The effect of different plant growth-promoting bacteria on the root system of Arabidopsis thaliana WT and PP2A signaling mutants.

Abstract

The development of new and better methods in agriculture, to increase crop yield, has become more and more important in the last years, as the world’s population is growing, and the demand for more food production increases. The traditional use of chemicals is damaging to the environment, and the focus on more eco-friendly methods has escalated in the last decades. One of these methods is the use of plant growth-promoting bacteria (PGPB). By inoculation of these bacteria to the soil, or other growth substrates, they may have a positive effect on the growth of the plants. In this thesis, seven bacteria, isolated by another research group, from roots of tomatoes grown in Italy; a well-known PGPB, Pseudomonas simiae WCS417; and a Paenibacillus sp. isolated from Solanum pennellii here at the University of Stavanger (UiS), were all used in different root growth assays with Arabidopsis thaliana. Mutated A. thaliana was also used to investigate whether some genes, which are important regulators of protein phosphatase 2A (PP2A), are involved in the interaction between the plant and the PGPB. This was done by looking at the effect on the root system, i.e. the effect on the primary root, lateral roots, and root hairs. The genes investigated were Phosphotyrosyl Phosphatase Activator (PTPA), Leucine Carboxyl Methyl Transferase (LCMT1), and Protein phosphatase 2A Methylesterase 1 (PME-1). Four different A. thaliana mutants were used; a PTPA over-expressor (ptpaox), a PTPA knock-down (ptpakd), a LCMT1 knockout (lcmt1), and a PME-1 knockout (pme1), in addition to wild type (WT). The Paenibacillus sp. did not appear to have a positive effect on the root system of A. thaliana plants, and the effects of the seven bacteria isolated by another research group were variable. Experiments with the known PGPB P. simiae WCS417 gave a similar effect on the root system of A. thaliana WT plants, previously described by others, with inhibition of primary root, increase of the numbers of lateral roots, and increase of root hair formation, compared to the control. This was also observed for ptpaox, and ptpakd, but ptpakd appeared to have a lower percent increase of lateral roots, compared to WT and ptpaox. However, this experiment was not repeated, and a definite conclusion of PTPA involvement in the interaction between A. thaliana and PGPB cannot be made. In addition, P. simiae WCS417 and the isolated Paenibacillus sp., were used for a “real-life” experiment. Solanum lycopersicum cv. Heinz and Moneymaker, grown in Vermiculite, were inoculated with the bacteria. This experiment did not show any effect of either bacteria. These experiments show that results obtained by others may be difficult to reproduce, and even though some bacteria show plant growth-promoting traits in vitro, many factors, e.g. competing microorganisms and conditions of the growth substrate, will influence their ability to implement these traits on the plants in the field.