Russian wheat aphid causes greater reduction in phloem transport capacity of barley leaves than bird cherry-oat aphid

The effects of feeding by the Russian wheat aphid (RWA), Diuraphis noxia Mordvilko and the Bird cherry-oat aphid (BCA), Rhopalosiphum padi L on the transport capacity of barley Hordeum vulgare L leaves were investigated and compared with a view to relating these effects to the visible symptoms shown by the respective infested plants. RWAcauses extensive chlorosis and necrosis on an infested plant whereas BCA causes no observable symptoms. Our results using the xenobiotic, phloem mobile fluorophore, 5, 6 carboxyfluorescein diacetate (5, 6-CFDA) revealed striking differences in damage to the transport of assimilates through the phloem by these two aphids. The result clearly suggests that short-term feeding by RWA causes a reduction in transport of assimilates and a more severe reduction or perhaps even permanent cessation of transport during long-term feeding. In contrast, feeding by BCA does not lead to a marked decrease in transport during short-term feeding period, however, a reduction in the transport was recorded during long-term feeding activities. These results perhaps suggest that damage to transport capacities of the barley leaves appears to be partly responsible for the observed symptoms in RWA-infested plants and the lack of them during BCAinfestations, symptoms such as reduction or cessation in transport of assimilates to growing tissues may lead to such observable symptoms

Russian wheat aphid causes greater reduction in phloem transport capacity of barley leaves than bird cherry-oat aphid

The effects of feeding by the Russian wheat aphid (RWA), Diuraphis noxia Mordvilko and the Bird cherry-oat aphid (BCA), Rhopalosiphum padi L on the transport capacity of barley Hordeum vulgare L leaves were investigated and compared with a view to relating these effects to the visible symptoms shown by the respective infested plants. RWAcauses extensive chlorosis and necrosis on an infested plant whereas BCA causes no observable symptoms. Our results using the xenobiotic, phloem mobile fluorophore, 5, 6 carboxyfluorescein diacetate (5, 6-CFDA) revealed striking differences in damage to the transport of assimilates through the phloem by these two aphids. The result clearly suggests that short-term feeding by RWA causes a reduction in transport of assimilates and a more severe reduction or perhaps even permanent cessation of transport during long-term feeding. In contrast, feeding by BCA does not lead to a marked decrease in transport during short-term feeding period, however, a reduction in the transport was recorded during long-term feeding activities. These results perhaps suggest that damage to transport capacities of the barley leaves appears to be partly responsible for the observed symptoms in RWA-infested plants and the lack of them during BCAinfestations, symptoms such as reduction or cessation in transport of assimilates to growing tissues may lead to such observable symptoms

Xylem as well as phloem sustains severe damage due to feeding by the Russian wheat aphid

Investigation of comparative effects of feeding damage by the Russian wheat aphid (RWA, biotype SA1, Diuraphis noxia Mordvilko) on leaf blades of susceptible and resistant wheat cultivars (Triticum aestivum L. var Betta and Betta-Dn1 respectively) were carried out to establish the level of ultrastructural damage caused by this aphid and the possible limitation of damage induced which could be ascribed to the resistance gene Dn1 over the susceptible cultivar. Ultrastructurally, Betta-Dn1 sustained less damage to the vascular tissue as well as to the mesophyll during the experimental period. Both inter- and intracellular probes resulted in considerable saliva deposition as the aphids probed for suitable feeding sites. Salivary tracks were observed between and within mesophyll, bundle sheath cells as well as the vascular tissue, including the xylem. Disruption of organelles and cytoplasm resulted from cell probing and sheath deposition. Cell and organelle damage was more evident in the non-resistant Betta cultivar. The aphids probed for and fed from thin-walled sieve tubes preferentially. Few thick-walled sieve tubes showed evidence of either aphid probing or feeding-related damage. Saliva was deposited when the aphids probed inter- and intracellularly for feeding sites. The aphids appeared preferentially to probe for and feed from thin-walled sieve tubes, as few thick-walled sieve tubes showed evidence of damage. Vessels, apparently probed for water, contained watery saliva that encased the secondary walls and sealed pit membranes between probed vessels and xylem parenchyma. The xylem probed by the RWA was rendered non-functional, probably contributing to symptoms of leaf roll, chlorosis and necrosis, which were observed within two weeks of infestation in the susceptible Betta cultivar. This damage was limited in the resistant Betta-Dn1 cultivar during the same time frame

Stronger induction of callose deposition in barley by Russian wheat aphid than bird cherry-oat aphid is not associated with differences in callose synthase or ≤-1,3-glucanase expression

The effects of infestation by the bird cherry-oat aphid (BCA), (Rhopalosiphum padi L) and the Russian wheat aphid (RWA) (Diuraphis noxia Mordvilko) on callose deposition and gene expression related to callose accumulation were investigated in barley (Hordeum vulgare L. cv. Clipper). The BCA, which gives no visible symptoms, induced very limited callose deposition, even after 14 days of infestation. In contrast, RWA, which causes chlorosis, white and yellow streaking and leaf rolling, induced callose accumulation already after 24h in longitudinal leaf veins. The deposition was pronounced after 72 h, progressing during 7 and 14 days of infestation. In RWA-infested source leaves, callose was also induced in longitudinal veins basipetal to the aphid-infested tissue, whereas in sink leaves, more callose deposition was found above the feeding sites. Nine putative callose synthase genes were identified in a data base search, of which eight were expressed in the leaves, but with similar level of expression in control and aphid-infested tissue. Four out of 12 examined β-1,3-glucanases were expressed in the leaves, and three of them were up-regulated in aphid-infested tissue. They were all more strongly induced by RWA than BCA. The results suggest that callose accumulation may be partly responsible for the symptoms resulting from RWA feeding and that a callose-inducing signal may be transported in the phloem. Furthermore it is concluded that the absence of callose deposition in BCA-infested leaves is not due to a stronger induction of callose-degrading β-1,3-glucanases in this tissue, as compared to RWA-infested leaves

Quantifying Vegetation Biophysical Variables from Imaging Spectroscopy Data: A Review on Retrieval Methods

An unprecedented spectroscopic data stream will soon become available with forthcoming Earth-observing satellite missions equipped with imaging spectroradiometers. This data stream will open up a vast array of opportunities to quantify a diversity of biochemical and structural vegetation properties. The processing requirements for such large data streams require reliable retrieval techniques enabling the spatiotemporally explicit quantification of biophysical variables. With the aim of preparing for this new era of Earth observation, this review summarizes the state-of-the-art retrieval methods that have been applied in experimental imaging spectroscopy studies inferring all kinds of vegetation biophysical variables. Identified retrieval methods are categorized into: (1) parametric regression, including vegetation indices, shape indices and spectral transformations; (2) nonparametric regression, including linear and nonlinear machine learning regression algorithms; (3) physically based, including inversion of radiative transfer models (RTMs) using numerical optimization and look-up table approaches; and (4) hybrid regression methods, which combine RTM simulations with machine learning regression methods. For each of these categories, an overview of widely applied methods with application to mapping vegetation properties is given. In view of processing imaging spectroscopy data, a critical aspect involves the challenge of dealing with spectral multicollinearity. The ability to provide robust estimates, retrieval uncertainties and acceptable retrieval processing speed are other important aspects in view of operational processing. Recommendations towards new-generation spectroscopy-based processing chains for operational production of biophysical variables are given

Mosquito-borne arboviruses of African origin: review of key viruses and vectors

Abstract Key aspects of 36 mosquito-borne arboviruses indigenous to Africa are summarized, including lesser or poorly-known viruses which, like Zika, may have the potential to escape current sylvatic cycling to achieve greater geographical distribution and medical importance. Major vectors are indicated as well as reservoir hosts, where known. A series of current and future risk factors is addressed. It is apparent that Africa has been the source of most of the major mosquito-borne viruses of medical importance that currently constitute serious global public health threats, but that there are several other viruses with potential for international challenge. The conclusion reached is that increased human population growth in decades ahead coupled with increased international travel and trade is likely to sustain and increase the threat of further geographical spread of current and new arboviral disease