Utilization of plant growth-promoting bacteria to ameliorate iron nutrition in legumes

Abstract

Iron (Fe) deficiency is an important micronutrient disorder that leads to severe yield losses and low nutritional crop value, particularly in calcareous soils. There is an urgent need to find sustainable and greener agricultural practices to achieve higher crop yields with higher nutritional value. Biofortification allows the increase of micronutrient concentrations in edible crop tissues and contributes to achieving such demands. The utilization of bioinoculants (BIs) with plant growth-promoting bacteria (PGPB) has been suggested as a promising approach for biofortification and prevention of Fe deficiency. So far, little work has been done on the role of PGPB in soybean (Glycine max) grown under alkaline conditions. The main purpose of this study was to test the potential of PGPB on plant growth and Fe uptake, unveiling mechanisms underlying Fe uptake and accumulation. Soybean was selected as a model species since it is severely affected by Fe deficiency and several underlying factors related to Fe homeostasis are identified. Firstly, an in-depth and critical literature review was conducted concerning the global importance of Fe and the Fe deficiency, the importance of soybean in the fulfillment of global policies, and the potential of PGPB as a sustainable approach to improve Fe nutrition and cope with Fe deficiency. Concerning the experimental work, the first study of this thesis aimed to evaluate the ability of 24 PGPB strains from a CBQF collection to enhance Fe uptake-related processes in soybean grown for 21 days in calcareous soil. Sphingobium fuliginis ZR 1-6 and Pseudomonas jessenii ZR 3-8 were selected based on their in vitro ability to produce indole-3-acetic acid (IAA), 1-aminocyclopropane-1- carboxylic acid (ACC) deaminase, siderophores, and organic acids, to tolerate high pH, and to reduce Fe3+. Bacterial isolates were inoculated singly and as a mixture, and a series of morphological, physiological, and molecular parameters were evaluated. S. fuliginis improved ferric chelate reductase (FC-R) activity (111 %), FRO2 expression (646 %), and root Fe (62 %); combined inoculation fostered Fe accumulation in trifoliates (144 %) and increased IRT1 (239 %) and FER4 expression (5036 %). Overall, S. fuliginis alone or in combination with P. jessenni were the best treatments. In a second study, PGPB were isolated from root tissues and rhizosphere of soybean grown in a Portuguese soil; 76 bacterial strains were isolated from roots (53 %), rhizosphere (29 %), and shoots (18 %), and 29 genera were identified. Two bacterial strains – B. licheniformis P2.3 and B. aerius S2.14 – were selected for in vivo experiments, and inoculated plants were grown to maturity. Photosynthetic parameters, chlorophyll content, total fresh weight, and Fe concentrations were not significantly affected by inoculation. Nevertheless, inoculation with B. licheniformis increased pod number (33 %), decreased FC-R activity (45 %), and increased expression of Fe-related genes; inoculation with B. aerius decreased root length (20 %), FC-R activity (55 %), and FRO2 expression, and increased expression of the remaining genes. Furthermore, inoculation with bacterial isolates improved the accumulation of Mn, Zn, and Ca in soybean tissues. In this study, B. licheniformis showed potential to be incorporated in formulations for improving soybean grown in calcareous soil. The formulation of BIs contemplate a series of requirements and their effective implementation is still challenging. However, they are a promising trend to the accomplishment of future global politics and present a series of advantages to greener agriculture practices that are critically reviewed in the last part of this thesis. In general, the results presented in this thesis contribute to better understand the mechanisms by which PGPB improve Fe uptake and plant growth, under alkaline conditions, and their potential as bioinoculants in a sustainable perspective.

Date of Award	6 Jul 2021
Original language	English
Awarding Institution	Universidade Católica Portuguesa
Supervisor	Marta Vasconcelos (Supervisor), Paula M. L. Castro (Supervisor) & Susana M. P. Carvalho (Supervisor)