Earthworms (Eisenia foetida) produce humic substances that can influence plant growth by mechanisms that are not yet clear. In this work, we investigated the effects of humic acids (HAs) isolated from cattle manure earthworm compost on the earliest stages of lateral root development and on the plasma membrane H ϩ -ATPase activity. These HAs enhance the root growth of maize (Zea mays) seedlings in conjunction with a marked proliferation of sites of lateral root emergence. They also stimulate the plasma membrane H ϩ -ATPase activity, apparently associated with an ability to promote expression of this enzyme. In addition, structural analysis reveals the presence of exchangeable auxin groups in the macrostructure of the earthworm compost HA. These results may shed light on the hormonal activity that has been postulated for these humic substances.

Increasing evidences have indicated that humic substances can induce plant growth and productivity by functioning as an environmental source of auxinic activity. Here we comparatively evaluate the effects of indole-3-acetic acid (IAA) and humic acids (HA) isolated from two different soils (Inseptsol and Ultisol) and two different organic residues (vermicompost and sewage sludge) on root development and on activities of plasmalemma and tonoplast H(+ )pumps from maize roots. The data show that HA isolated from these different sources as well as low IAA concentrations (10(-10) and 10(-15) M) improve root growth through a markedly proliferation of lateral roots along with a differential activation not only of the plasmalemma but also of vacuolar H(+)-ATPases and H(+)-pyrophosphatase. Further, the vacuolar H(+)-ATPase had a peak of stimulation in a range from 10(-8) to 10(-10) M IAA, whereas the H(+)-pyrophosphatase was sensitive to a much broader range of IAA concentrations from 10(-3) to 10(-15) M. It is proposed a complementary view of the acid growth mechanism in which a concerted activation of the plasmalemma and tonoplast H(+ )pumps plays a key role in the root cell expansion process driven by environment-derived molecules endowed with auxinic activity, such as that of humic substances.

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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.