the stem was directly and univocally correlated to the observed growth spurts. The observations in this process‐based study show that FWU is an important water acquisition mechanism under certain circumstances and might be of ecological importance for the establishment of A. marina seedlings. Distribution of mangrove trees might hence be more significantly disturbed by climate change‐driven changes in rainfall patterns than previously assumed.     

Priming of Wheat with the Green Leaf Volatile Z-3-Hexenyl Acetate Enhances Defense against Fusarium graminearum But Boosts Deoxynivalenol Production

Priming refers to a mechanism whereby plants are sensitized to respond faster and/or more strongly to future pathogen attack. Here, we demonstrate that preexposure to the green leaf volatile Z-3-hexenyl acetate (Z-3-HAC) primed wheat (Triticum aestivum) for enhanced defense against subsequent infection with the hemibiotrophic fungus Fusarium graminearum. Bioassays showed that, after priming with Z-3-HAC, wheat ears accumulated up to 40% fewer necrotic spikelets. Furthermore, leaves of seedlings showed significantly smaller necrotic lesions compared with nonprimed plants, coinciding with strongly reduced fungal growth in planta. Additionally, we found that F. graminearum produced more deoxynivalenol, a mycotoxin, in the primed treatment. Expression analysis of salicylic acid (SA) and jasmonic acid (JA) biosynthesis genes and exogenous methyl salicylate and methyl jasmonate applications showed that plant defense against F. graminearum is sequentially regulated by SA and JA during the early and later stages of infection, respectively. Interestingly, analysis of the effect of Z-3-HAC pretreatment on SA- and JA-responsive gene expression in hormone-treated and pathogen-inoculated seedlings revealed that Z-3-HAC boosts JA-dependent defenses during the necrotrophic infection stage of F. graminearum but suppresses SA-regulated defense during its biotrophic phase. Together, these findings highlight the importance of temporally separated hormone changes in molding plant health and disease and support a scenario whereby the green leaf volatile Z-3-HAC protects wheat against Fusarium head blight by priming for enhanced JA-dependent defenses during the necrotrophic stages of infection.Biogenic volatile organic compounds (BVOCs) are known regulators of communication of sedentary plants with their direct environment (Dudareva et al., 2006). Besides attracting pollinators (Pichersky and Gershenzon, 2002), repelling insect herbivores (Birkett et al., 2010), and exerting direct antimicrobial properties (Friedman et al., 2002), BVOCs can act as an alarm signal to warn neighboring plants of an imminent herbivorous or pathogen attack (Heil and Ton, 2008) or serve as an intraplant signal for the induction of resistance (Karban et al., 2006). Engelberth et al. (2004) found that maize (Zea mays) seedlings emitted the green leaf volatiles (GLVs) Z-3-hexenal, Z-3-hexenol (Z-3-HOL), and Z-3-hexenyl acetate (Z-3-HAC) after they had been infested with caterpillars of Spodoptera exigua. Neighboring uninfested seedlings that had been exposed to these GLVs subsequently showed a considerable higher production of the plant defense hormone jasmonic acid (JA) after treatment with caterpillar regurgitant. This form of induced resistance is called priming. Plants in a primed state display faster and/or stronger activation of defense pathways when challenged by microbial pathogens, herbivorous insects, or abiotic stresses (Conrath, 2009). Exposure to these priming signals does not entail a direct activation of costly defense mechanisms but rather a stronger up-regulation of defense pathways when the plant is actually under attack (van Hulten et al., 2006). Besides resulting in a stronger induction of the JA pathway, priming also has been shown to enhance defense associated with the salicylic acid (SA) pathway, which plays a critical role in plant defense against biotrophic pathogens (Conrath et al., 2006; Jung et al., 2009).The lion’s share of attention on the use of GLVs in induced resistance has been directed to plant-insect interactions. However, the literature regarding priming by GLVs in plant-pathogen interactions remains scarce (Heil, 2014). Few studies have been performed investigating the effect of priming by GLVs on plant-fungus interactions (Scala et al., 2013a, and refs. therein). For example, hexanoic acid, a molecule with a similar structure to GLVs, has been shown to act as a priming agent in tomato (Solanum lycopersicum) plants against an infection by the necrotrophic fungus Botrytis cinerea, leading to a reduced accumulation of reactive oxygen species in primed plants (Vicedo et al., 2009; Kravchuk et al., 2011; Finiti et al., 2014). Since the GLVs E-2-hexenal (E-2-HAL), Z-3-HOL, E-2-hexenol, and Z-3-HAC also have been reported to be emitted by perennial ryegrass (Lolium perenne) after infection with Fusarium poae (Pańka et al., 2013) and by wheat (Triticum aestivum) seedlings after infection with Fusarium graminearum (Piesik et al., 2011), one may speculate that GLVs not only serve as a priming agent against the impending threat of herbivorous insects but rather constitute a general warning and priming mechanism against insects, bacteria, and fungi alike.Fusarium head blight (FHB) is an important disease in cereals caused by a complex of Fusarium spp., of which the hemibiotroph F. graminearum is one of the most prevalent (Parry et al., 1995; Goswami and Kistler, 2004; Audenaert et al., 2009). Besides yield losses of up to 40%, FHB also confers quality losses because of the production of mycotoxins such as deoxynivalenol (DON; Parry et al., 1995; Bottalico and Perrone, 2002; Vanheule et al., 2014).The hemiobiotrophic nature of F. graminearum entails that its lifestyle is characterized by a biotrophic phase followed by a necrotrophic phase. During the biotrophic phase, spores will germinate and hyphae will grow extracellularly and intercellularly. To counteract fungal colonization during the biotrophic phase, the host plant will accumulate hydrogen peroxide (H₂O₂) to induce programmed cell death. However, H₂O₂ acts as a signal for F. graminearum to produce DON, which in turn creates a positive feedback loop leading to increased H₂O₂ and DON production, clearing the way for F. graminearum to further colonize the host plant (Desmond et al., 2008). Plant defense against the biotrophic and necrotrophic phases generally has been linked to SA- and JA-related pathways, respectively (Glazebrook, 2005). This was also found in the study by Ding et al. (2011). They reported higher endogenous SA concentrations during the first hours of infection, followed by a rise in JA concentrations later on. However, plant defense against pathogens is regulated by a whole array of plant hormones, between which an intricate cross talk exists (Pieterse et al., 2012). One of the best-studied antagonistic signaling pathways is between SA and JA (Thaler et al., 2002; Pieterse et al., 2012). Research investigating the hormonal modulation of plant immunity has been done primarily in dicots. The negative relationship between SA and JA also seems to be conserved in rice (Oryza sativa), another monocot (De Vleesschauwer et al., 2013). Because of the presence of this possible antagonistic signaling and the hemibiotrophic lifestyle of F. graminearum, it is important to look more closely to the effect of priming on these two defense pathways in wheat.Here, we show that preexposure of wheat to the GLV Z-3-HAC primes wheat plants for an enhanced defense against a future infection with F. graminearum. Furthermore, our results indicate that pretreatment with Z-3-HAC leads to a stronger activation of JA-related defense while exerting suppressive effects on SA-responsive gene expression. Lastly, we found evidence that enhanced plant defense led to increased DON production by F. graminearum.     

Model-assisted evaluation of crop load effects on stem diameter variations and fruit growth in peach

Key message

The paper identifies and quantifies how crop load influences plant physiological variables that determine stem diameter variations to better understand the effect of crop load on drought stress indicators.

Abstract

Stem diameter (D ^stem) variations have extensively been applied in optimisation strategies for plant-based irrigation scheduling in fruit trees. Two D ^stem derived water status indicators, maximum daily shrinkage (MDS) and daily growth rate (DGR), are however influenced by other factors such as crop load, making it difficult to unambiguously use these indicators in practical irrigation applications. Furthermore, crop load influences the growth of individual fruits, because of competition for assimilates. This paper aims to explain the effect of crop load on DGR, MDS and individual fruit growth in peach using a water and carbon transport model that includes simulation of stem diameter variations. This modelling approach enabled to relate differences in crop load to differences in xylem and phloem water potential components. As such, crop load effects on DGR were attributed to effects on the stem phloem turgor pressure. The effect of crop load on MDS could be explained by the plant water status, the phloem carbon concentration and the elasticity of the tissue. The influence on fruit growth could predominantly be explained by the effect on the early fruit growth stages.