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⁺-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⁺-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.

The effects of humic acids (HA) isolated from seven different Oxisols from Brazilian tropical and subtropical regions on root growth of maize seedlings were investigated. The HA were characterized by elemental composition, ¹³C solid-state nuclear magnetic resonance (NMR) (¹³C cross-polarization magic angle spinning-NMR), diffusion-ordered NMR spectroscopy, and high-performance size exclusion chromatography. The HA differed in hydrophobicity and molecular dimension of their alkyl, carbohydrate-like, and aromatic C components. Changes in plasma membrane H⁺-ATPase activity and expression in root cells were considered physiological markers of HA bioactivity. All HA promoted root growth and proton pump activation in maize vesicles. Maize plants treated for 7 days with 20 mg C HA L⁻¹ increased adenosine triphosphate hydrolysis from 72% to 392%, and H⁺-ATPase expression from 111% to 495%. Within a general stimulation of root areas over control, some HA preferentially increased root length, whereas others enhanced root density. Root growth enhancement was significantly correlated to HA hydrophobicity. Although acting synergistically with hydrophobicity, the relationship to bioactivity of HA apparent molecular size and carbon content was of a lesser significance. The conformational dynamics of humic hydrophobic associations in the rhizosphere may release auxin-like plant growth promoters and actively enhance plants' biochemical activities.

A bioatividade de ácidos húmicos (AH) isolados de lodo da estação de tratamento de esgoto (AHL) e de vermicomposto (AHV) foi avaliada pela ação dessas substâncias sobre o transporte de prótons através da membrana plasmática de células de raízes de café e milho e sua relação com o desenvolvimento dessas espécies. Houve estímulo da área superficial radicular em ambas as espécies cultivadas com ambos AH, mostrando uma concentração ótima em torno de 40 mg L-1. Nessa condição, os tratamentos com AHL e AHV estimularam a H+-ATPase de membrana plasmática em plântulas de café e milho. Os AHL foram mais efetivos na promoção desses efeitos do que os AHV. A modificação do perfil cromatográfico dos AH em solução antes e após o cultivo das plântulas revelou que a interação planta-AH promoveu uma redistribuição das massas moleculares dessas substâncias, sugerindo uma dinâmica de mobilização de subunidades funcionais dos AH por exsudatos das raízes. A análise estrutural dos AH detectou a presença de grupamentos de auxina. A análise comparativa da ação desses dois AH sobre as espécies representantes de plantas monocotiledôneas (milho) e dicotiledôneas (café) apontam para a ativação da H+-ATPase de plasmalema como possível marcador metabólico de bioatividade dos ácidos húmicos.

Many cellular components are transported using a combination of the actin- and microtubule-based transport systems. However, how these two systems work together to allow well-regulated transport is not clearly understood. We investigate this question in the Xenopus melanophore model system, where three motors, kinesin II, cytoplasmic dynein, and myosin V, drive aggregation or dispersion of pigment organelles called melanosomes. During dispersion, myosin V functions as a “molecular ratchet” to increase outward transport by selectively terminating dynein-driven minus end runs. We show that there is a continual tug-of-war between the actin and microtubule transport systems, but the microtubule motors kinesin II and dynein are likely coordinated. Finally, we find that the transition from dispersion to aggregation increases dynein-mediated motion, decreases myosin V–mediated motion, and does not change kinesin II–dependent motion. Down-regulation of myosin V contributes to aggregation by impairing its ability to effectively compete with movement along microtubules.