Modulation of tomato root architecture and root hair traits by Pseudomonas brassicacearum and Variovorax paradoxus containing 1-aminocyclopropane-1-carboxylate deaminase

By decreasing root 1-aminocyclopropane-1-carboxylate (ACC) content and plant ethylene production, the microbial enzyme ACC deaminase is a widespread beneficial trait of plant growth-promoting rhizobacteria (PGPR), ameliorating ethylene-mediated root growth inhibition. However, relatively little is known about whether bacterial ACC deaminase modulates root architecture and root hair traits. Thus the dwarf tomato (Solanum lycopersicum) cultivar Micro-Tom was inoculated in vitro with Pseudomonas brassicacearum Am3, its ACC deaminase deficient mutant T8-1, a known PGPR strain Variovorax paradoxus 5C-2 or chemically treated with agents that promoted or inhibited ethylene production or sensitivity (Ag+, Co2+, and ACC). ACC treatment reduced both root elongation and the number of lateral roots, while ethylene inhibitors (Ag+, Co2+) and V. paradoxus 5C-2 promoted primary root elongation, but differentially affected lateral root length and number. Ag+ stimulated lateral root development, while Co2+ and V. paradoxus 5C-2 did not. Inoculation with P. brassicacearum Am3 and T8-1 inhibited elongation of the primary and lateral roots at a high inoculum concentration (106 cells cm3). All bacterial strains significantly increased the length and number of root hairs, with these effects more pronounced in P. brassicacearum Am3 than in the mutant T8-1. Treatment with Ag+ inhibited root hair formation and elongation, while Co2+ had the opposite effects. ACC treatment had no effect on root hair elongation but increased root hair density. While root growth inhibition caused by P. brassicacearum Am3 was independent of ACC deaminase, the promotion of root hair elongation and density by this strain was augmented by ACC deaminase activity. Thus ACC deaminase can modulate the morphological impacts of bacteria on root hair response by affecting plant ethylene content. © 2022, Institute of Experimental Botany, ASCR. All rights reserved

Metabolism by Rhizobacteria of Abscisic Acid Deuterated in the Cyclohexene Part

The required amount of abscisic acid containing 4 – 5 D atoms ([2H]ABA) in the cyclohexene fragment of the molecule was produced via isotope exchange in 100% deuterated D2O at 220°C in the presence of diisopropyl ethylamine. The yield of labeled compound was 45 – 80%. A complex of biological studies was carried out using the D-labeled preparation. It was established that [2H]ABAserved as a growth substrate for soil bacteria, which included the isotopic label in the composition of cell metabolites. The characteristics of D-labeled metabolites allowed their reliable identification in mass spectra of the total bacterial metabolome. Three intermediates of a new metabolic pathway of microbial degradation of this phytohormone were identified using the developed technique

Rhizosphere bacterium rhodococcus sp. P1y metabolizes ab-scisic acid to form dehydrovomifoliol

The phytohormone abscisic acid (ABA) plays an important role in plant growth and in response to abiotic stress factors. At the same time, its accumulation in soil can negatively affect seed germination, inhibit root growth and increase plant sensitivity to pathogens. ABA is an inert compound resistant to spontaneous hydrolysis and its biological transformation is scarcely under-stood. Recently, the strain Rhodococcus sp. P1Y was described as a rhizosphere bacterium assimilat-ing ABA as a sole carbon source in batch culture and affecting ABA concentrations in plant roots. In this work, the intermediate product of ABA decomposition by this bacterium was isolated and purified by preparative HPLC techniques. Proof that this compound belongs to ABA derivatives was carried out by measuring the molar radioactivity of the conversion products of this phytohor-mone labeled with tritium. The chemical structure of this compound was determined by instrumen-tal techniques including high-resolution mass spectrometry, NMR spectrometry, FTIR and UV spec-troscopies. As a result, the metabolite was identified as (4RS)-4-hydroxy-3,5,5-trimethyl-4-[(E)-3-oxobut-1-enyl]cyclohex-2-en-1-one (dehydrovomifoliol). Based on the data obtained, it was con-cluded that the pathway of bacterial degradation and assimilation of ABA begins with a gradual shortening of the acyl part of the molecule