The production of enzymes capable of degrading the cell walls of invading phytopathogenic fungi is an important component of the defense response of plants. The timing of this natural host defense mechanism was modified to produce fungal-resistant plants. Transgenic tobacco seedlings constitutively expressing a bean chitinase gene under control of the cauliflower mosaic virus 35S promoter showed an increased ability to survive in soil infested with the fungal pathogen Rhizoctonia solani and delayed development of disease symptoms.

https://science.sciencemag.org/content/sci/254/5035/1194.full.pdf

Transgenic Plants with Enhanced Resistance to the Fungal Pathogen Rhizoctonia solani

Plant chitinases

Radish seeds have previously been shown to contain two homologous, 5-kD cysteine-rich proteins designated Raphanus sativus-antifungal protein 1 (Rs-AFP1) and Rs-AFP2, both of which exhibit potent antifungal activity in vitro. We now demonstrate that these proteins are located in the cell wall and occur predominantly in the outer cell layers lining different seed organs. Moreover, Rs-AFPs are preferentially released during seed germination after disruption of the seed coat. The amount of released proteins is sufficient to create a microenvironment around the seed in which fungal growth is suppressed. Both the cDNAs and the intron-containing genomic regions encoding the Rs-AFP preproteins were cloned. Transcripts (0.55 kb) hybridizing with an Rs-AFP1 cDNA-derived probe were present in near-mature and mature seeds. Such transcripts as well as the corresponding proteins were barely detectable in healthy uninfected leaves but accumulated systemically at high levels after localized fungal infection. The induced leaf proteins (designated Rs-AFP3 and Rs-AFP4) were purified and shown to be homologous to seed Rs-AFPs and to exert similar antifungal activity in vitro. A chimeric Rs-AFP2 gene under the control of the constitutive cauliflower mosaic virus 35S promoter conferred enhanced resistance to the foliar pathogen Alternaria longipes in transgenic tobacco. The term "plant defensins" is proposed to denote these defense-related proteins.

Small cysteine-rich antifungal proteins from radish: their role in host defense.

The nature of induced resistance
Resistance, according to Agrios (1988) is the ability
of an organism to exclude or overcome, completely
or in some degree, the effect of a pathogen or other
damaging factor Disease resistance in plants is manifested
by limited symptoms, reflecting the inability
of the pathogen to grow or multiply and spread, and
often takes the form of a hypersensitive reaction (HR),
in which the pathogen remains confined to necrotic
lesions near the site of infection Induced resistance is
the phenomenonthat a plant, once appropriately stimulated,
exhibits an enhanced resistance upon 'challenge'
inoculation with a pathogen Although induced resistance
has been attracting attention recently (Ryals et
al, 1994; Hammerschmidt and Kuc, 1995), the first
systematic enquiry into induced resistance was made
by Ross (1961a,b) He observed that the inducible
resistance response to tobacco mosaic virus (TMV)
in N gene-containing, hypersensitively reacting tobacco
was not confined to the immediate vicinity of the
resulting local necrotic lesions, but extended to other
plant parts A ring of tissue around the developing
lesions became fully refractory to subsequent infection
(localized acquired resistance; Ross, 1961a), whereas
challenge inoculation of distant tissues resulted in
much smaller, and occasionally fewer, lesions (systemic
acquired resistance (SAR); Ross, 1961b) than in
non-induced plants Even leaves thatweremere initials
at the time of the primary inoculation became induced,
suggesting that as a result of the initial infection, a
signal was generated, transported and propagated, that
primed the plant to respond more effectively to subsequent
infection (Bozarth and Ross, 1964) Treatments that influenced lesion size after primary infection had
similar effects on lesions developing upon challenge
inoculation (Ross, 1966), leading to the conclusion
that the mechanisms responsible for resistance expression
were the same under both conditions Only upon
challenge inoculation, defense mechanisms appeared
to be expressed earlier and to a greater extent (De Laat
and Van Loon, 1983; Dean and Kuc, 1987)

/pdf/induced-resistance-in-plants-and-the-role-of-pathogenesis-40mag32efs.pdf

Induced resistance in plants and the role of pathogenesis-related proteins

Disease resistance in transgenic plants has been improved, for the first time, by the insertion of a gene from a biocontrol fungus. The gene encoding a strongly antifungal endochitinase from the mycoparasitic fungus Trichoderma harzianum was transferred to tobacco and potato. High expression levels of the fungal gene were obtained in different plant tissues, which had no visible effect on plant growth and development. Substantial differences in endochitinase activity were detected among transformants. Selected transgenic lines were highly tolerant or completely resistant to the foliar pathogens Alternaria alternata, A. solani, Botrytis cinerea, and the soilborne pathogen Rhizoctonia solani. The high level and the broad spectrum of resistance obtained with a single chitinase gene from Trichoderma overcome the limited efficacy of transgenic expression in plants of chitinase genes from plants and bacteria. These results demonstrate a rich source of genes from biocontrol fungi that can be used to control diseases in plants.

/pdf/genes-from-mycoparasitic-fungi-as-a-source-for-improving-2uc2xwflge.pdf

Genes from mycoparasitic fungi as a source for improving plant resistance to fungal pathogens.

Endochitinases (E.C. 3.2.14, chitinase) are believed to be important in the biochemical defense of plants against chitin-containing fungal pathogens. We introduced a gene for class I (basic) tobacco chitinase regulated by Cauliflower Mosaic Virus 35S-RNA expression signals into Nicotiana sylvestris. The gene was expressed to give mature, enzymatically active chitinase targeted to the intracellular compartment of leaves. Most transformants accumulated extremely high levels of chitinase-up to 120-fold that of non-transformed plants in comparable tissues. Unexpectedly, some transformants exhibited chitinase levels lower than in non-transformed plants suggesting that the transgene inhibited expression of the homologous host gene. Progeny tests indicate this effect is not permanent. High levels of chitinase in transformants did not substantially increase resistance to the chitin-containing fungus Cercospora nicotiana, which causes Frog Eye disease. Therefore class I chitinase does not appear to be the limiting factor in the defense reaction to this pathogen.

Ursula Hinz

Papers

High-level expression of a tobacco chitinase gene in Nicotiana sylvestris. Susceptibility of transgenic plants to Cercospora nicotianae infection.