Toxin-producing endosymbionts shield pathogenic fungus against micropredators

The phytopathogenic fungus Rhizopus microsporus harbours a bacterial endosymbiont (Mycetohabitans rhizoxinica) for the production of the toxin rhizoxin, the causative agent of rice seedling blight. This toxinogenic bacterial-fungal alliance is, however, not restricted to the plant disease, but has been detected in numerous environmental isolates from geographically distinct sites covering all five continents. Yet, the ecological role of rhizoxin beyond rice seedling blight has been unknown. Here we show that rhizoxin serves the fungal host in fending off protozoan and metazoan predators. Fluorescence microscopy and co-culture experiments with the fungivorous amoeba Protostelium aurantium revealed that ingestion of R. microsporus spores is toxic to P. aurantium. This amoebicidal effect is caused by the bacterial rhizoxin congener rhizoxin S2, which is also lethal towards the model nematode Caenorhabditis elegans. By combining stereomicroscopy, automated image analyses, and quantification of nematode movement we show that the fungivorous nematode Aphelenchus avenae actively feeds on R. microsporus that is lacking endosymbionts, while worms co-incubated with symbiotic R. microsporus are significantly less lively. This work uncovers an unexpected ecological role of rhizoxin as shield against micropredators. This finding suggests that predators may function an evolutionary driving force to maintain toxin-producing endosymbionts in non-pathogenic fungi.

Toward understanding the impact of nuisance algae bloom on the reduction of rice seedling emergence and establishment

California rice (Oryza sativa L.) production has been recently challenged by the early season bloom of nuisance algae. The algal community in rice is a complex of green algae (Nostoc spongiforme Agardh ex Bornet) and cyanobacteria species that could develop a thick algal mat on the surface of the water and interfere with the emergence and establishment of rice seedlings. The objective of this research was to determine the impact of algae infestation level on rice seedling emergence. A mesocosm study was conducted in 57 L tubs. Three levels of algae infestation (low, medium, and high) were produced by adding fertilizer N:P amount into the tubs including 0:0, 75:35, and 150:70 kg-1ha. Sixty rice seeds (M-206) were soaked for 24 hours and spread into tubs filled with water. Photosynthetic Active Radiation (PAR), Chlorophyll a concentration as the quantitative measure of algae, number of emerged rice seedlings, and their dry biomass were studied during the experiment. Results showed that algae infestation can directly change the amount of light received into the water. Minimum, maximum and mean percentage of PAR inside the water declined by the increase of algae infestation level. As a consequence, rice seedling emergence dropped under the high algae pressure. At very high algae infestation (i.e. chlorophyll a concentration of above 500 µg ml-1), rice seedling emergence reduced up to 90%. Furthermore, rice seedling emergence was delayed under algae infestation. When algae infestation was low, time to 50% of rice seedling emergence (t50) ranged between five and ten days, while at high algae infestation t50 ranged between twelve and twenty days. Moreover, individual rice seedling biomass reduced from one gram to 0.01 gram by the increase of algae infestation. The results from this study indicate that uncontrolled algae at the beginning of the rice-growing season could reduce rice seedling emergence, establishment, and rice stand. Given that algae infestation in field has a patchy pattern, loss of rice stand in these patches could provide empty niches for other weeds to grow.

Rhizo-inoculation of phosphate solubilizing bacteria strains to improve rice (Oryza sativa L. var. FARO 44) growth under ferruginous ultisol conditions.

Background The research investigated the possibility of phosphate solubilizing bacteria (PSB) with plant growth-promoting (PGP) capabilities to improve growth properties of rice plant under ferruginous ultisol (FU) condition through rhizo-inoculation strategy. The PSB with PGP properties used in this research were Bacillus cereus strain GGBSU-1, Proteus mirabilis strain TL14-1 and Klebsiella variicola strain AUH-KAM-9 that were previously isolated and characterized following the 16S rRNA gene sequencing. The rice seeds were sown in a composite FU soil sample and a humus soil (control) and then rhizo-inoculated along the root region of the growing rice seedling at 16 days after sowing. The rice plant was studied for differences in morphological, physiological and biomass parameters for 16 weeks after rhizo-inoculation. Results The composite FU soil used in the study had high pH, low bioavailable phosphorus, low water holding capacity and high iron levels which has led to a low growth properties of rice seeds sown in FU soil without rhizo-inoculation. After rhizo-inoculation, a significant increase in morphological, biomass and physiological parameters were observed in the rice plant grown in the FU soil as against the control and the rice plant in FU soil without inoculation except for terpenoid which is usually known to signify biotic stress and as part of plant defense mechanism. Generally, Bacillus cereus strain GGBSU-1 showed higher improvement compared to other PSBs used in the present study. This is due to a more improved growth properties observed. Conclusion Rhizo-inoculation of rice seedling with Bacillus cereus strain GGBSU-1, Proteus mirabilis strain TL14-1 and Klebsiella variicola strain AUH-KAM-9 under FU soil condition significantly improved growth properties of the rice plant. This suggest the ability of PSB to solubilize and mineralize soil phosphate and improve its availability for plant use in phosphate stressed soil, thereby improving plant growth properties.

Screening of plant growth-promoting bacterial endophytes and rhizobacteria isolated from Curcuma xanthorrhiza

Bacterial endophyte and rhizobacteria were reported to associate with medicinal plants including Zingiberaceae plants and involved in growth promotion. These beneficial bacteria are promising candidates as biostimulants because of their ability in promoting plant growth. This study aims to evaluate the activity of endophytic and rhizosphere bacteria isolated from Curcuma xanthorrhiza (Javanese turmeric) in promoting rice seedling and Javanese turmeric growth. Fifty-seven of 150 total bacterial isolates with negative hemolysis and hypersensitivity reactions were characterized to investigate their plant growth-promoting (PGP) traits. Ten selected bacteria (two bacterial endophytes and eight rhizobacteria) with multiple PGP traits were inoculated to rice seed with seed treatment and inoculated to Javanese turmeric rhizome with seed treatment and seed treatment+soil soil drenching. Our results showed that bacterial isolates tested on rice seed promoted rice seedling growth significantly. A total of five, three, six, and three bacterial isolates could increase leaf number, root length, fresh shoot weight, and fresh root weight of rice seedling (p<0.05), respectively. In contrast, all of the bacterial isolates tested on Javanese turmeric rhizomes showed a non-significant effect on the plant growth. Further studies should be considered to investigate the effect of formulated potential bacterial isolates with different application frequencies and environmental conditions on the harvest yield of rice and Javanese turmeric as well as active compounds of Javanese turmeric.