K.A. El-Tarabily

Promotion of plant growth by an auxin-producing isolate of the yeast Williopsis saturnus endophytic in maize (Zea mays L.) roots

A plant-growth-promoting isolate of the yeast Williopsis saturnus endophytic in maize roots was found to be capable of producing indole-3-acetic acid (IAA) and indole-3-pyruvic acid (IPYA) in vitro in a chemically defined medium. It was selected from among 24 endophytic yeasts isolated from surface-disinfested maize roots and evaluated for their potential to produce IAA and to promote maize growth under gnotobiotic and glasshouse conditions. The addition of l-tryptophan (L-TRP), as a precursor for auxins, to the medium inoculated with W. saturnus enhanced the production of IAA and IPYA severalfold compared to an L-TRP-non-amended medium. The introduction of W. saturnus to maize seedlings by the pruned-root dip method significantly (P<0.05) enhanced the growth of maize plants grown under gnotobiotic and glasshouse conditions in a soil amended with or without L-TRP. This was evident from the increases in the dry weights and lengths of roots and shoots and also in the significant (P<0.05) increases in the levels of in planta IAA and IPYA compared with control plants grown in L-TRP-amended or non-amended soil. The plant growth promotion by W. saturnus was most pronounced in the presence of L-TRP as soil amendment compared to seedlings inoculated with W. saturnus and grown in soil not amended with L-TRP. In the glasshouse test, W. saturnus was recovered from inside the root at all samplings, up to 8 weeks after inoculation, indicating that the roots of healthy maize may be a habitat for the endophytic yeast. An endophytic isolate of Rhodotorula glutinis that was incapable of producing detectable levels of IAA or IPYA in vitro failed to increase the endogenous levels of IAA and IPYA and failed to promote plant growth compared to W. saturnus, although colonization of maize root tissues by R. glutinis was similar to that of W. saturnus. Both endophytic yeasts, W. saturnus and R. glutinis, were incapable of producing in vitro detectable levels of gibberellic acid, isopentenyl adenine, isopentenyl adenoside or zeatin in their culture filtrates. This study is the first published report to demonstrate the potential of an endophytic yeast to promote plant growth. This is also the first report of the production of auxins by yeasts endophytic in plant roots.

Promotion of tomato (Lycopersicon esculentum Mill.) plant growth by rhizosphere competent 1-aminocyclopropane-1-carboxylic acid deaminase-producing streptomycete actinomycetes

The ability of streptomycete actinomycetes to promote growth of tomato through the production of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase was evaluated under gnotobiotic and greenhouse conditions. To achieve this, 64 isolates of Streptomyces spp. obtained from a tomato rhizosphere in the United Arab Emirates were initially selected for their ability to produce ACC deaminase as well as indole-3-acetic acid (IAA) and subsequently for their rhizosphere competence as root colonizers. Of the two selected ACC deaminase-producing isolates showing exceptional rhizosphere competence, S. filipinensis no. 15 produced both ACC deaminase and IAA, whilst S. atrovirens no. 26 did not produce IAA. Under greenhouse conditions, the application of S. filipinensis no. 15 or S. atrovirens no. 26 resulted in the reduction of the endogenous levels of ACC, the immediate precursor of ethylene, in both roots and shoots and increased plant growth. Plant growth promotion was most pronounced in the presence of S. filipinensis no. 15 compared to S. atrovirens no. 26. This relative superiority in performance shows the advantage conferred to S. filipinensis no. 15 due to its ability to produce both IAA and ACC deaminase. In comparison, an ACC deaminase-producing isolate of S. albovinaceus no. 41 which was neither rhizosphere-competent nor capable of producing IAA, failed to promote plant growth compared to S. filipinensis no. 15 or S. atrovirens no. 26 although the growth promotion obtained by S. albovinaceus no. 41 was significant compared to control. The application of S. globosus no. 8, which was not rhizosphere-competent and did not produce detectable levels of ACC deaminase or IAA did not promote plant growth. These results indicate the importance of rhizosphere competence. In conclusion I report the production of ACC deaminase by streptomycete actinomycetes and its ability to enhance plant growth through reduction in the in planta levels of endogenous ACC and the consequent lowering of endogenous ethylene levels in plant tissues.

Plant growth promotion and biological control of Pythium aphanidermatum, a pathogen of cucumber, by endophytic actinomycetes

Aims:  To evaluate the potential of Actinoplanes campanulatus, Micromonospora chalcea and Streptomyces spiralis endophytic in cucumber roots, to promote plant growth and to protect seedlings and mature plants of cucumber from diseases caused by Pythium aphanidermatum, under greenhouse conditions.

Growth promotion of bean (Phaseolus vulgaris L.) by a polyamine-producing isolate of Streptomyces griseoluteus

Of seventy-five actinomycetes isolated from a bean rhizosphere in the United Arab Emirates, an isolate of Streptomyces griseoluteus (WT) was found to be capable of producing relatively high levels of putrescine on decarboxylase agar medium and to produce putrescine, spermidine and spermine in liquid decarboxylase medium. In the glasshouse, the application of the WT strain to soil amended with arginine (as a precursor for putrescine) significantly (P < 0.05) promoted the growth of bean plants and increased the fresh and dry weights and lengths of roots and shoots, compared with control plants. Infestation of soil with the WT strain resulted in a significant (P < 0.05) increase in the levels of putrescine, spermidine and spermine, certain endogenous plant growth regulators (PGRs) (indole-acetic acid, and gibberellic acid), chlorophylls (a, b) and carotenoids with a concomitant reduction in the level of abscisic acid in bean plants, compared with control plants. A polyamine non-producing mutant strain (PNPM) obtained from the wild-type isolate (WT), however, failed to promote plant growth. There were no significant (P > 0.05) differences between the levels of polyamines, endogenous PGRs, chlorophylls (a, b), and carotenoids between plants that were not exposed to either of the strain (control) and those grown in soil with the PNPM strain. Both WT and PNPM strains were incapable of producing in vitro detectable levels of PGRs, indole-acetic acid, indole-pyruvic acid, gibberellic acid, isopentenyl adenine and zeatin in the culture filtrates. This study is the first to demonstrate the potential of a polyamine-producing actinomycete to promote plant growth. In addition, it is also the first published report of the production of polyamines by streptomycete actinomycetes.

Suppression of Rhizoctonia solani diseases of sugar beet by antagonistic and plant growth‐promoting yeasts

Aims:  Isolates of Candida valida, Rhodotorula glutinis and Trichosporon asahii from the rhizosphere of sugar beet in Egypt were examined for their ability to colonize roots, to promote plant growth and to protect sugar beet from Rhizoctonia solani AG‐2‐2 diseases, under glasshouse conditions.

Microbiological differences between limed and unlimed soils and their relationship with cavity spot disease of carrots (Daucus carota L.) caused by Pythium coloratum in Western Australia

Application of lime (4000 kg ha-1) to a soil used for commercial carrot production (pH 6.9) significantly (p<0.05) reduced the incidence of cavity spot disease of carrots compared to unlimed soil (pH 5.1). It significantly (p<0.01) increased soil microbial activity as measured by the hydrolysis of fluorescein diacetate and arginine ammonification. The application of lime resulted in a significant (p<0.01) increase in the total numbers of colony forming units (efu) of aerobic bacteria, fluorescent pseudomonads, Gram negative bacteria, actinomycetes and a significant (p<0.01) decrease in the cfu of filamentous fungi and yeasts compared to unlimed soil. Liming also increased the cfu of non-streptomycete actinomycetes rarely reported in similar studies. These non-streptomycete actinomycetes were estimated and isolated using polyvalent Streptomyces phages and the dry heat technique to reduce the dominance of streptomycetes on isolation plates. The non-streptomycete actinomycetes isolated included species of Actinoplanes, Micromonospora, Streptoverticillium, Nocardia, Rhodococcus, Microbispora, Actinomadura, Dactylosporangium and Streptosporangium. The numbers of actinomycetes antagonistic to Pythium coloratum, a causal agent of cavity spot disease of carrots increased in soil amended with lime. Application of lime also reduced the isolation frequency of P. coloratum from asymptomatic carrot roots grown in soil artificially infested with the pathogen, 3, 4 and 5 weeks after sowing.

Isolation and characterisation of sulfur-oxidising bacteria, including strains of Rhizobium, from calcareous sandy soils and their effects on nutrient uptake and growth of maize (Zea mays L.)

Four sulfur-oxidising bacteria were selected among 427 bacterial isolates from calcareous sandy soils in the United Arab Emirates (UAE). These isolates were selected based on their strong ability to oxidise elemental sulfur (S°) in vitro and were identified as Paracoccus versutus CBS 114155, Paracoccus pantotrophus CBS 114154, and 2 strains as Rhizobium spp. NCCB 100053 and NCCB 100054. This is the first published report of a Rhizobium species capable of S° oxidation and also the first record of sulfur-oxidising bacteria from UAE soils. These isolates were tested in a greenhouse in the presence and absence of S° to study their effects on maize growth. Best growth was observed in the treatment with P. versutus application combined with S°, which significantly reduced soil pH, increased soil SO4 level and the uptake of N, S, Fe, Mn, and Zn in maize roots and shoots. The P and Cu uptake in the shoots of maize plants was not significant compared with the treatment that received the application of S° alone. There was no response in plant growth to treatments that included the application of S° combined with P. pantotrophus or Rhizobium strain NCCB 100053 compared with the treatment that received the application of S° alone. There was significant growth inhibition of maize plants in the treatment receiving Rhizobium strain NCCB 100054 with or without the application of S° compared with the treatment that included the application of S° alone. This growth inhibition was associated with a significant decrease in the levels of N, P, S, Fe, Mn, Zn, and Cu in roots and shoots in the absence of S°. Rhizobium strain NCCB 100054 applied with S° significantly decreased the levels of N, S, and Fe in the roots and the levels of N, P, S, Fe, Mn, and Cu in the shoots of maize, with no significant differences in the levels of P and Mn in the roots and in the levels of Zn in the shoots, compared with the treatment with S° alone. These results indicate that the treatment P. versutus combined with S° can be effective as a soil conditioner for horticultural production in calcareous sandy soils.

Fish emulsion as a food base for rhizobacteria promoting growth of radish (Raphanus sativus L. var. sativus) in a sandy soil

Commercial fish emulsion was evaluated as a plant growth medium and as a nutrient base to enhance radish (Raphanus sativus L. var. sativus) growth by bacterial and actinomycete isolates. Six bacterial isolates including three actinomycetes were selected from a screening of 54 bacteria (including 23 actinomycetes) based on their ability to produce plant growth regulators (PGRs) and to colonize radish roots. These isolates were tested in the presence and absence of autoclaved or non-autoclaved fish emulsion or inorganic fertilizers. The nutrient contents and types and levels of PGRs in tissues of treated plants were assayed to determine the basis of growth promotion. Fish emulsion was found to support plant growth in a sandy soil as effectively as an applied inorganic fertilizer. The plant growth promotion by bacterial and actinomycete isolates was most pronounced in the presence of autoclaved or non-autoclaved fish emulsion than in the presence of the inorganic fertilizers. The bacterial and actinomycete isolates were capable of producing auxins, gibberellins and cytokinins and appeared to use fish emulsion as a source of nutrients and precursors for PGRs. PGR levels in planta following combined treatments of the bacterial and actinomycete isolates and fish emulsion were found to be significantly enhanced over other treatments. The effect of fish emulsion appears to be more related to its role as a nutrient base for the bacterial and actinomycete isolates rather than to the increased activity of the general microflora of treated soil. This is the first report of fish emulsion as a nutrient base for plant growth promoting rhizobacteria. These results also indicate that the successful treatment can be effective and economical for horticultural production in sandy soils such as those found in the United Arab Emirates where fish emulsion is already in use as a substitute or supplement for inorganic fertilizer.

Enhancement of morphological, anatomical and physiological characteristics of seedlings of the mangrove Avicennia marina inoculated with a native phosphate-solubilizing isolate of Oceanobacillus picturae under greenhouse conditions.

One hundred and twenty-nine rock phosphate (RP) solubilizing bacteria were isolated from the mangrove Avicennia marina rhizosphere sediment deficient in available phosphorus (P), in the United Arab Emirates (UAE). Among these isolates, an outstanding isolate of Oceanobacillus picturae grown in Pikovskaya’s liquid medium amended with RP, caused the highest percentage of RP solubilization (97%), the most pronounced drop in the medium pH and produced a variety of organic acids, as well as acid and alkaline phosphatases. Greenhouse experiments were carried out to investigate the effect of sediment inoculation with O. picturae (Op) with or without RP amendment on the growth, nutrient uptake, conducting tissue and photosynthetic gas exchange characteristics of mangrove seedlings. The application of Op to sediments amended with RP significantly promoted the growth of roots and shoots of seedlings compared with those grown in sediment amended with RP only. O. picturae significantly increased available sediment P, decreased sediment pH, positively enhanced nutrient uptake parameters in roots and shoots, increased stem circumference, number of xylem vessels, mean xylem diameter, and the hydraulically weighted xylem vessel diameter, compared with plants grown in non-inoculated sediment amended with only RP. Mechanistic analysis of A:Ci response curves (Assimilation versus sub-stomatal CO2 concentration) showed that photosynthetic gas exchange characteristics were significantly enhanced by Op + RP treatment compared with RP alone or control treatment. The maximal Rubisco-catalyzed carboxylation velocity (Vc,max), increased by more than two fold in plants treated with RP + Op, over the control. Application of Op + RP greatly increased the maximal electron transport rate values (Jmax) by 185%, over the control plants. In addition, the triose phosphate utilization rate (VTPU) increased three fold in plants treated with RP + Op over the control. To the best of our knowledge, this is the first published report of Op from mangrove sediments and also the first report of this organism as a phosphate-solubilizing bacterium. These results have clearly shown that Op has the potential to be applied as an effective and economical treatment in association with RP amendment for mangrove reforestation programs in arid environments such as those found in the UAE.

Application of actinomycetes to soil to ameliorate water repellency

Aims: The aim of this study was to develop a novel isolation technique using a mixture of Bacillus and Streptomyces phages to selectively isolate wax‐utilizing non‐streptomycete actinomycetes effective in ameliorating water repellency in a problem soil.