Coinoculation of soybean plants with Bradyrhizobium japonicum and Trichoderma harzianum: Coexistence of both microbes and relief of nitrate inhibition of nodulation.

Coinoculation of plants with mixtures of beneficial microbes sometimes produces synergistic effects. In this study, the effect of soybean coinoculation with the N2-fixing Bradyrhizobium japonicum E109 and the biocontrol fungus Trichoderma harzianum Th5cc was analyzed. Nodulation by E109 was not hampered by Th5cc, which antagonized five out of seven soybean pathogens tested. Furthermore, Th5cc relieved nitrate-inhibition of nodulation, enabling the formation of nodules containing infected cells with bacteroids in the presence of the otherwise inhibitory 10 mM KNO3. Th5cc released micromolar amounts of auxin, and addition of 11 µM indoleacetic acid to soybean plants inoculated with E109 in the absence of Th5cc also induced nodulation in the presence of 10 mM KNO3. Thus, Th5cc may release auxins into the soybean rhizosphere, which hormones might participate in overcoming the nitrate-inhibition of nodulation. Our results suggest that soybean plants coinoculated with these microorganisms might benefit from biocontrol while contributing to soil-nitrogen preservation.

Characterization of novel Trichoderma spp. isolates as a search for effective biocontrollers of fungal diseases of economically important crops in Argentina.

Monoconidial cultures of 33 isolates of Trichoderma from Buenos Aires Province, Argentina were characterized on the basis of twenty eight morphological, physiological and biochemical features. All of them were screened for proteinase, endochitinase and ß-1,3 glucanase activity. Universally primed PCR (UP-PCR) and inter-simple sequence repeat (ISSR) techniques were used to examine the genetic variability among isolates, which resulted in 127 bands for the total number of isolates. These results were subjected to numerical analysis revealing 20 haplotypes grouped in five clusters. The ability of Trichoderma isolates to antogonize soil-borne fungal plant pathogens using a dual culture assay was done against five fungal species: Alternaria sp., Bipolaris sorokiniana, Fusarium graminearum, F. solani, and Pyricularia oryzae. The highest inhibition values (85% RI) were obtained against B. sorokiniana and P. oryzae. Three isolates of T. harzianum named as FCCT2, FCCT3 and FCCT9 were capable of causing a high growth inhibition on four of the fungal species assayed, which was in agreement with their higher extracellular hydrolytic activity. Our results suggest that these isolates have the potential to be effective agents for biocontrol of cereal and tomato fungal pathogens.