Distribution, population dynamics, and characteristics of ice nucleation-active bacteria in deciduous fruit tree orchards

Deciduous fruit tree orchards located in the Pacific Northwest were surveyed over a 3-year period for the presence of ice nucleation-active (INA) bacteria. In the Yakima Valley, only about 30% of the fruit tree orchards contained INA bacteria (median population ca. 3 x 10 CFU/g [fresh weight]) in contrast to nearly 75% of the orchards in the Hood River Valley (median population ca. 5 x 10 CFU/g [fresh weight]). These INA populations ranged from less than 10 to over 10 CFU/g (fresh weight) of blossoms and, in Hood River Valley orchards, generally comprised over 10% of the total bacterial population. Populations of INA bacteria fluctuated during the year with highest levels developing on buds and flowers during the cool, wet spring, followed by a drop in populations during the warmer, drier, summer months and finally a gradual increase in the autumn. The INA bacteria persisted on dormant buds from which they again colonized young developing vegetative tissues. All INA bacteria were identified as Pseudomonas syringae. The frequency of ice nucleation at -5 degrees C for these strains ranged from nearly every cell being INA to less than 1 in 10 cells. The median frequency of ice nucleation at -5 degrees C was 10 cells per ice nucleus. The INA P. syringae strains from individual orchards were diverse with respect to bacteriocin typing and in ice nucleation frequency. The consistent absence of detectable INA bacteria or presence of low populations in most of the orchards surveyed during periods when critical temperatures (i.e., -2 to -5 degrees C) were common indicated a limited role for INA bacteria in frost susceptibility of most Pacific Northwest orchards.

Characterization of Pyoverdin pss , the Fluorescent Siderophore Produced by Pseudomonas syringae pv. syringae

Pseudomonas syringae pv. syringae B301D produces a yellow-green, fluorescent siderophore, pyoverdin pss , in large quantities under iron-limited growth conditions. Maximum yields of pyoverdin pss of approximately 50 μg/ml occurred after 24 h of incubation in a deferrated synthetic medium. Increasing increments of Fe(III) coordinately repressed siderophore production until repression was complete at concentrations of ≥ 10 μM. Pyoverdin pss was isolated, chemically characterized, and found to resemble previously characterized pyoverdins in spectral traits (absorbance maxima of 365 and 410 nm for pyoverdin pss and its ferric chelate, respectively), size (1,175 molecular weight), and amino acid composition. Nevertheless, pyoverdin pss was structurally unique since amino acid analysis of reductive hydrolysates yielded β-hydroxyaspartic acid, serine, threonine, and lysine in a 2:2:2:1 ratio. Pyoverdin pss exhibited a relatively high affinity constant for Fe(III), with values of 10 25 at pH 7.0 and 10 32 at pH 10.0. Iron uptake assays with [ 55 Fe]pyoverdin pss demonstrated rapid active uptake of 55 Fe(III) by P. syringae pv. syringae B301D, while no uptake was observed for a mutant strain unable to acquire Fe(III) from ferric pyoverdin pss . The chemical and biological properties of pyoverdin pss are discussed in relation to virulence and iron uptake during plant pathogenesis.

Outer membrane protein mediating iron uptake via pyoverdinpss, the fluorescent siderophore produced by Pseudomonas syringae pv. syringae

In an iron-limited environment Pseudomonas syringae pv. syringae B301D produces a yellow-green fluorescent siderophore called pyoverdinpss which functions in high-affinity iron transport. Two-dimensional electrophoretic comparisons of the outer membrane proteins of strain B301D identified nine proteins which were expressed at low (50 nM) but not at high (10 microM) iron concentrations. Except for the minor protein 8e, the iron-regulated proteins exhibited high molecular weights ranging from approximately 74,000 to 80,000. A mutant of strain B301D incapable of iron uptake (Iu-) from ferric pyoverdinpss lacked the 74,000-molecular-weight protein 4a, which was the major iron-regulated outer membrane protein. In contrast, a nonfluorescent mutant (Flu-) unable to synthesize pyoverdinpss showed no quantitative or qualitative difference in its outer membrane profile from that of the wild-type strain. In plant pathogenicity tests the Iu- and Flu- strains caused typical brown necrotic and sunken lesions in immature sweet cherry fruit which were indistinguishable from those of the wild-type strain. Thus, excretion of pyoverdinpss and subsequent Fe(III) uptake do not have a determinative role in the pathogenicity or virulence of P. syringae pv. syringae.