Several previous reports also demonstrated that low levels of IAA stimulated primary root growth. Similar to our observations, the Burkholderia sp. SSG that was isolated from boxwood leaves produced 2.9 to 4.5 µg mL−1 of IAA with tryptophan and had plant growth promotion ability in three boxwood varieties. Additionally, Burkholderia phytofirmans strain PsJN, which was isolated from onion roots, showed higher IAA production, around 12 µg mL−1 , with the addition of tryptophan and improved the growth of potato, tomato, maize, and grapevines. Other Burkholderia seminalis strains can also synthesize IAA and have been reported to increase rice and tomato seedling growth. These previous studies, along with our observations, suggest that B. seminalis strain 869T2 may be similar to other Burkholderia species and other plant-growth-promoting bacteria that utilize IAA to increase root growth, which may assist host plants in taking up nutrients from the surrounding environment and improve aerial tissue growth. Consistent with this hypothesis, we observed that plant size, height, fresh weight, dry weight, and total leaf areas of several tested plant species all significantly increased after inoculation with B. seminalis strain 869T2. It is known that the IAA can positively affect cell division, enlargement, tissue differentiation, root formation, and the control process of nutrition growth. The IAA can also function as a signal molecule to influence the expression of various genes involved in energy metabolism and other plant hormone synthesis, such as gibberellin and ethylene. Interestingly, we observed earlier flowering in the 869T2-inoculated hot pepper and okra plants, square plastic plant pots suggesting that acceleration of plant growth rates might occur in these plants.
In the future, transcriptome analysis of plant hormone response genes and energy-metabolic-related genes in the 869T2-inoculated plants might help us further decipher the possible mechanism of plant growth promotion ability of strain 869T2. From the results of our study, we observed that B. seminalis strain 869T2 had a better IAA yield at a temperature range of 25 ◦C to 37 ◦C and pH of 6 to 9. Similarly, Burkholderia pyrrocinia strain JK-SH007 reached the maximum production of IAA at 37 ◦C and pH 7.0. Several other plant-growth-promoting bacteria, including Bacillus siamensis, Bacillus megaterium, Bacillus subtilis, and Bacillus cereus, had relatively higher IAA yields at temperatures of 2–135 ◦C and pH 7–8. Three different bacteria isolated from therhizosphere of Stevia rebaudiana also exhibited greater production of IAA at a pH range of 6–9 and a temperature of 35 ◦C to 37 ◦C; these bacteria also increased the root and shoot bio-masses of wheat and mung bean. Various carbon sources are used as an energy source for IAA production and could enhance recycling of cofactors in bacterial cells. Our results revealed that IAA yields of B. seminalis strain 869T2 were slightly better when glucose and fructose were used in media. Several previous publications also indicated that the ability of plant-growth-promoting bacteria to produce IAA was different, depending on the carbon source used in the media. Results from these studies and our study demonstrated that IAA production by different plant-growth promoting bacteria can be influenced by various factors, such as temperature, pH, carbon sources, culture conditions, and bacterial species. In this study, we utilized the colorimetric method to estimate the IAA amounts of B. seminalis strain 869T2 when grown in various in vitro conditions and media.
Because the available tryptophan in the rhizosphere and root exudates of plants might be relatively lower than the tryptophan used in the media, the IAA production of B. seminalis strain 869T2 when grown in inoculated plants shall be determined with more sensitive and accurate methods, such as high-performance liquid chromatography or ultra-performance liquid chromatography systems. Apart from the IAA production ability of B. seminalis strain 869T2, this bacterium exhibited siderophore production and phosphate solubilization activities. Iron is an important element for many biological processes in plant growth and development. Most iron in soils is present in the highly insoluble ferric form, which is unavailable for plant absorption. Endophytic bacteria can yield iron-chelating agents such as siderophores, which bind ferric iron and help transport it into plant cells via root-mediated degradation of organic chelate, ligand exchange, or other mechanisms. Phosphorus is another essential macronutrient for numerous metabolism processes in plants, such as biosynthesis of macromolecules, signal transduction, photosynthesis, and respiration. Most of the phosphorus in soil is insoluble and not available for root uptake to support plant growth. In order to increase the bio-availability of phosphorus for plants, certain endophytic bacteria turn insoluble phosphate into soluble forms via the processes of chelation, ion exchange, acidification, or production of organic acids. Previous studies have also correlated siderophore production and phosphate solubilization abilities with the plant growth promotion traits of other Burkholderia species, such as the Burkholderia sp. SSG isolated from boxwood and the Burkholderia sp. MSSP isolated from root nodules of Mimosa pudica. Burkholderia cenocepacia strain CR318, which was isolated from maize roots, significantly enhanced maize plant growth by solubilizing inorganic tricalcium phosphate. Other studies have revealed that additional Burkholderia species also have the ability to solubilize inorganic phosphate to increase available phosphorous in agricultural soils and improve agricultural production.
In summary, both previous studies and our results suggest that the IAA synthesis, siderophore production, and phosphate solubilization abilities of B. seminalis strain 869T2 may collectively contribute to the growth enhancement observed in the several plant species tested here. We successfully inoculated and reisolated B. seminalis strain 869T2, which was originally isolated from the monocot plant vetiver grass, in several eudicot plant species of the Brassicaceae, Asteraceae, Amaranthaceae, Solanaceae, and Malvaceae families. Strain 869T2 can significantly improve the growth of both the roots and aerial parts of Arabidopsis and several leafy vegetables, including ching chiang pak choi, pak choi, loose-leaf lettuce, romaine lettuce, red leaf lettuce, and Chinese amaranth. These results suggest that the endophytic bacterium strain 869T2 may have a wide host range. A similar observation was reported for Burkholderia phytofirmans strain PsJN, first isolated from onion roots, which enhanced the growth of Arabidopsis, switch-grass, potato, tomato, maize, wheat, and grapevines. We did not observe significant growth improvement in hot pepper or okra plants after inoculation with strain 869T2; however, we did observe early flowering and better fruit development in these tested plants. These results suggest that the plant growth promotion abilities of strain 869T2might be more apparent in crops with a shorter life cycle or that the latter two tested host plant species might not be fully compatible with this bacterium. The plant colonization process and growth promotion abilities of endophytic bacteria seem to be active processes that are regulated by different characteristics of both the host plants and bacteria. In conclusion, our study revealed the potential of Burkholderia seminalis strain 869T2 for use as a bio-inoculant in agriculture to improve plant growth and production.Tropical fruit flies , such as the Mediterranean fruit fly from Africa, square pot plastic the oriental fruit fly from Asia and the Mexican fruit fly from the Americas, are recognized by entomologists as among the most destructive agricultural insect pests in the world. Because of tephritids’ economic importance, US states such as California—considered by both the US Department of Agriculture and the California Department of Food and Agriculture to be free of these pests, but with climates favourable to their establishment—invest heavily in measures to keep tephritids from becoming established. These steps include restricting importation of commodities that originate in regions with ongoing tephritid outbreaks, requiring post-harvest treatments for imported fruits and vegetables grown in areas where the pests are endemic or established, maintaining large-scale monitoring programmes for early detection, supporting preventive release programmes of sterile flies to pre-empt establishment, and launching eradication campaigns to eliminate pest populations once discovered. Indeed, 90% of the eradication projects initiated in California between 1982 and 2007 were directed against tropical fruit flies . The historical challenges posed by the fruit fly threat to California are similar to those posed by many other invasive insect species. For example, the propagule pressure of fruit flies resulting from the ever-increasing movement of people and products is an ongoing challenge posed by all invasive species. Similarly, global warming has resulted in the expansion of pest ranges worldwide. Fewer frost days, longer growing seasons, more heat waves and greater frequency of warm nights in California, combined with an abundance of suitable hosts in both urban and commercial environments, create ideal conditions for a wide range of species, particularly tropical tephritids, to successfully invade. Two aspects of California’s fruit fly invasions are unique, however. First, in most years and locations, fruit fly detections are extremely rare because of a combination of the slow population growth of newly introduced species and of population suppression from intervention programmes.
This combination of elements makes it difficult to decipher patterns in detections, because there are few ‘dots’ to connect, and small numbers of captures separated in both time and space may give the illusion that previously detected populations have been eliminated. Second, an unprecedented number of pest tephritids have been detected in California in recent decades, including a more than eight fold increase in the number of species , and thousands more flies have been captured in California than in all other US mainland states combined. We are unaware of any other single taxonomic group that consists of such a large number of economically important invasive species that are continually reappearing in the same region. Our broad goal in this paper is to bring principles of invasion biology, mapping techniques and quantitative methods to bear on detection and interception data to answer questions about the residency status of tropical fruit flies captured in California. We show that, despite the due diligence, quick responses and massive expenditures of government agencies to prevent entry and establishment of these pests, virtually all of the species against which eradication projects were directed have been reappearing; several species reappear annually, and several others every 2–5 years. The preponderance of evidence supports the hypothesis that at least five and as many as nine species are established in the state. Tephritids have been detected in nearly all regions of California where conditions are favourable for fruit fly establishment . Although the largest numbers of detections by far have been in the greater metropolitan areas of southern California, including the Los Angeles Basin and San Diego, a substantial number of flies were also detected in northern California, in the San Francisco Bay Area. Tephritids also began appearing in the state’s main agricultural growing region, the Central Valley, which includes the Sacramento and San Joaquin Valleys, and the Imperial Valley. The non-random pattern of the invasions is reflected in the fact that 100% of first records for all species were in southern California , all but one of which were found in two regions: Los Angeles and San Diego. These regions contain only around one-third of the state’s population, yet account for 100% of the tephritid first records. California was free of any tropical fruit fly species before the mid-1950s , despite the rapid growth of the fruit industry in the late nineteenth century and first half of the twentieth century, as well as relatively lax regulatory protocols at ports of entry. Two species were detected during the 1950s , followed by four more in the 1960s and 1970s. The tephritid situation in the state changed drastically in the 1980s because of: continued reappearances and spread of previously detected species in metropolitan Los Angeles and San Diego; seven new species detected, raising the total in the state from six to 13 species ; and first tephritid detections in northern California , including a massive, widespread medfly outbreak in the Bay Area. Three new tephritid species captured in the 1990s raised the total in the state to 16. The economic stakes were elevated to a new level when one of these, the olive fly , was declared established, and several previously detected species appeared in the Central Valley growing region. Even though only one new species has been captured during the past 12 years, nine previously detected species have been recaptured repeatedly over ever-expanding areas , including seven that have been captured multiple times during the past 3 years . The magnitude and geographical scope of the recurrent detections are evident in maps in figure 1b, showing the historical records of the hundreds of state-wide, regional and local detections of the four most frequently captured species.