Contribution of lipoxygenase metabolism to the brassinosteroid signaling pathway.

Abstract

Brassinosteroids (BSs) are phytohormones, the chemical structure of which is closest to the chemical structure of animal steroid hormones. It was shown by a number of researchers that BSs contributed to plant growth and development [1]. It was also demonstrated that BS caused an increase in the plant resistance to pathogens [2] and adverse climatic factors [3]. Receptors of BS were found to be located in plasmalemma [4]. Phosphorylation of certain proteins was demonstrated to be an early BS-induced response [5]. These findings suggest that BSs are incorporated into the signaling systems of plant cells. However, it is still unclear which types of signaling systems may incorporate BSs. It was also suggested that BSs constituted a previously unknown signaling system [2]. Because it is well known that the lipoxygenase signaling system is involved in cell response to various pathogens, mechanical injuries, elicitors, and some other primary signals [6], the goal of this work was to study the possibility of the brassinosteroid-induced activation of this signaling system. Effects of 24-epibrassinosteroid (EPB), the most active form of BS, on concentrations of oxygenated derivatives of linoleic acid (indicators of lipoxygenase activity) and on the level of phosphorylation of soluble proteins of pea leaves (it is well known that reactions of protein phosphorylation/dephosphorylation are a very important component of signaling systems) were studied. It was suggested that the analysis of lipoxygenase activity and the use of phospholipase A 2 (PLA 2 ) inhibitor provided an opportunity to elucidate the contribution of the lipoxygenase metabolism to the initial phase of the EPB-induced response of the cells. Bromphenylacyl bromide (BPB) was used as a phospholipase A 2 inhibitor decreasing the rate of release of unsaturated fatty acids (lipoxygenase substrate). The lipoxygenase activity level was assessed using the methodological approach based on the appearance of two conjugated bonds and absorption maximum at 232 nm during the reaction of formation of oxygenated derivatives of linoleic acid.