Surveys conducted during the first two years of the study throughout the entirety of the six research blocks showed that the prevalence of PD in control vines actually declined slightly from the first to the second year , but not due to an increase in replanting efforts or vine death , Rather, this decline in prevalence likely reflects overwinter recovery of mild cases of the disease . Thus, the observed return of symptoms in most severely pruned vines does not appear to be explained by reinfection with X. fastidiosa after clearing of infection during the severe-pruning process. Our results indicate that the apparent effectiveness of severe pruning depended on the initial disease severity, and the effectiveness weakened over time. This suggests at least two constraints exist regarding the general utility of pruning as a PD management tool. First, severe pruning does not appear to be useful for mild cases of PD, as many of those same vines would recover from the infection over the winter . Second, there appears to be little value in pruning severely diseased vines; the high frequency of symptom return within a few years indicates that even severe pruning does not clear most vines of X. fastidiosa infection.
That leaves a statistically significant window with respect to intermediate severity cases,blueberry grow bag size which may benefit from severe pruning. The apparent benefit for this category of diseased vines would stem from infections that are not so localized that they are highly susceptible to natural recovery over the winter, but also not fully systemic such that the infection has developed below the pruning point . Reliable identification of this narrow class of diseased vines may require substantial experience with PD scouting, detailed record keeping, and an appreciation for variability in symptoms or infection dynamics based on grapevine cultivar and environmental conditions . Research in other bacterial plant pathosystems has evaluated the potential benefit of pruning and whether pruning extent is related to its effectiveness at clearing hosts of infection . A study of the citrus disease huanglongbing, associated with infection by Candidatus Liberibacter spp., evaluated two levels of pruning severity, neither of which showed promise as a disease management tool . In this pathosystem, it is plausible that a very protracted incubation period may undermine the effectiveness of pruning, because by the time the first symptoms are visible, the infection may have already moved throughout much of the tree. Collectively, our results are more similar to a study of citrus variegated chlorosis .
In this study, the presence of X. fastidiosa in plant tissues at different distances from symptomatic leaves was determined for varying levels of disease severity. X. fastidiosa was more widely distributed in trees with severe disease symptoms compared to those with early stage foliar symptoms. Although ColettaFilho et al. did not test whether pruning at various distances proximal to symptomatic leaves would eliminate X. fastidiosa infections, the current recommendation is to prune citrus material if early symptoms are present, and to not prune plants with severe disease symptoms . Citrus plant age is also an important consideration; Coletta-Filho and de Souza recommend that symptomatic citrus trees up to three-years-old be removed rather than pruned, whereas trees four-years-old or older should be pruned. We did not examine vine age as a factor in this study, but the biology of citrus and grape differ in terms of the overwinter recovery that can occur in grape and the apparently slower movement of X. fastidiosa in citrus compared to grape. Anecdotally, the two most mature plots in our study showed the most rapid return of disease, and the youngest plot showed the slowest return. More studies of the effect of vine age are needed before concluding that interactive effects of plant age and pruning differ between the PD and citrus variegated chlorosis pathosystems.The length of lifespan of a particular normal cell of any organism is predetermined. Similarly, the length of lifespan of all organisms is pre-determined by their genetic makeup and their external and internal environments and diet-related factors specific to an organism.
Therefore, the length of lifespan can be increased or decreased by manipulating the environment, diet and genetic factors only by small extent. The length of lifespan of the organisms can also be impacted by differential rates of senescence of cells and organs that ultimately lead to the death of the organisms. The differential rates of cellular senescence are influenced by several confounding factors, such as external and internal environments, diet and genetic factors. Because of these confounding factors that can impact rates of progression of degenerative changes in the organs, it is almost impossible to study aging in the absence of organ pathology. Based on numerous studies on aging in vertebrates and invertebrates, a recent informative review has suggested that oxidative stress theory of aging can only be applied to conditions in which age associated pathologies are included. Furthermore, it was suggested that in environment with minimal stress, oxidative damage plays little role in aging. This suggestion can be argued on the fact that little oxidative damage may take longer time to deregulate protective transcription factors, adaptive responses to stressors, and repair mechanisms, and thereby extending the lifespan of the organisms more than that produced by higher oxidative damage which can deregulate above biological functions in shorter time. Using vertebrate and invertebrate models, some major biochemical and genetic factors that are associated with aging processes have been identified. They include increased oxidative stress and chronic inflammation, decreased adaptive response to stressors, post-translational protein modifications, mitochondrial dysfunction, decreased of proteasome and lysosomal-mediated proteolytic activity, shortening of telomeres and transcriptional deregulation.
Among these, the theory of oxidative stress is most extensively investigated in various experimental models, using pharmacological agents, antioxidants, anti-inflammatory agents and deletion of one or more antioxidant enzymes as well as of mitochondrial complexes. Depending upon the experimental models, experimental designs, and substrate used to assay oxidative stress and criteria of oxidative stress, the role of oxidative stress in aging has been substantiated or questioned. We hypothesized that increased oxidative stress may be one of the primary early events that causes chronic inflammation, transcriptional deregulation, post-translational protein modifications, mitochondrial dysfunction, decreased of proteasome and lysosomal-mediated proteolytic activity and shortening of telomeres. Invertebrate models, such as Caenorhabditis eleganshas been extensively used to evaluate the role of oxidative stress in aging primarily due to shorter lifespan of about 3 days and ease of genetic manipulation. This review analyzes recent published studies on C. elegans on the role of oxidative stress in determining the length of lifespan by generating mutants that show suppression of mitochondrial function or lack of superoxide dismutase . Caenorhabditis elegans has been extensively used to investigate the role of oxidative stress in aging by measuring the length of lifespan. Mitochondria are considered the major sites for the production Reactive oxygen species ,blueberry box although ROS are also produced outside the mitochondria. In order to demonstrate the impact of oxidative stress, several mutants of C. elegans were generated. They include mutations in four clock genes , mutation in the iron sulfur protein of mitochondrial complex III, mutation in the gene NUO-6 and mutation in the gene daf-2. The effects of mutations on oxidative stress and lifespan are summarized in Table 1. The clk-1 gene encodes an enzyme that is necessary for the biosynthesis of ubiquinone that is required by the mitochondria to generate energy. Mutation in the clk-1 gene increases the life span by slowing down mitochondrial activity due to reduced availability of ubiquinone. This slowing of the electron transport chain would reduce oxidative stress. The role of reduced oxidative stress in extending the lifespan is further supported by the fact that overexpression of clk-1 gene in wild-type C. elegans increased mitochondrial activity and shortened the lifespan. A mutation in the iron sulfur protein of mitochondrial complex III causes low oxygen consumption, reduced oxidative stress and increased lifespan. Mutation in the daf-2 gene which codes for a member of insulin receptor family increased lifespan and enhanced resistance to oxidative stress. In this daf-2 mutant, expression of the SOD-3 gene, which encodes mitochondrial Mn-superoxide dismutase, was much higher than in the wild type. This implies that the increased levels of SOD-3 in the daf-2 mutant reduced oxidative stress and thereby increased lifespan. Mutation in the gene NUO-6 which encodes complex I of mitochondria increases life span of C. elegans by decreasing the mitochondrial function. Mutation in the age-1 increased lifespan by two folds. This mutant wormhad increased catalase and Cu/Zn SOD activities which may account for the increased resistance to the paraquat, a superoxide generating chemical.
The mutants C. elegans support the view that the levels of oxidative stress is one of the important determinant factors in determining the length of lifespan.Superoxide anions are produced enzymatically outside the mitochondria by different oxidases and nonenzymatically inside the mitochondria. SOD detoxifies superoxide to hydrogen peroxide , which is converted to water and oxygen by catalase. There are five superoxide dismutase isoforms SOD-1, SOD-2, SOD-3, SOD-4 and SOD-5 in C. elegans. However, in most organisms there are only 3 SODs. SOD-1 is present in the cytoplasm and represents the majority of SOD activity, whereas SOD-2 and SOD-3 are present in mitochondria. Increased levels of superoxide were observed in SOD deleted wild type worms or in ISP-1 and NOU-6 mutants. The effect of deletion of SOD on C. elegans lifespan is shown in Table 2. The impact of SOD deletion on the lifespan appears to be contradictory, depending upon the C. elegans model used. For example, SOD2 deletion markedly increased the lifespan of mutant clk-1 worms, but it decreased the lifespan of mutant isp-1worms. In addition, deletion of individual SOD genes from wild type C. elegans did not decrease the lifespan of these worms. This is in sharp contrast to other model, such as yeast, flies, and mice in which deletion of cytoplasmic or mitochondrial SOD caused decreased in the lifespan. In another study, it was demonstrated that the levels of superoxide were elevated in nou-6 mutant and isp-1 mutant and they lived longer than the wild type, however, the oxidative stress was low and overall levels of ROS did not change. From these results, it was concluded that elevation of superoxide is sufficient to increase the lifespan of these mutant worms. Based on these results, the role of oxidative stress in aging was questioned. It should be pointed out that if superoxide is precursor of ROS, the levels of ROS and oxidative stress should have been increased. This was not observed in the above study, suggesting that reduced oxidative stress possibly due to adaptive response by other antioxidant enzymes, such as catalase and glutathione peroxidase and improved repair mechanisms was responsible for the extension of lifespan of these mutant worms. In the same article, it was observed that addition of N-acetylcysteine and vitamin C individually abolished the effect of superoxide on life extension and other associated changes in nou-6 and isp-1 mutants. The antioxidant effect of NAC is mediated via glutathione, which in the presence of the high superoxide environment of mutant worms can be oxidized and then act as a pro-oxidant. Therefore, observed abolition of the effect of superoxide on life extension could be related to pro-oxidant effects of NAC and vitamin C. In order to assess the role of SOD on life extension further, a model of C. elegans was developed in which all five SODs were deleted, was established. The results showed that SOD 12345 worms were viable and exhibited a normal lifespan similar to that of wild-type despite increased sensitivity to multiple stressors. However, these SOD lacking worms showed reduced fertility, slow development, slower defecation cycle and decreased movement . From these results, it was concluded that SOD is dispensable for normal lifespan of C. elegans. This is in sharp contrast to mammals in which SOD is considered indispensable for survival. Thus, the results obtained on some genetic models of C. elegans cannot readily be extrapolated to the genetic models of mammals. If SOD is dispensable for the survival and lifespan of C. elegans as reported recently, overexpression of SOD should have no impact on the lifespan of these worms. On the contrary, it was reported that over expression of the major cytosolic Cu/Zn-SOD increased lifespan of wild type worms which was not related to reduced lipid oxidation or glycation.