Re-start of olive waste vermicompost through addition of tryptophan and its effects on indole-3-acetic acid in pepper rhizosphere when used as soil amendment

- A new set of primers targeting bacterial ALDH-encoding genes was designed.
- ALDH-encoding genes are present in most of vermicompost-borne bacteria.
- ALDH-encoding genes are not specific for bacterial indole-3-acetic acid synthesis.

In this work, we aimed to test the capacity of an exogenous source of tryptophan to stimulate the bacterial biosynthesis of the phytohormone indole-3-acetic acid (IAA) in agro-industrial by-products, trying to gain insight into the role of key genes involved in the indole-3-pyruvic acid pathway (IPA) for bacterial IAA synthesis. Sunflower-oil cake was mixed with a 9-months-old vermicompost olive wastes and then vermicomposted for 60 days using earthworms Eisenia foetida. Synthetic tryptophan and no tryptophan source were used as controls. For this, a novel set of primers targeting bacterial aldehyde-dehydrogenase (ALDH)-encoding genes involved in the last step of the IPA route was designed and tested, and a method involving ultrasound-assisted extraction of IAA and subsequent analysis by liquid chromatography-tandem mass spectrometry was optimized. Re-starting the vermicomposting process increased bacterial abundance and the number and the expression of ALDH genes but not the IAA content in the final vermicomposts, meaning that the transcription of the ALDH genes could be not quite high enough to change the overall level of the existent IAA. After that, the effects of the resulting vermicompost were evaluated on the bacterial microbiome of the pepper (Capsicum annuum L.) rhizosphere. When used as amendments, ALDH gene expression was detected in the pepper's rhizosphere after 3 months of growth only when vermicompost incorporating sunflower cake was used, where the highest levels of IAA were also detected. The results indicate the presence of the ALDH genes in the chromosome of most of vermicompost-borne bacteria, probably not specific for IAA synthesis, and that the IPA pathway may not be the main biosynthesis pathway for IAA in vermicompost-borne bacteria.