Fatty acids and lysophospholipids as potential second messengers in auxin action. Rapid activation of phospholipase A2 activity by auxin in suspension‐cultured parsley and soybean cells

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Summary: Activation of cytosolic phospholipase A2 is a typical signal transduction reaction in animal cells and occurs in plants in response to auxin, elicitors and wounding. Exogenously added fluorescent bis‐BODIPY‐phosphatidylcholine was taken up and hydrolysed by a cellular phospholipase A2. Rapid activation of a phospholipase A2 by auxin in suspension‐cultured parsley ( Petrosilenum crispum L.) and soybean ( Glycine max L.) cells was shown by detection and quantification of fluorescent reaction products of phospholipase A2. Hormone‐triggered fluorescent fatty acid accumulation could be detected as early as 5 min. Auxins at 2 μM or higher concentrations activated phospholipase A2 and fluorescent fatty acids accumulated 1.1‐ to threefold after 90–120 min, depending on the auxin concentration. Fluorescent lysolipid did not accumulate up to 150 μM auxin. Known inhibitors of phospholipase A2 inhibited hormone‐dependent fluorescent fatty acid accumulation in cell cultures and, previously, elongation growth in etiolated zucchini hypocotyl segments ( Scherer & Arnold (1997 ) Planta 202, 462–469). When lipids were labeled by [14C]‐choline and [14C]‐ethanolamine the corresponding lysophospholipids could be quantified in cell extracts. Radioactive lysophospholipids accumulated as rapidly as 1–2 min after auxin treatment but only at concentrations well above 100 μM auxin. We hypothesize that phospholipase A2 activation is an early intermediate step between receptor and downstream responses. We hypothesize that fatty acid(s) could be second messengers in several auxin functions, especially in cell elongation. Lysophospholipids seem to be indicators or second messengers for stress caused by high auxin concentrations or may have different auxin‐linked functions and are also known to accumulate during elicitor action.

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