Earthworm humic matter produces auxin-like effects on Daucus carota cell growth and nitrate metabolism

While earthworms are known to improve plant growth by improving the structure of the soil, recent work has suggested that earthworms also produced humic substances endowed with hormone-like activity. Suspensions of Daucus carota (carrot) cells were treated with auxin derivatives (2,4-D=2,4-dichlorophenoxyacetic acid, IAA=indole-3-acetic acid and NAA=1-naphthylacetic acid) and a humic substance of low molecular weight (HEf), obtained from the faeces of the earthworms Nicodrilus caliginosus and Allolobophora rosea, which have auxin-like activity. HEf, at a concentration of 200 μg C l−l, caused an increase in carrot cell growth, determined by packed cell volumes, similar to that of 2,4-D, and morphological changes induced by HEf were similar to those induced by IAA. Humic matter and the tested auxin derivatives stimulated carrot cell glutamate dehydrogenase (GDH), glutamine synthetase (GS) and malic dehydrogenase (MDH) to various extents. When carrot cells were treated with HEf or IAA, GDH and MDH isoenzymes separated by isoelectrofocusing showed similar polymorphism. The addition of HEf or auxin derivatives to carrot cells caused an increase in free amino acids belonging to the oxaloacetate and α-ketoglutarate pathways. Our results showed that HEf affects nitrate metabolism, has auxin-like activity, was strongly acidic and had elevated peptide, carbohydrate and aliphatic C contents. Our work has confirmed that the effective materials in the HEf were highly acidic and of low molecular weight. We conclude that, in addition to the already well established modes of action, earthworms are important in humogenesis because their faeces contained humic substances that can influence plant growth via physiological effects.