Aspects of chemotaxis in zoospores of Phytophthora cinnamomi
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. Motility of zoospores of Phytophthora cinnamomi in distilled water, and 40 ?M, 1 mM or 25 mM ethanol was studied with dark-field photomicrography using a stroboscopic technique with coded sequences. Ethanol in isotropic solutions did not affect the average speed, velocity, amplitude or frequency of the helical path, but did suppress spontaneous turning activity. However, when zoospores encountered gradients of ethanol they turned, moved up the gradient and accumulated in numbers proportional to the concentration of ethanol at the source. Three methods, designated the ‘swim-in test’, the ‘swim-out test’, and the ‘rhizosphere model’, were devised for examining chemotactic responses of zoospores to ethanol in a soil pore system simulated by glass capillaries. The rhizosphere model allowed maintenance of desired concentration gradients of test materials. In the absence of chemotaxis, zoospores entering capillaries were continually disorientated through collision with the walls with the result that penetration beyond the entrance was limited. In the rhizosphere model, chemotactic reactions became increasingly evident as the concentration gradient within capillaries was increased from 0.2 to 25 mM per (16mm), resulting in a greater degree and speed of penetration. In ‘swim-out’ tests the threshold gradient for increased active movement out of capillaries was greater for tubes 190 and 280 ?m than for tubes 440 and 800 ?m in diameter. Positive chemotaxis to methanol, n-propanol, n-butanol and acetaldehyde was also observed. Of several Phytophthora spp. examined, only P.megasperma var. sojae responded to ethanol in a manner similar to P.cinnamomi. When zoospores ultimately encysted they germinated in gradients of ethanol with germ tubes directed towards the source. Ethanol was detected at a concentration of approximately 250 ?g/g fresh weight (ca. 5 mM) in radicles of Lupinus angustifolius that had been grown in wet but well-drained soil, and (also at 5 mM) in soil that had been saturated for 12-24 h. The results are considered in relation to the production of ethanol by plant roots during brief periods of fermentative metabolism, the distribution of exudate concentration about plant roots, and the opportunity for zoospores to accumulate at sites of ethanol exudation in saturated soil. The ‘swim-out’ test was used to examine negative chemotaxis in zoospores of P.cinnamomi with the zoospores in this case remaining in the tubes and retreating inwards from the ends of capillaries. Zoospores responded to various acids and salts by exhibiting acute, repetitive turning movements at a well-defined point in a concentration gradient. For zoospores swimming in glass distilled water of pH 6.0, this point always corresponded to a critical absolute concentration of cation which was calculated for H+ to be 37 ?M, for K+ to be 628 ?M, for NH4+ to be 5040 ?M, and for Na+ to be 6480 ?M when Cl- was the coionic species. The net result of the chemotactic reactions was that zoospores swam away from the source of diffusing chemical, regaining normal motility quickly as they returned to regions of lower concentration of cation. Zoospores did not regain normal motility as quickly after turning in response to H+ when glutamate was the coion as when malate or chloride were the coionic species. Incorporation of 10mM ethanol isotropically into both zoospore suspension and test solution did not alter the critical concentration at which zoospores turned in response to diffusing HCl or KCl. However, when a concentration gradient of ethanol was established simultaneously with that of L-glutamic acid, zoospores turned at a position closer to the source of acid than when ethanol was absent. On the other hand, when concentration gradients of ethanol and either L-malic or hydrochloric acids were established simultaneously, zoospores turned in response to the acid before experiencing a significant stimulation from the ethanol present. Zoospores encysted immediately when suspensions were mixed with acids or salts having a concentration in the mixture above the concentration which induced the turning reaction. Zoospores also encysted within a short time of exposure to a concentration of cation slightly below the critical concentration for a chemotactic response. The observations provide strong evidence of a threshold effect in negative chemotaxis to cations, but consideration is given to observations that engender caution in accepting an all-or-none hypothesis. The relative tolerance of zoospores to the various cations follows the classic Hoffmeister lyotropic series for cation exchange reactions, and negative chemotaxis is interpreted as a surface exchange phenomenon. By contrast, the effects of ethanol on chemotactic reactions in zoospores appear to be unrelated to the surface of the plasma-membrane. The same negative chemotactic reactions to various acids occurred in zoospores swimming in aqueous soil extract as was observed for those swimming in glass distilled water. The responses of zoospores to the various compounds in vitro are discussed in relation to the behaviour of zoospores that might encounter these same compounds when suspended or swimming in flowing or stationary water in soil.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1973