In vivo evidence for the involvement of phospholipase A and protein kinase in the signal transduction pathway for auxin-induced corn coleoptile elongation

Abstract

In vivo evidence for the involvement of phospholipase A and protein kinase in the signal transduction pathway for auxin-induced com coleoptiie elongation Auxin-induced elongation of com coleoptiles is accompanied by cell wall acidification, which depends upon H"^-pump activity. We tested the hypothesis that phospholipase A and a protein kinase are involved in the pathway of auxin signal transduction leading to H^ secretion, and elongation of com coleoptiles. initiaOy. the pH of the bath solution at 50^100 pm from the surface of a coleoptiie segment (pH^,) ranged between 4.8 and 6.6 when measured with an H"-sensiti^'e microelectrode. Twenty or 50 ]iM lysophosphatidylcholine, 50 pM linolenic acid or 50 ]iM arachidonic acid induced a decline in pH^ by 0.3 to 2.1 units. The effect was blocked by 1 mM vanadate, suggesting that ]y.sophosphaEidylcholine or hnolenic acid induced acidification of the apoplast by activating the H^-pump. Lysophosphatidylcholine and linolenic acid also accelerated the elongation rate of the coleoptiles. While hnolenic acid and arachidonic acid, highly unsaturated fatty acids, promoted pH^^ decrease and coleoptiie elongation, linoleic acid, oleic acid, and stearic acid, fatty acids with a lesser extent of unsaturation, had no such effects. The effects of lysophosphatidylcholine, linolenic acid, and arachidonic acid on H* secretion were not additive to that of indoleacetic acid (IAA), suggesting that lysophosphoiipids, fatty acids and auxin use similar pathwaj^s for the activation of the H^-pump. The phospholipase Ai inhibitors, aristolochic acid and manoalide, inhibited the IAA-induced pH,^ decrease and coleoptiie elongation. The general protein kinase inhibitors., H-7 or staurosporine, blocked the IAA-or lysophosphatidylcholine-induced decrease in pH". H-7 also inhibited the eoleoptile elongation induced by LAA or lysophosphatidylcboline. These results support the hypothesis that phospholipase A is activated by auxin, and that the products of the enzvine, lysophosphoiipids and fatty acids, induce acidification of the apoplast by activating the H^pump through a mechanism involving a protein kinase, which in turn promotes com eoleoptile elongation.

Lipid deacylating enzymes in plants: Old activities, new genes

“…In maize (Zea mays), the activation of a PLA in response to indole-3-acetic acid was shown to be indispensable for coleoptile elongation, which occurs through a decrease in pH caused by H þ secretion [122]. Furthermore, the inhibitors aristolochic acid and manoalide inhibited this auxin effect [122].…”

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

“…These lipid products are thought to be responsible for transmitting signals needed for the induction of functional responses. Both fatty acids and lysophospholipids have been shown to produce auxin-like effects in maize coleoptiles (Yi et al, 1996) and to activate the H + -ATPase in vitro (Palmgren et al, 1988). However, only fatty acids accumulated significantly after treatment of cultured plant cells with physiologically relevant concentrations of auxin (2 lM); lysophospholipids did not accumulate at auxin concentrations below 100 lM (Paul et al, 1998).…”

“…Rapid activation of PLA 2 by auxin has been demonstrated in isolated microsomes and cultured cells (Paul et al, 1998;Andre´, 1989, 1993). In addition, inhibitors of animal PLA 2 blocked auxin-induced growth of Zucchini hypocotyls and maize coleoptiles (Scherer and Arnold, 1997;Yi et al, 1996). A PLA 2 enzyme is defined by its ability to catalyse the hydrolysis of the middle (sn-2) ester bond of substrate phospholipids to release lysophospholipids and free fatty acids.…”

CYP94A1, a plant cytochrome P450-catalyzing fatty acid ω-hydroxylase, is selectively induced by chemical stress in Vicia sativa seedlings

“…The rapid onset of transcription in response to these chemicals is in favor of activation mechanisms (Harper et al 1993 ;Van der Hoeven et al 1996;Yi et al 1996) rather than de novo synthesis of elements of the perception, the signal transduction pathways and of the transcription machinery. Actually, even in the presence of cycloheximide, CYP94A1 mRNAs accumulate in response to clofibrate (unpublished data), reinforcing the hypothesis of the involvement of activation reactions for its transcriptional regulation.…”