This paper examines induced resistance (SAR) in plants against various insect and pathogenic invaders. SAR confers quantitative protection against a broad spectrum of microorganisms in a manner comparable to immunization in mammals, although the underlying mechanisms differ. Discussed here are the molecular events underlying SAR: the mechanisms involved in SAR, including lignification and other structural barriers, pathogenesis-related proteins and their expression, and the signals for SAR including salicylic acid. Recent findings on the biological role of systemin, ethylene, jasmonates, and electrical signals are reviewed. Chemical activators of SAR comprise inorganic compounds, natural compounds, and synthetic compounds. Plants known to exhibit SAR and induced systemic resistance are listed.

Systemic acquired resistance

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

The concept that photosynthetic flux is influenced by the accumulation of photo-assimilate persisted for 100 years before receiving any strong experimental support. Precise analysis of the mechanisms of photosynthetic responses to sink activity required the development of a battery of appropriate molecular techniques and has benefited from contemporary interest in the effects of elevated CO2 on photosynthesis. Photosynthesis is one of the most highly integrated and regulated metabolic processes to maximize the use of available light, to minimize the damaging effects of excess light and to optimize the use of limiting carbon and nitrogen resources. Hypotheses of feedback regulation must take account of this integration. In the short term, departure from homeostasis can lead to redox signals, which cause rapid changes in the transcription of genes encoding photosystems I and II. End-product synthesis can exert short-term metabolic feedback control through Pi recycling. Beyond this, carbohydrate accumulation in leaves when there is an imbalance between source and sink at the whole plant level can lead to decreased expression of photosynthetic genes and accelerated leaf senescence. In a high CO2 world this may become a more prevalent feature of photosynthetic regulation. However, sink regulation of photosynthesis is highly dependent on the physiology of the rest of the plant. This physiological state regulates photosynthesis through signal transduction pathways that co-ordinate the plant carbon : nitrogen balance, which match photosynthetic capacity to growth and storage capacity and underpin and can override the direct short-term controls of photosynthesis by light and CO2. Photosynthate supply and phytohormones, particularly cytokinins, interact with nitrogen supply to control the expression of photosynthesis genes, the development of leaves and the whole plant nitrogen distribution, which provides the dominant basis for sink regulation of photosynthesis.

Sink regulation of photosynthesis

In the spectrum of plant-microbe interactions disease is a rare outcome. In many interactions complex, integrated defense mechanisms prevent infection and disease. These defensive systems include preformed physical and chemical barriers as well as inducible defenses such as the strengthening of cell walls or synthesis of antimicrobial compounds (i.e., phytoalexins) and proteins.1,2 In certain cases plants react to pathogen attack by developing long-lasting, broad-spectrum systemic resistance to later attacks by pathogens. This phenomenon, termed systemic acquired resistance (SAR), has been observed in many species and may be ubiquitous among higher plants. In the last five years progress has been made toward understanding the molecular basis of SAR. In this review we first provide a brief history of SAR research, then describe our present knowledge of the manifestation and induction of SAR. We discuss recent findings that indicate a central role for the SAR pathway in plant health and finally present our current working model of SAR induction.

/pdf/systemic-acquired-resistance-2qr1m6rcwh.pdf

Systemic Acquired Resistance

Plant chitinases

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.

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

Cytokinins and phytochrome have both been reported to promote chloroplast development, and possible interactions between the two have been suggested We have examined the effects of red light (R) and a cytokinin, benzyladenine (N6-benzylaminopurine; BA), on the levels of four mRNAs coding for chloroplast proteins in Lemna gibba L The amounts of hybridizable RNA coding for both the major chlorophyll a/b-binding protein and for the small subunit of ribulose-1,5-bisphosphate (RuBP) carboxylase decrease to a low level when white-light-grown L gibba plants are placed in the dark We have previously shown that a subsequent R treatment causes a several-fold increase in the levels of these two messages, and this increase is phytochrome-mediated We have now found that addition of submicromolar concentrations of BA to plants kept in total darkness also results in an increase in levels of these two mRNAs Furthermore, BA treatment magnifies the extent of the response to R treatment However, the levels of mRNAs encoding the large subunit of RuBP carboxylase and the 32-kDa herbicide-binding protein, which are both chloroplastsynthesized messages, are not significantly altered by either R or BA treatment during the same time period The relative amount of β-actin mRNA, a nuclear-encoded message for a cytoplasmic protein, is also not altered either by R or BA treatment Thus, BA treatment does not simply alter the proportion of mRNA to total RNA This conclusion is also supported by the observation that levels of mRNA hybridizing to a sequence abundant in dark-treated plants are not altered by BA treatment The amplification by BA of the R-induced increase in the level of chlorophyll a/b-binding protein mRNA, consistently seen in total RNA, is not observed in RNA isolated from nuclei from plants receiving the same treatments We therefore suggest that cytokinin is regulating expression of this message at a post-transcriptional level, possibly by affecting the stability of the RNA

Benzyladenine modulation of the expression of two genes for nuclear-encoded chloroplast proteins in Lemna gibba: Apparent post-transcriptional regulation.

When white-light-grown Lemna gibba plants are placed in the dark, the levels of mRNAs for two nuclear-encoded chloroplast proteins, the small subunit of ribulose-1,5-bisphosphate carboxylase (SSU) and the major chlorophyll a/b-binding protein of light-harvesting complex II (LHCP), decline to a small fraction of their previous level. We have reported [4] that red light (R), acting through phytochrome, and benzyladenine (BA), a synthetic cytokinin, independently stimulate accumulation of both mRNAs in the dark. Here, we have analyzed the products of transcription in isolated nuclei to determine if cytokinins act primarily through stimulation of transcription or if post-transcriptional processes are involved. We find that BA pretreatment may slightly stimulate transcription of LHCP RNA either with or without a red-light treatment. However, the effects of BA on the LHCP RNA accumulation were much greater than on transcription. Two naturally occurring cytokinins are also effective in increasing the mRNA abundance. We therefore conclude that, in Lemna, post-transcriptional processes are important in regulation of the LHCP RNA by cytokinins.

Cytokinin modulation of LHCP mRNA levels: the involvement of post-transcriptional regulation.

Vacuolar class I β-1,3-glucanases (EC 3.2.1.39) are believed to be important in the induced defense reaction of plants to fungal infection. We used antisense transformation to test this hypothesis and to identify other possible physiological functions of this enzyme. Nicotiana sylvestris plants were transformed with antisense constructions containing the region from position 27 to 608 of the coding sequence of the basic, vacuolar β-1,3-glucanase gene GLA of tobacco regulated by cauliflower mosaic virus 35S RNA expression signals. Plants homozygous for this transgene showed a marked, ca. 20-fold reduction in the constitutive expression of class I β-1,3-glucanase antigen in their leaves. RNA blot analysis indicated that the antisense plants expressed low levels of the sense transcript of the host β-1,3-glucanase gene and the antisense transcript of the transgene. Immune blot analysis of plant extracts indicated that only expression of the N. sylvestris homologue of class I tobacco β-1,3-glucanase and not the acidic, class II isoforms of the enzyme was blocked in the antisense plants. Class I isoforms of β-1,3-glucanase and chitinase were coordinately induced in leaves of untransformed and empty-vector-transformed N. sylvestris plants treated with ethylene or infected with the fungal leaf pathogen Cercospora nicotianae. In antisense plants, chitinase but not β-1,3-glucanase was induced under these conditions indicating that antisense transformation effectively blocks constitutive as well as induced expression of class I β-1,3-glucanase. Under greenhouse conditions, antisense plants developed normally and were fertile. The plants did not exhibit increased susceptibility to C. nicotianae infection. These results suggest that expression of the β-1,3-glucanase isoform blocked by antisense transformation is not necessary for ‘house-keeping’ functions of N. sylvestris nor defense against the fungal pathogen tested.

The function of vacuolar β-1,3-glucanase investigated by antisense transformation. Susceptibility of transgenic Nicotiana sylvestris plants to Cercospora nicotianae infection

We have isolated a cDNA clone, called pLg106, which corresponds to a negatively light-regulated mRNA in the aquatic monocot Lemna gibba. Lg106 mRNA is more abundant in dark-treated plants than in plants grown under continuous white light. The levels of Lg106 mRNA reached a maximum 12–24 hours after plants had been placed in the dark. In vitro-labelled transcripts from nuclei isolated from white light-grown and darktreated Lemna hybridized equally to Lg106 DNA, indicating that transcription of the Lg106 gene was similar in both white light and darkness. Therefore, the higher level of Lg106 mRNA in the dark was not attributable to a change in transcription. We suggest that the stability of Lg106 mRNA is affected by changes in illumination. The steady-state level of Lg106 did not appear to be under phytochrome regulation, as the abundance of Lg106 mRNA from plants grown in intermittent red light or intermittent red followed immediately by far-red light was not significantly different from that of dark-treated plants. The size of the mRNA was estimated to be 650–700 nt. Genomic Southern blots indicated the presence of a single gene for Lg106. The Lg106 cDNA was sequenced and examined for similarities to nucleotide sequences in GenBank.

/pdf/characterization-of-a-negatively-light-regulated-mrna-from-48b1lk4of8.pdf

Susan Flores

Papers

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

Benzyladenine modulation of the expression of two genes for nuclear-encoded chloroplast proteins in Lemna gibba: Apparent post-transcriptional regulation.

Cytokinin modulation of LHCP mRNA levels: the involvement of post-transcriptional regulation.

The function of vacuolar β-1,3-glucanase investigated by antisense transformation. Susceptibility of transgenic Nicotiana sylvestris plants to Cercospora nicotianae infection

Characterization of a negatively light-regulated mRNA from Lemna gibba.