Plant growth promoting bacteria and humic substances: crop promotion and mechanisms of action

Abstract

High-external input agriculture is one of the most disruptive human activities, which have been justified by the current economic paradigm due to high productivity and the need to feed a growing population. However, we are dangerously close to the edge of the planet resources and both hunger and food insecurity has increased. Limiting the use of non-renewable chemical fertilizers and pesticides, changing water management, enhancing diversity and considering the often-neglected social dimension of agriculture are the bases to other chemical and biological technologies to agriculture. Biological inputs can stimulate the substitution of chemical inputs without questioning the current fundaments or can be adopted as a turning point to intensify the harsh processes of transition to more environmental friendly agriculture. The debate is open and our contribution is to develop the scientific basis for biological inputs that, unlike soluble fertilizers and pesticides, depend on a number of factors for its success in promoting crop yield. In this review, we showed the results obtained with the combined use of diazotrophic endophytic bacteria and humic substances in diverse crops (sugarcane, maize, tomato, common beans and pineapple), presenting the main morphological and physiological changes induced by biological technology. A snapshot of the state of the art of the use of plant growth promoting bacteria together with humic substances was provided, showing their potential especially when plants are subjected to moderate to severe abiotic stress. The number of studies reporting the combined use of plant growth promoting bacteria and humic substances is surprisingly low. There is an open avenue for research and encouraging debate is the goal. To overcome the conventional agriculture, maintaining productivity levels is more than scientific challenge, is a humanitarian duty. The biological inputs can help in this purpose.

Humic Substances and Efficient Microorganisms: Elicitation of Medicinal Plants—A Review

“…Researches have been done about the effect of bioestimulants such as microorganisms, rhizobacterium and humic substances on the production of medicinal plants in vitro (Halpern et al, 2015;Canellas et al, 2015;Olivares, 2017). Therefore, it is necessary to know if the content of secondary metabolites changes when the elicitor agents are applied in vivo and if its application in agroecological systems are economically and socially viable, overcoming the conventional agriculture and keeping the levels of productivity.…”

“…These changes in the physical, chemical and microbiological structure of the soil have the purpose of greater water retention, aeration, directly influencing root growth, nutrient absorption, primary and secondary metabolic pathways Canellas et al, 2015;Olivares et al, 2017;Ozfidan-Konakci et al, 2018), hormone signaling and, consequently, higher yield of the medicinal plants.…”

“…Therefore, the application of humic substances in the in vivo cultivation of medicinal plants opens up the opportunity for the development of organic fertilizers for agroecological systems, aiming at good quality raw material without pesticides with increased concentration of secondary metabolites biologically and pharmacologically interesting (Canellas et al, 2015;Halpern et al, 2015, Olivares et al, 2017.…”

“…Thus, the application of fungi and bacterium as elicitors in the in vivo cultivation of medicinal plants is an important tool in organic and agroecological production, which contributes to the production of raw material with quality and pharmacological interest. They can also improve the performance and optimization of the use of nutrients, opposing the application of chemicals in the production of medicinal plants, conferring quality production with respect to the man and to the environment (Adesemoye et al, 2009;Olivares et al, 2017).…”

Alkamides: a new class of plant growth regulators linked to humic acid bioactivity

“…These compounds participate in cell-to-cell signaling, usually referred to as quorum sensing, which is a fundamental step in endophytic bacteria biofilm formation and host colonization [24]. Considering the effects of biostimulants manufactured using humic acids (HA) and plant growth-promoting bacteria on plant physiology [25][26][27], as well as the wide distribution of these lipids in unbound soil fractions and compost humeomes [28][29][30][31][32], we determined whether (i) this class of compounds was linked to HA bioactivity levels and whether (ii) alkamides are present in the supramolecular structure of HA.…”