Stomatal Responses to Humic Substances and Auxin are Sensitive to Inhibitors of Phospholipase A2

Abstract

Two fractions of earthworm humic substances, differing in nominal molecular weight, containing a very low amount of free IAA, and exhibiting auxin-like properties, were prepared and characterised by infrared and 1 H-NMR spectroscopy. In this study we investigate their effects on stomatal opening, as influenced by phospholipase A 2 , in leaf of the Argenteum mutant of pea (Pisum sativum L.) Both of the humic fractions caused stomatal opening in the epidermal peels. The response showed a broad biphasic dose dependence and the effective concentrations were similar for the two fractions. The maximal stomatal apertures in response to both humic substances were similar to that caused by IAA and somewhat less than the response to white light or fusicoccin. Two inhibitors of phospholipase A 2 selectively blocked the response of stomata to both IAA and humic substances, without affecting the response to light or fusicoccin. We conclude that stomatal opening in response to auxin and humic substances involves activation of a phospholipase A 2 that is not involved in signalling the response to light or fusicoccin.

Biological Activities of Humic Substances

“…In a recent work, Russell et al (2006) confi rmed previous fi ndings, assaying two fractions of earthworm humic substances, differing in nominal molecular weight, on epidermal peels of the Argenteum mutant of pea ( Pisum sativum L.), where IAA receptors were present. Both of the humic fractions caused stomatal opening in epidermal peels.…”

Humic substances stimulate maize nitrogen assimilation and amino acid metabolism at physiological and molecular level

“…The presence of low amount of IAA in different soil humic substances has been reported [33,44]. Moreover, these substances seem to have a physiological effect on both a maize isoform of H + ATPase Mha2 [34,37,38], which is a specific auxin target, and a pea phospholipase A 2 [45], which is a component of an auxin-dependent signalling.…”

Mixed rhizobia and Herbaspirillum seropedicae inoculations with humic acid-like substances improve water-stress recovery in common beans

“…Stomatal closure and transpiration reduction, as responses to water deficiency, have been long recognized because the opening and closing of stomata results from turgor differences between guard cells and the surrounding subsidiary or epidermal cells [20]. Like auxins, HA-like substances can also induce stomatal movement [21]. Another important factor involved in turgor maintenance is the cell's concentration of soluble compounds.…”