Date of Award:

5-1-1997

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Committee Chair(s)

Anne J. Anderson

Committee

Anne J. Anderson

Committee

Bradley R. Kropp

Committee

Richard J. Mueller

Committee

Jeanette Norton

Committee

Gregory J. Podgorski

Abstract

Mycorrhizal hyphae were directed in their growth pattern in roots. Arbuscules produced by a single isolate of Glomus intraradices Schenck & Smith in roots of corn, wheat, carrot, clover, mung bean, and bean (Phaseolus vulgaris L.) were consistently limited to cortical cells immediately adjacent to the endodermis. Two hypotheses for controlled growth of arbuscular mycorrhizal fungi in plants were examined: (1) activation of plant defenses by the fungus, and (2) availability of host-derived carbon to the fungus. Accumulations of mRNAs for the defense-related genes phenylalanine ammonia-lyase (PAL), chalcone synthase, chitinase (CHT), glucanese, and hydroxyproline-rich glycoprotein were examined in the roots of P. vulgaris colonized by G. intraradices. In situ hybridizations of colonized roots using probes for PAL and CHT showed accumulations of both transcripts changed only in arbusculated cells. Both young, finely branched arbuscules and older, clumped arbuscules displayed PAL and CHT message accumulations. PAL and CHT mRNA accumulations were greater in cortical cells containing young arbuscules than in cells containing clumped arbuscules. Intercellular hyphae and vesicles elicited no defense response. Because arbuscules in cells surrounding the endodermis are uniquely positioned to intercept the lateral movement of photosynthate out of the vascular cylinder, transcript levels for sucrose-metabolizing enzymes were examined in arbusculated cells. A cDNA was cloned from roots of P. vulgaris encoding a soluble acid invertase with potential vacuolar targeting. This clone and cDNAs for a Rhizobium-responsive sucrose synthase of soybean and a cell wall acid invertase of carrot were used as nucleic acid probes in in situ hybridizations of mycorrhizal roots. Transcript levels for cell wall acid invertase in arbusculated cells of carrot remained unchanged. Sucrose synthase mRNA levels were high in phloem tissues of the vascular cylinder of nonmycorrhizal and mycorrhizal roots of P. vulgaris. Strong endodermal cell-specific expression of the soluble acid invertase cDNA was observed in noncolonized and arbuscular mycorrhizal P. vulgaris roots. In addition, mycorrhizal roots of P. vulgaris showed increased transcript accumulation for the sucrose synthase and soluble acid invertase cDNAs that was limited to arbusculated cells.

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