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Cluster Roots (cluster + root)
Selected AbstractsPlant responses to drought and phosphorus deficiency: contribution of phytohormones in root-related processesJOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 4 2005Lutz Wittenmayer Abstract Environmental stresses are one of the most limiting factors in agricultural productivity. A large portion of the annual crop yield is lost to pathogens (biotic stress) or the detrimental effects of abiotic-stress conditions. There are numerous reports about chemical characterization of quantitatively significant substrate fluxes in plant responses to stress factors in the root-rhizosphere system, e.g., nutrient mobilization, heavy-metal and aluminum immobilization, or establishment of plant-growth-promoting rhizobacteria (PGPR) by exudation of organic anions, phytosiderophores, or carbohydrates into the soil, respectively. The hormonal regulation of these responses is not well understood. This paper highlights this complex process, stressing the involvement of phytohormones in plant responses to drought and phosphorus deficiency as examples. Beside ethylene, abscisic acid (ABA) plays an important role in drought-stress adaptation of plants. This hormone causes morphological and chemical changes in plants, ensuring plant survival under water-limited conditions. For example, ABA induces stomata closure, reduction in leaf surface, and increase in root : shoot ratio and, thus, reduction in transpiration and increase in soil volume for water uptake. Furthermore, it supports water uptake in soil with decreasing water potential by osmotic adjustment. Suitability of hormonal parameters in the selection for improving stress resistance is discussed. Auxins, ethylene, and cytokinins are involved in morphological adaption processes to phosphorus (P) deficiency (increase in root surface, e.g., by the formation of more dense root hairs or cluster roots). Furthermore, indole-3-acetic acid increases root exudation for direct and indirect phosphorus mobilization in soil. Nevertheless, the direct use of the trait "hormone content" of a particular plant organ or tissue, for example the use of the drought-stress-induced ABA content of detached leaves in plant breeding for drought-stress-resistant crops, seems to be questionable, because this procedure does not consider the systemic principle of hormonal regulation in plants. Reaktionen von Pflanzen auf Trockenstress und Phosphormangel: Die Rolle von Phytohormonen in wurzelbezogenen Prozessen Umweltstress stellt den wesentlichsten Limitierungsfaktor für die landwirtschaftliche Produktion dar. Ein erheblicher Teil der jährlichen Ernten geht durch pathogene Organismen (biotischer Stress) oder durch die verheerende Wirkung abiotischer Stressoren verloren (v. a. Trockenstress und Nährstoffmangel). Es gibt zahlreiche Untersuchungen zur stofflichen Charakterisierung der pflanzlichen Stressreaktion an der Wurzel, z.,B. Nährstoffmobilisierung, Schadstoffimmobilisierung oder Etablierung von wachstumsfördernden Rhizobakterien durch Wurzelabscheidungen. Die hormonelle Steuerung dieser Prozesse ist bisher weniger erforscht. Der Artikel geht dieser Problematik am Beispiel von Trockenstress und Phosphormangel unter besonderer Berücksichtigung von Phytohormonen nach. Bei der Anpassung von Pflanzen an Wassermangelbedingungen spielt neben Ethylen das Phytohormon Abscisinsäure (ABA) eine wichtige Rolle. Es induziert morphologische und chemische Veränderungen in der Pflanze, die ein Überleben unter Wassermangelbedingungen ermöglichen. Beispielsweise induziert die ABA den Stomataschluss, eine Verringerung der Blattoberfläche sowie eine Erhöhung des Wurzel:Spross-Verhältnisses und bewirkt dadurch eine verringerte Transpiration und Vergrößerung des Bodenvolumens zur Erschließung von Wasservorräten. Darüber hinaus kann eine ABA-induzierte Anreicherung von osmotisch wirksamen Verbindungen zur Wasseraufnahme bei sinkendem Wasserpotential im Boden beitragen. Bei Phosphat (P)-Mangel sind vor allem Auxine, Cytokine und Ethylen an der morphologischen Anpassung der Wurzeln (Vergrößerung der Wurzeloberfläche durch verstärkte Bildung von Wurzelhaaren oder Proteoidwurzeln) beteiligt. Darüber hinaus bewirkt Indolyl-3-Essigäure eine Intensivierung der Abgabe von Wurzelabscheidungen zur direkten oder indirekten P-Mobilisierung in der Rhizosphäre. Trotzdem wird die unmittelbare Verwendung des Indikators "Hormongehalt" eines bestimmten Pflanzenorganes, beispielsweise der trockenstressinduzierte ABA-Gehalt von abgeschnittenen Blättern, für die Züchtung auf Stressresistenz als problematisch angesehen, da sie das systemische Prinzip der Hormonregulation nicht berücksichtigt. [source] Citrate exudation from white lupin induced by phosphorus deficiency differs from that induced by aluminumNEW PHYTOLOGIST, Issue 3 2007B. L. Wang Summary ,,Both phosphorus (P) deficiency and aluminum (Al) toxicity induce root exudation of carboxylates, but the relationship between these two effects is not fully understood. Here, carboxylate exudation induced by Al in Lupinus albus (white lupin) was characterized and compared with that induced by P deficiency. ,,Aluminum treatments were applied to whole root systems or selected root zones of plants with limited (1 µm) or sufficient (50 µm) P supply. ,,Aluminum stimulated citrate efflux after 1,2 h; this response was not mimicked by a similar trivalent cation, La3+. P deficiency triggered citrate release from mature cluster roots, whereas Al stimulated citrate exudation from the 5- to 10-mm subapical root zones of lateral roots and from mature and senescent cluster roots. Al-induced citrate exudation was inhibited by P limitation at the seedling stage, but was stimulated at later growth stages. Citrate exudation was sensitive to anion-channel blockers. Al treatments did not affect primary root elongation, but inhibited the elongation of lateral roots. ,,The data demonstrate differential patterns of citrate exudation in L. albus, depending on root zone, developmental stage, P nutritional status and Al stress. These findings are discussed in terms of possible functions and underlying mechanisms. [source] White lupin has developed a complex strategy to limit microbial degradation of secreted citrate required for phosphate acquisitionPLANT CELL & ENVIRONMENT, Issue 5 2006LAURE WEISSKOPF ABSTRACT White lupins (Lupinus albus L.) respond to phosphate deficiency by producing special root structures called cluster roots. These cluster roots secrete large amounts of carboxylates into the rhizosphere, mostly citrate and malate, which act as phosphate solubilizers and enable the plant to grow in soils with sparingly available phosphate. The success and efficiency of such a P-acquisition strategy strongly depends on the persistence and stability of the carboxylates in the soil, a parameter that is influenced to a large extent by biodegradation through rhizosphere bacteria and fungi. In this study, we show that white lupin roots use several mechanisms to reduce microbial growth. The abundance of bacteria associated with cluster roots was decreased at the mature state of the cluster roots, where a burst of organic acid excretion and a drastic pH decrease is observed. Excretion of phenolic compounds, mainly isoflavonoids, induced fungal sporulation, indicating that vegetative growth, and thus potential citrate consumption, is reduced. In addition, the activity of two antifungal cell wall-degrading enzymes, chitinase and glucanase, were highest at the stage preceding the citrate excretion. Therefore, our results suggest that white lupin has developed a complex strategy to reduce microbial degradation of the phosphate-solubilizing agents. [source] Physiological changes in white lupin associated with variation in root-zone CO2 concentration and cluster-root P mobilizationPLANT CELL & ENVIRONMENT, Issue 10 2005M. D. CRAMER ABSTRACT White lupin (Lupinus albus L.) mobilizes insoluble soil phosphorus through exudation of organic acids from ,cluster' roots. Organic acid synthesis requires anaplerotic carbon derived from dark CO2 fixation involving PEP-carboxylase. We tested the hypothesis that variation in root-zone CO2 concentration would influence organic acid synthesis and thus P mobilization. Root-zone CO2 concentrations and soil FePO4 concentrations supplied to sand-grown white lupin (cv. Kiev Mutant) were varied. More biomass accumulated in plants supplied with 360 µL L,1 CO2 to the root-zone, compared with those aerated with either 100 or 6000 µL L,1 CO2. Increased FePO4 in the sand resulted in greater leaf P concentrations, but root-zone [CO2] did not influence leaf P concentration. Suppression of cluster-root development in plants supplied with 100 µL L,1 root-zone CO2 was correlated with increased leaf [P]. However, at both 360 and 6000 µL L,1 CO2, cluster-root development was suppressed only at the highest leaf P concentration. Phloem sap [P] was significantly increased by greater [FePO4] in the sand, but was reduced with increased root-zone [CO2], and this may have triggered increased cluster-root initiation. Succinate was the major organic acid (carboxylate) in the phloem sap (minor components included malate, citrate, fumarate) and was increased at greater [FePO4], suggesting that this shoot-derived carboxylate might provide an important source of organic acids for root metabolism. Since cluster root development was inhibited by increasing concentrations of FePO4 in the sand, it is possible that succinate was utilized for the functioning of the root-nodules. [source] | ||||||||||