Huanglongbing (yellow dragon disease), previously known as citrus greening disease, is one of the worst diseases of citrus trees worldwide. It is caused by the bacterial disease Candidatus Liberibacter asiaticus that spreads through the tree canopy, causing decline and then death of the tree.
There is no cure – the only way to stop the disease is to destroy all infected trees and replace them.
The disease huanglongbing originated from China, with its vectors from Asia (Asiatic citrus psyllid) and Africa (African citrus psyllid). Depending on the species, the disease and its vectors can now be found throughout:
North, Central and South America
South East Asia, including Indonesia and East Timor
Papua New Guinea.
The islands of Torres Strait provide a potential pathway for the movement of serious pests into Australia, such as huanglongbing and the Asian citrus psyllid, present in countries to our north.
How to identify Huanglongbing (Candidatus Liberibacter asiaticus)
Everyone needs to keep an eye out for symptoms of huanglongbing.
Huanglongbing is spread by the movement of infected plants and plant propagative material and by sap sucking insects. These insects – the Asiatic citrus psyllid (Diaphorina citri) and African citrus psyllid (Trioza erytreae) – are not present in Australia and are of major concern due to their ability to spread huanglongbing.
Adults of the Asiatic citrus psyllid are 3-4 millimeters long with brown markings on the wings. When feeding on the veins of the young leaves, they adopt a ‘head-down, tail-up’ position.
Juvenile psyllids are yellow and commonly found feeding on young, soft shoots.
The African citrus psyllid is similar but larger with a light brown-grey body and black head, and large transparent forewings.
Huanglongbing causes yellowing of citrus plant leaves and in some instances deformed, sour and bitter fruit.
Symptoms on leaves are subtle and hard to pick but one key sign is a blotchy yellowing that is not symmetrical or mirrored on both sides of the leaf.
Later, new young leaves are small, upright and yellow, with green bands around the veins.
In well-managed orchards, a yellowing that spreads slowly over the tree and through an orchard is an easily seen sign. The spreading yellowing effect can be especially hard to see in neglected backyard citrus trees growing in poor soils.
Infected trees have a blotchy yellowing that is not symmetrical or mirrored on both sides of the leaf Source: DAWR
Fruit from infected trees can be misshapen or lopsided, and when cut lengthwise, the arrangement of internal tissues may be irregular Source: DAWR
Methylation Vegetable oil can improve the spreading area, adhesion and permeability of droplets on the surface of the crop, and promote the absorption and conduction in crop.
Besides, the methylated vegetable oil can prevent the liquid droplets from drying too fast, thereby enhancing the absorption of droplets through pores and the stratum corneum, and enhances efficacy for herbicides.
Tea Saponin, a glycoside compound extracted from camellia tea seeds, is excellent natural nonionic active surfactant. It can be widely used in pesticide, cultivation, textile, daily chemicals, arthitectural field, medical field and so on.
Tea saponin is triterpenoid saponin, it tastes bitter and spicy. It stimulates mucous membrane of nose to lead to sneeze. The pure product is fine white column-shape crystalloid with strong moisture absorption ability. It presents apparent acidity to methyl red. It’s easy to be dissolved in water, water-contained methanol, water-contained ethanol, glacial acetic acid, acetic anhydride and pyridine etc. Its melting point: 224.
Chemical name : N,N’-Dimethylolurea
Formula : C3H8N2O3
Molecular Weight : 120.1
CAS No. : 140-95-4
Dimethylolurea is used to treat textiles and wood, and is mixed with fillers for use in molding adhesives. And used in disinfectants and other biocidal products, as an in-can preservative, as a preservative for liquid-cooling and processing systems, and as a slimicide. Dimethylolurea is also used as a preservative in metal-working fluids, as a developer of photographic film, and as a cleaning agent and disinfectant.
Tomato plants (Lycopersicum esculentum Mill) grown under tropical field conditions were treated with an alkaline seaweed extract made from Ascophyllum nodosum (ASWE).
Two field experiments and one greenhouse experiment were conducted to evaluate methods of application, dosage of application, and the impact of each on plant growth parameters and on the quality and yield of fruit.
Field experiment 1 included 0.2 % ASWE spray, 0.2 % ASWE root drench, fungicide spray and combinations of the above. Plants foliar-sprayed with 0.2 % ASWE had significantly increased plant height (10 %) and plant fruit yield (51 %) when compared to control plants. Similar results were observed for ASWE spray alternated with fungicide or with ASWE root drench. Field experiment 2 included 0.5 % ASWE spray, fungicide spray and ASWE spray alternated with fungicide. The higher concentration of ASWE resulted in a significant increase in plant height (37 %) and plant fruit yield (63 %) compared to control plants. The third experiment under greenhouse conditions also showed that 0.5 % ASWE spray caused a significant increase in plant height (20 %) and plant fruit yield (54 %) compared to control plants.
In the greenhouse, ASWE-treated plants had larger root systems and increased concentrations of minerals in the shoots. Fruit from plants treated with ASWE showed significant increases in quality attributes including, size, colour, firmness, total soluble solids, ascorbic acid levels and mineral levels.
Overall, the use of ASWE resulted in clear improvements in tomato fruit yield and quality under tropical growing conditions.
The fact that the biosynthesis of active GAs (see Glossary) is a complex, multistepped process with diverse intermediates (Figure 1) makes it difficult to pinpoint the exact tissue or organ in which GAs are synthesized and localize to. Studies focusing on the spatial organization of the GA biosynthesis pathway, characterizing the expression patterns of different GA biosynthetic enzymes using GUS as a reporter, have led to several insights. First, GA biosynthesis genes are differentially expressed among different tissues, cell types, and developmental stages . Second, several members of the GA3ox family, which catalyze the final step in the synthesis of bioactive GAs, are expressed in growing and elongating shoot and root organs . Third, although there are several examples of tissues in which the expression of GA biosynthesis genes co-localizes with GA perception genes (e.g., in inflorescence meristem and developing leaves), there are also examples where these two groups do not overlap (e.g., GA-biosynthesis genes are not expressed in the aleurone cells of the endosperm but GA signaling genes are) . Such spatial separation between genes involved in GA biosynthesis and perception suggests the requirement for GA movement. Finally, levels of expression of genes constituting the GA biosynthetic pathway itself do not always coincide . For example, the expression of the late stage GA biosynthesis genes AtGA3ox1 and AtGA3ox2 in germinating embryos is spatially different from that of the early GA biosynthesis gene AtCPS. This and other examples suggest that the location of GA precursors could play an important role in regulating GA responses.
A recent study, combining mathematical and experimental approaches, compared the putative GA response, represented by the expression pattern of the SCR3 GA responsive gene (pSCR3:GUS reporter) and GA perception sites, represented by the expression pattern of GA perception proteins (GID1 and DELLA). The study demonstrated that alternating temperatures act as an instructive signal in the embryonic root tip in Arabidopsis dormant seeds . The modeling nicely showed that the process of dormancy break in the seed is defined by the distribution of the plant hormones GA and abscisic acid (ABA) . This spatial separation of ABA and GA responses suggests that crosstalk between ABA and GA is non-cell-autonomous and is controlled at the level of hormone movement between spatially separated signaling centers .
It should be noted that the observations and interpretations regarding GA localization are limited by several factors. First, the spatiotemporal resolution of the studies, using GUS reporters or mRNA expression, is relatively low. It would be constructive to increase the resolution of such studies through dynamic monitoring of fluorescent reporters. Second, only a few of the GA biosynthesis genes families, and only a few members from those families, have been analyzed so far. In order to draw a comprehensive map of the spatial distribution of GA biosynthesis, a concurrent characterization of the whole pathway will be required. Third, studies to date have usually analyzed expression of GA biosynthetic genes at the mRNA level. As it is possible that these enzymes are subjected to post-translational modifications and non-cell-autonomous movement, it will be important to examine their localization as translational fusions. The ultimate goal should be to generate specific sensors that will provide a readout for the enzyme family activity. This would allow a specific readout of the final enzymatic biochemical activity and overcome redundancies. It is reasonable to assume that GA localization is also regulated by catabolism, conjugation, and transport steps . Thus, expression patterns of GA biosynthesis genes will not necessarily enable identification of all sites of active GA localization and response.
In order to overcome several of the limitations illustrated above, a novel fluorescence resonance energy transfer-based GA biosensor (termed GPS1) was developed. The GPS1 biosensor, constructed by fusing GID1 variants to the DELLA N-termini, showed an increased emission ratio in response to nanomolar concentrations of GA4. With the exception of a few limitations such as nonreversible response to GA4, phenotypic hypersensitivity to a GA biosynthesis inhibitor, and a limited response to GA3 and GA1, this biosensor should be a useful new tool for identifying GA response sites. For example, GPS1 revealed that GA response is higher in the elongation zone compared with the root meristematic zone. GA localization correlated with cell length when GA4 was exogenously applied, suggesting that rapid transport or catabolism of GA in the root may generate local GA gradients independently of GA biosynthesis. In addition, the GPS1 sensor indicated that high levels of GA4 in the elongating hypocotyl depend on darkness . GPS1 should find broad utility in exploration of GA distribution and transport mechanisms and is expected, for example, to shed light on GA distribution in known and novel GA transporter loss-of-function and gain-of-function lines reported for the nitrate transporter 1/peptide transporter family (NPF) and SWEET families (further discussed below).
A different approach to address the question of GA distribution and accumulation sites utilized fluorescently labeled versions of GA3 and GA4 (termed GA-Fl). Combining imaging of GA-Fl localization with information on transporter expression levels and genetics showed that the TEMPRANILLO (TEM) proteins play an essential role not only in GA biosynthesis but also in regulating GA distribution in the mesophyll, which, in turn, regulates epidermal trichome formation . In roots, GA-Fl accumulated specifically in the elongating endodermal cells of Arabidopsis roots. The localization of GA-Fl in the elongating cells is consistent with GFP-RGA levels and with other studies indicating that GA activity is necessary for root elongation and gravitropic response , but only partially overlaps with GPS1 signal, which was not restricted to the endodermis. Since GA-Fl is highly specific to NPF3 , it is possible that it represents only a subset of GA forms. Alternatively, it is possible that GPS1 and GA-Fl report on GA levels on different sites within the cell; whereas GPS1 mainly responds to nuclear GA4 levels, the fluorescent GA4 reports on transport and localization of exogenously applied GA that eventually localizes to the vacuole. Since the GA response was shown to be restricted to the root endodermis cell layer, it will be important to evaluate the distribution of active GA and its precursors at a cellular resolution, as has been successfully carried out for the plant hormones auxin and cytokinin .
Cells entering the elongation zone increase in length by approximately 10-fold over 5 hours. Such a rapid expansion is expected to result in a rapid intracellular dilution of GA, practically reducing its effective concentration. Modeling of this process suggests a correlation between GA distribution and root cell growth, and the study authors posit that cellular GA levels decrease at the elongation zone due to cytosolic dilution . Independent analyses of the GPS1 sensor response and GA-Fl distribution indicate that GA levels are higher in the root elongation zone compared with the meristematic zone; therefore, GA is probably either synthesized locally or imported from surrounding tissues to compensate for dilution.
Gibberellin (GA) was first identified in the pathogenic fungus Gibberella fujikuroi, which causes a disease in rice called ‘foolish-seedling’.
By producing large quantities of GA, the plants become long and slender, are incapable of supporting their own weight, and are chlorotic and partially infertile .
Further research established GA as a hormone that is essential for many developmental processes in plants, among them are seed germination; organ elongation and expansion through cell growth; trichome development; transition from vegetative to reproductive growth; and flower, seed, and fruit development .
GA manipulation in agriculture is common practice; the best-known contribution of GA manipulations to agriculture is the introduction of dwarfing alleles into staple crops. This manipulation resulted in one of the cornerstones of the so-called ‘green revolution’ and led to a massive increase in global wheat and rice yields. Identification of the genes responsible for these traits showed that the encoded proteins interfere with the action or production of GA .
Among more than 130 GAs identified in plants, fungi, and bacteria to date, only a subset, namely GA1, GA3, GA4, and GA7 are thought to function as bioactive hormones .
Additional forms of GA that exist in plants are precursors of the bioactive forms or deactivated metabolites .
Natural and post-harvest ethylene-induced pigment changes in the rind of Satsuma mandarin (Citrus unshiu Marc.) fruits respond differently to the exogenous application of growth regulators.
Both gibberellin A3 and the synthetic cytokinins N6-benzyladenine and kinetin opposed the ethylene-induced chlorophyll destruction, while the loss of chlorophyll during natural maturation was retarded by the gibberellin but not by the cytokinins.
This different behaviour suggests that ethylene may not be playing a central role in the endogenous control of ripening.
Carotenoid accumulation during natural maturation is apparently controlled through a different mechanism than chlorophyll loss since it is reduced both by the cytokinins and gibberellin A3.
Kinetin and gibberellin A3 increased to a similar extent the accumulation of reducing sugars and free amino acids, and reduced that of non-reducing sugars in the peel during natural maturation.
Their differential effect on chlorophyll loss may not be explained through their effects on sugar accumulation.