AcKNowlEdGEMENTSM. Schulz thanks the University of Bonn and the Faculty of Agronomy for financial support (PRIOR, project A1). We thank Prof. Schellander and his coworkers for support with the ABI Prism 7000 SDS instrument equipment. ABSTrAcTExposure to the allelopathic monoterpenes camphor (100 mg/10 L) and menthol (50 mg/10 L) for 24 h enhanced transpiration of Arabidopsis thaliana fully developed rosette leaves similar to de-waxing. As ascertained by ESEM analyses the leaf surfaces were spotted with platelet like structures which seem to be partly mixed with the lipophilic epicuticular layers. The structures are supposed to contain the condensed monoterpenes, which could be identified by GC. Long term exposure (more than 48 h) to 100 mg/ 50 mg killed the plants by desiccation, a 24 h exposure caused necrotic spots that became visible one to two days after the treatment. Examinations of the stomatal apertures indicated that monoterpenes induced stomatal opening followed by extreme swelling and a final break down of the protoplasts. Exposure of Arabidopsis thaliana to volatiles of Mentha piperita, Lavandula latifolia and Artemisia camphorata resulted in a dramatic increase of the stomata aperture but swelling of the protoplasts was less exhibited.In contrast to de-waxing, expression of the fatty acid condensing enzyme encoding CER6 gene and de novo synthesis of CER6 protein was not induced after 24 h of exposure to the monoterpenes.The aim of the study was to demonstrate that the lipophilic layers of the leaf surface and the stomata are primary targets of monoterpene allelopathic attack. Enhanced transpiration results from a combination of affected lipophilic wax layers and a disturbed stomata function.
Monoterpenes at high atmospheric concentrations are strong growth inhibitors in allelopathic interactions. Effects depend on dose, molecular structure of the monoterpene and on the species of the receiver plant. Stomata are among the first targets affected by camphor and menthol. Previously, it could be demonstrated that the compounds induce swelling of the protoplasts, prevent stomatal closure and enhance transpiration. In this study, we show that the block of stomatal closure is accompanied by changes to the cytoskeleton, which has a direct role in stomatal movements. Although MPK3 (MAP3 kinase) and ABF4 gene expressions are induced within six hours, stomatal closure is prevented. In contrast to ABF4, ABF2 (both transcription factors) is not induced. MPK3 and ABF4 both encode for proteins involved in the process of stomatal closure. The expression of PEPCase, an enzyme important for stomatal opening, is down regulated. The leaves develop stress symptoms, mirrored by transient changes in the expression profile of additional genes: lipoxygenase 2 (LOX2), CER5, CER6 (both important for wax production) and RD29B (an ABA inducible stress protein). Non-invasive methods showed a fast response of the plant to camphor fumigations both in a rapid decrease of the quantum yield and in the relative growth rate. Repeated exposures to the monoterpenes resulted finally in growth reduction and a stress related change in the phenotype. It is proposed that high concentrations or repeated exposure to monoterpenes led to irreversible damages, whereas low concentrations or short-term fumigations may have the potential to strengthen the plant fitness.
In order to gain further insight into the signal transduction pathway concerning gravitropism, we studied the expression profiles of mRNA in etiolated sunflower (Helianthus annuus L.) seedlings. Differential-display reverse transcriptase PCR product assayed by capillary electrophoresis revealed the small GTPase Ran, regulating nuclear import and export of proteins. Parallel analysis of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) release by a highly advanced system of metal-dye detection combined with high-performance liquid chromatography provided evidence that the second messenger Ins(1,4,5)P3 is modulated by changes of the gravity vector. Investigations by fast clinorotation and sounding rockets established a positive correlation between the Ins(1,4,5)P3 level and the expression rate of Ran mRNA during simulated and real microgravity. Since an asymmetric distribution of auxin during graviresponse is suggested to induce differential cell elongation, additional information on the perception and transduction pathways was achieved by auxin stimulation experiments. While we were able to demonstrate an auxin-dependent production of Ins(1,4,5)P3, the expression of Ran mRNA was not affected by auxin. Finally, besides the phosphoinositide system as one element of the signal transduction chain linking graviperception to graviresponse, a Ran-mediated interaction model of extracellular microgravity signal perception and intercellular transduction pathway is proposed.
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