Nuevas perspectivas para la horticultura urbanaevaluación de un biofilm bacteriano como promotor del crecimiento vegetal

Abstract

The application of plant growth promoter bacteria has significant benefits such as the improvement of the physicochemical properties and the health of the soil, as well as the growth and productivity of crops. This practice has become extremely relevant as it is much more environmentally friendly and an invaluable alternative to the use of pesticides and fertilizers of chemical origin known for their negative impact on the environment. The aim of this doctoral thesis was to produce and analyze the efficacy of a biofilm of bacterial origin as a promoter of plant growth in Lactuca sativa and Solanum lycopersicum grown on substrates that resemble soils of urban gardens and on substrates contaminated with metal. Bacterial strains of the Bacillus genus were evaluated: B. amylolyticus, B. subtilis subsp. spizizeni, Bacillus subtilis var. natto, B. liquefaciens 235 and B. liquefaciens 157 as “biological control agents” against phytopathogenic fungi: Fusarium solani, Fusarium graminearum, Fusarium oxysporum, Pythium ultimum and Rhizoctonia solani. The selected bacteria was Bacillus subtilis subsp. spizizenii, which under laboratory conditions released protein metabolites and other metabolites with biosurfactant and emulsifying properties capable of desestabilizing the membranes of pathogenic microorganisms. This strain acted as a “biofertilizer”, due to its ability to solubilize and mineralize phosphorus, but it did not show nitrogen-fixing activity. Likewise, it turned out to be a “phytostimulant”, as it produced hormones such as indole acetic acid, abscisic acid and zeatin riboside. Therefore, B. subtilis subsp. spizizenii was positioned as a promising alternative to be used as a seeds inoculant for the promotion of plant growth. Subsequently, the culture conditions were optimized in the laboratory for the bacteria to develop a robust biofilm, to be used as a novel seed inoculation method. For this purpose, complex nitrogenous sources based on peptone and yeast extract and simple nitrogen sources with different amino acids were evaluated, the most suitable being Lglutamic acid. Different carbonate sources of five and six carbons were tested, the most suitable being glycerol. Other conditions that were appropriate for the development of the biofilm were: temperature of 30 ° C and cultivation under static conditions for 96 hours. Under these conditions, spores also developed in the biofilm. This was of utmost importance because in this way the viability of the bacteria is not only ensured under the structure of the biofilm, which constitutes a protected environment, but also with the presence of spores, which are resistance structures. The growth promoting effect of B. subtilis subsp. spizizenii was confirmed on three varieties of Lactuca sativa: L. sativa var Crimor, L. sativa var. Waldman Green and L. sativa var. Grand Rapid and on two varieties of Solanum lycopersicum: S. lycopersicum var. Platense and S. lycopersicum var. Río Grande. Two methods of inoculation of the bacteria were compared in L. sativa var. Gran Rapid and S. lycopersicum var. Río Grande: on the one hand, the conventional method with a liquid inoculum (planktonic form) and on the other, the biofilm. Growth parameters (aerial and root biomass) were evaluated as well as yield and fruit quality parameters in the case of S. lycopersicum. For both plant species the application of the biofilm turned out to be the most effective. In substrates contaminated with copper and zinc, the application of the biofilm also showed a beneficial effect in these horticultural species both in the growth parameters and in some parameters of yield and fruit quality. The present work highlights the importance of the incorporation of biofilms as an innovate method for the inoculation of bacteria with growth-promoting activity in plant species.