After the initiation of asymmetric division, the primordia emerge, form active meristems, and break through the epidermal cells to become new lateral roots. Auxin is essential for various steps in the course of root development—from cell fate acquisition to meristem initiation, emergence, and elongation . In Arabidopsis, auxin is mainly synthesized in young apical tissues of the shoots and roots . Indole-3-acetic acid is considered the major form of auxin, with tryptophan being its precursor . Among the four pathways of IAA biosynthesis from Trp, the indole-3-pyruvic acid pathway is the major pathway in Arabidopsis . In the IPyA pathway, tryptophan aminotransferases convert Trp into IPyA, and YUCCAs synthesize IAA from IPyA, a rate-limiting step for the pathway . In rice, FISH BONE encodes a Trp aminotransferase; loss of function results in pleiotropic abnormal phenotypes, which include small leaves with large lamina joint angles, unusual vascular development, and defects in root development, which are all consistent with a decrease in internal IAA levels . Mutations in CONSTUTIVELY WILTED1result in narrow and rolled leaves, in addition to the decreased growth of lateral and crown roots . Conversely, the over expression of OsYUC1 causes an increase in IAA accumulation, and auxin-overproducing phenotypes are observed . Such phenotypes are subject to the presence of the transcription factor WUSCHEL-RELATED HOMEOBOX 11 , a key regulator of root development . In rice,vertical farming aeroponics auxin induces WOX11 transcription, which establishes the YUCCA–auxin–WOX11 module for root development . Ethylene also controls root development. Treatment with low concentrations of an ethylene precursor, 1-aminocyclopropane- 1-carboxylic acid , promotes the initiation of lateral root primordia.
In contrast, exposure to higher ACC concentrations inhibits such initiation considerably, while also promoting the growth of already existing lateral root primordia . The regulation is linked tightly with auxin . For example, ethylene application results in the accumulation of auxin at the tip of Arabidopsis primary roots through the promotion of auxin synthesis mediated by WEAK ETHYLENE INSENSIVE2/ANTHRANILATE SYNTHASE α1 and WEI7/INSENSIVE2/ ANTHRANILATE SYNTHASE β1 . WEI2 and WEI7 encode the α and β subunits, respectively, of anthranilate synthase , a rate-limiting enzyme in the biosynthesis of the auxin precursor Trp . In rice, ethylene also increases endogenous IAA concentrations in the roots; however, the effect is minimized in mutants defective in YUC8/REIN7, which participates in auxin biosynthesis . The homeobox genes are critical for growth and development because they regulate cell fate and plant specificity . A family of zinc-finger homeodomain proteins has an N-terminal conserved domain containing several cysteine and histidine residues for potential zinc binding, in addition to a C-terminal domain containing a homeodomain . Most ZF-HD proteins do not have an intrinsic activation domain, which suggests that interactions with other factors are necessary for transcriptional activation . In addition, all 14 members of the ZF-HD gene family in Arabidopsis are predominantly expressed in floral tissues and play key roles in their development . One member, AtHB33, which is negatively regulated by ARF2, is required for seed germination and primary root growth . Among the 11 ZF-HD genes in rice, the over expression of OsZHD1 and OsZHD2 induces leaf curling by controlling the number and arrangement of bulliform cells . Here, we report that the overexpression of OsZHD2 in rice improves root growth by enhancing meristem activity. We demonstrated that the homeobox protein elevated ethylene concentrations by increasing the transcript levels of ethylene biosynthesis genes. We further obtained ChIP assay data that revealed an interaction between OsZHD2 and the chromatin of ACS5. Analyses of transgenic rice plants carrying DR5::GUS and DR5::VENUS revealed that the expression of the DR5 reporter gene was induced following treatment with ACC, an ethylene precursor.
The results suggest that OsZHD2 increases the biosynthesis of ethylene and subsequently auxin, which stimulates root growth.Untreated irrigation water runoff and untreated recycled irrigation water have been shown to introduce and spread microbial pathogens, such as Pythium and Phytophthora, in commercial nurseries .These pathogens belong to the Oomycota and are often referred to as water molds that are most active during wet and humid periods and produce flagellated spores, called zoospores that can spread through the water. Phytophthora sp. cause diseases in agriculture, arboriculture and natural ecosystems, and the estimated losses associated with these pathogens are in the billions of dollars . Of particular concern in California is the spread of P. ramorum, causal agent of Sudden Oak Death . Costs associated with SOD incorporate costs to property owners, ornamental nursery industry, and the state and federal government. There are hundreds of thousands of susceptible oak trees located on near developed communities. Infected trees in these areas will need to be removed, disposed of, and replaced at the cost of the landowner or local government. Kovacs et al. estimated that this would amount to a discounted cost of$7.5 million and an associated $135 million in losses to property values for single family homes in California. Sudden Oak Death was first noticed in the 1990s when California hikers along the central coast reported oaks suddenly dying . This die off was frequently found along the interfaces between urban and natural areas. In 2000 the pathogen causing SOD, P. ramorum, was isolated and researchers soon noticed that it belonged to the same species as a newly described pathogen from diseased rhododendrons and viburnums in European nurseries in 1993 . P. ramorum causes foliar and shoot blight on many plants, including important ornamental species, and bleeding cankers on the tree trunks leading to the death of the plant on relatively few hosts, like coast live oak and tan oak . This disease is responsible for the death of tens of thousands of trees in California and Oregon and most strongly affects tanoak, coast live oak, California black oak, and Shreve’s oak . P. ramorum also causes severe damage on plantations of the non-native Japanese larch in the United Kingdom and Ireland; the disease on this new host was named ‘Sudden larch death’ . Abundant inoculum can be produced on foliar hosts and spread from there to both foliar and non-foliar hosts.
Due to this mode of transportation, there are instances of the Phytopthora species moving from ornamental nurseries to the natural environment due to uncontrolled contamination . Nevertheless, few nurseries currently treat their irrigation water . Instead, nurseries use fungicides to control the spread of fungal pathogens and oomycetes, which increases costs and promotes the growth of resistant plant pathogens. In addition, these plant pathogens may be suppressed when under the presence of fungicides and proliferate when the fungicide is discontinued . For these reasons, various techniques, including the use of chlorine, ozone and UV light, have been used to mitigate the spread of the pathogen, but each technique has its drawbacks. Most methods for the treatment of irrigation water can be costly for small operations. The most common treatment method used is liquid chlorine injection. This technique requires consistent addition, monitoring of chlorine concentrations, assessment of the system water quality and on-site storage . The chlorine dose needed is dependent on water quality because the high nitrogen and organic content in the dissolved and suspended matter incorporated in irrigation run-off increases the chlorine demand on the system. Ozone has similar limitations to using a liquid chlorine injection, but it can be generated on demand. Nevertheless, generally ozone has higher capital costs compared to the use of liquid chlorine and ultra-violet light treatment . Ultraviolet light works as a disinfectant by exciting the nucleic acids in DNA and RNA. This excitation results in the dimerization of adjacent nucleic acids and prevents the further transcription of the DNA or RNA and inhibits replication. Since UV disinfection is a physical treatment process, it avoids generating toxic by-products caused by the use of oxidizing chemical disinfectants, such as chlorine. Similarly, there is no additional smell or taste added to the water, no danger of overdosing the disinfectant,vertical indoor hydroponic system and no need to store hazardous materials on site. Another benefit of UV disinfection over chlorine and ozone is that it alone is effective against both bacterial and viral pathogens. The ability of a UV system to disinfect agricultural water run-off is dependent on its ability to deliver a UV dose sufficient to inactivate the pathogens of concern. The UV dose depends on the intensity of the UV light emitted by the lamps, on the flow rate of the water through the system, and on the UV transmittance of the water. Factors that determine the UVT include the level of suspended solids, the turbidity, color, and the concentration of soluble organic matter. The run-off from agricultural activities is often high in dissolved and suspended matter and the resulting turbidity can thus be sufficiently high as to lower the water’s transmittance of UV radiation thereby increasing, sometimes by more than 4-fold, the UV dose that is required to inactivate Pythium and Phytophthora . Thus it is important that the water quality parameters of the untreated influent be quantified to ensure that the UVT does not fall below a level that would reduce UV light penetration and thus inhibit its germicidal ability. It is also important that the UV system itself, through aspects of its design, remains capable of delivering sufficient UV dose even when the UVT is low. One way to increase the UV dose is to reduce the flow rate of the water through the system thereby increasing the time in which the pathogens are exposed to the UV light.
In many commercial applications, this is not an acceptable remedy due to the large quantities of runoff water generated in daily operations. Moreover, by decreasing the flow rate, the resulting flow can become laminar leading to significant reduction in mixing. When mixing is reduced, the disinfection efficiency in conditions of low UVT is also reduced since the pathogens that are not brought sufficiently close to the source of the UV light can leave the system without having received sufficient dose for inactivation. Recently, a novel system for water disinfection with UV light was developed and tested with success in the treatment of effluent from a number of municipal waste water treatment plants . This system, by virtue of a number of features unique to its design, has the potential of being particularly suitable for use in agricultural and horticultural operations where the water run-off is often low in UV transmittance. The new system also benefits from being free of the problem of ‘lamp fouling’ that is present in most commercially-available UV systems. In such systems, the UV-emitting lamps are immersed within the water being treated and hence, with time, become covered with bio-film and mineral deposits that reduce the intensity of the emitted light and necessitate frequent system shut downs for cleaning. The objective of this study was to introduce this system to the plant-pathology community by testing it, in situ, at a facility for the study of diseases of ornamental plants to assess its performance in inactivating P. ramorum in actual irrigation run-off water representative of that obtained in commercial nurseries.The UV system presented here is shown in Fig. 1. Untreated water enters the system into a pressure vessel located at the bottom of the column and exits this vessel through a series of nozzles arranged along the circumference of a circle. Affixed to the top of this vessel is aquartz cylinder; quartz being one of the few materials that allow UV-C radiation to pass through. The manner in which the water enters this cylinder causes it to rotate as it rises to the top. The rotating motion, coupled with the presence of a drain port at the cylinder base, lead to the formation of a central air vortex in the shape of a cone with its broad base located at top of the cylinder. The UV lamps are arranged around the outside of the quartz cylinder and thus do not come in contact with the water being treated. In this way, the problem of lamp ‘fouling’ does not arise. The combined rotational and axial motions of the water rising induce high levels of shear stress on the inside walls of the quartz cylinder. These high levels of shear stress provide a self-cleansing mechanism in that they prevent material from adhering to the inside of the cylinder which thus remains free of ‘fouling’.