Fluctuations in the fluorescent pseudomonad and actinomycete populations of rhizosphere and rhizoplane during the growth of spring wheat

Abstract

The total bacterial populations, fluorescent pseudomonads, and actinomycetes in the rhizosphere, rhizoplane, and endorhizosphere of two wheat lines (C-R5B and C-R5D) were investigated weekly during plant growth. The total numbers of bacteria (colony-forming units) were counted on a low-nutrient agar medium, while the numbers of fluorescent pseudomonads and actinomycetes were determined by means of selective media. Fluorescent pseudomonads generally constituted less than 0.5% of the total bacterial numbers and the percentage significantly decreased with time in the rhizosphere, rhizoplane, and root-free soil. The percentages of fluorescent pseudomonads were lower in the rhizoplane than in the rhizosphere, and in the endorhizosphere they were barely detectable. The rapid decline of fluorescent pseudomonads frequently observed in survival studies may be explained by the inability of the plant to support this group of bacteria during later development of its rhizosphere. Actinomycetes generally represented no more than 7% of the total bacterial numbers. Actinomycete percentages were highest in the rhizosphere and root-free soil and lowest in the endorhizosphere. Only minor differences were found in total bacterial numbers and in the numbers of fluorescent pseudomonads between the two wheat lines. However, numbers of actinomycetes were significantly different over the whole experiment between wheat line C-R5B and C-R5D in the rhizosphere, rhizoplane, and endorhizosphere. Key words: actinomycetes, bacteria, fluorescent pseudomonads, rhizosphere, sampling time.

Molecular Analysis of the Nitrate-Reducing Community from Unplanted and Maize-Planted Soils

“…Thus, comparison of the narG restriction profiles of the PCR products obtained from DNA extracted from soil and from the clone libraries indicated that no important bias occurred during the cloning step. These data are in accordance with the results of Miller et al (15), who found 10 times more actinomycetes in wheat rhizosphere than in unplanted soil. More recently, after finding that a high proportion of dominant populations in the rhizosphere belonging to high-GϩC gram-positive bacteria, Smalla et al underlined that high GϩC gram-positive bacteria might be more dominant in the rhizosphere than previously suggested (27).…”

“…Soil samples from all pots of the same treatment were mixed and then subsampled for DNA extraction. DNA was extracted from three 250-mg aliquots of soil from the unplanted treatment (samples BSA, BSB, and BSC) and the planted treatment (samples PSD, PSE, and PSF) according to the method of Martin-Laurent et al (15). Briefly, samples were homogenized in 1 ml of extraction buffer for 30 s at 1,600 rpm in a mini-bead beater cell disruptor (Mikro-Dismembrator S; B. Braun Biotech International).…”

Succession of Indigenous Pseudomonas spp. and Actinomycetes on Barley Roots Affected by the Antagonistic Strain Pseudomonas fluorescens DR54 and the Fungicide Imazalil

“…Furthermore, the level of Pseudomonas CFU in bulk soil throughout the experiment and the level of Pseudomonas DNA copies in bulk soil at day 0 (Table 2) were significantly lower than those in the root base rhizosphere. This supports the view that Pseudomonas bacteria are rhizosphere competent (25,34) and hence confirms that the agar plate results in the literature are not biased by cultivation. We found fewer Pseudomonas organisms in the root tip rhizosphere (Table 2) than at the root base, probably because the root tips represented young rhizosphere in which the Pseudomonas population had not yet proliferated.…”

“…Pseudomonas strains are mostly associated with fresh organic matter, rich in easily degradable compounds (14,20,34,38), while actinomycetes traditionally are considered to be most active late in the decomposition process, where they are strong competitors for complex organic compounds (4,19). Pseudomonas typically has a considerably higher occurrence in the rhizosphere than in bulk soil (25,34), while actinomycetes have been found to have both higher and lower occurrences in rhizosphere soil than in bulk soil, depending on the plant species (5,25).…”

Isolation and Identification of Actinobacteria from Surface-Sterilized Wheat Roots

“…Studies of the interactions between different groups of endophytic microorganisms within the host wheat plant have not yet been carried out but are necessary in order to obtain data on the colonization processes that occur in nature. Studies on the external rhizosphere populations during the growth of wheat plants have determined the fluctuations in actinobacterial and pseudomonad populations of the rhizoplane and rhizosphere (31,32) and the increases in populations of Pseudomonas, Chryseobacterium, and Flavobacterium spp. in the presence of Take-all disease (29).…”