ABSTRACT

Plants, for hundreds of millions of years, have been evolving defensive strategies to protect themselves against herbivores and pathogens. As plant species evolved within their ecological niches, they developed their own unique arrays of defensive chemicals against the predators they confronted. Virtually thousands of chemicals are present among living plant species that are thought to be defense-related. The advantages of inducible defensive chemicals to plants is not totally clear, but it can be speculated that many plant species have evolved in very hostile environments in which their nutrients were limited, and the need to conserve energy in diverse ecological systems was essential. The inducible defenses have been of particular interest to biochemists and molecular and cell biologists because of the challenges to understand their complex signaling systems, and because of their potential for genetically regulating defensive genes to improve plant productivity.Defensive chemicals are synthesized in many plant species in response to damage inflicted by attacking herbivores (1, 2). In tomato plants, a signal (or signals) originates at the wound site that is transported throughout the plant where it activates the synthesis of defensive proteins that interfere with the digestive systems of the attacking herbivores (Fig. 1). These inducible defensive proteins have been identified as serine, cysteine and aspartyl proteinase inhibitors (2-4) and polyphenol oxidase (5). These proteins interact with the proteins and proteinases of herbivore guts and adversely affect proteolysis of the ingested food, reducing the availability of essential amino acids and retarding the growth and development of the herbivores (2, 5). The net effect of this process in a natural ecosystem is likely to result in a reduction of damage to the plant, either by killing the predator or by providing a longer period of exposure of surviving herbivores to their natural predators. Research on inducible plant defenses has been primarily concerned with defense against insect predators, but more recently, proteinase inhibitors in sedges and grasses have been found to be a major factor in regulating fluctuating populations of lemmings (6). The initial search for the systemic signal in tomato plants resulted in the finding that oligogalacturonides derived from the plant cell wall were inducers of the defensive proteinase inhibitor genes in excised tomato leaves (7). Subsequently, chitin and chitosan oligomers derived from fungal cell walls were also found to be active inducers (8). However, both classes of oligosaccharides were active only at relatively high concentrations (several hundred micrograms per plant) and did not move to distal leaves when placed on leaf wounds (9). The oligosaccharides are therefore considered to be among the signals produced at sites of pathogen attacks where they help produce localized defensive chemicals. A renewed search for the systemic signal in the soluble components of tomato leaves resulted in the isolation o...

No abstract

Oligogalacturonide fragments that activate defensive genes in plant leaves heretofore have been thought to be generated only by pathogen-derived pectin-degrading enzymes, because polygalacturonase (PG) activity has not been reported in leaves. Here, we report that mRNAs encoding a PG catalytic subunit protein and its regulatory (␤-subunit) protein are expressed in tomato leaves in response to wounding, systemin, and oligosaccharide elicitors. Synthesis of the two subunits in response to wounding is systemic and is accompanied by an increase in PG activity in extracts from both wounded and unwounded leaves. The finding that PG subunit mRNAs and PG enzyme activity are induced by wounding indicates that herbivore attacks can produce endogenous oligogalacturonide elicitors that may be involved in the local and systemic activation of defense responses against both herbivores and pathogens.

During the course of characterization of the wound-response related proteins from tomato (Lycopersicon esculentum Mill.) leaves, a serine carboxypeptidase (EC 3.4.16.1) was identified. An increase in peptidase activity in response to wounding, and the isolation of a protein with carboxypeptidase (CP) activity from tomato leaves had been reported previously, but the mRNA coding for the enzyme was not identified. We now report the isolation of a tomato leaf type I serine-CP cDNA whose corresponding mRNA is induced by wounding, systemin and methyl jasmonate. The protein sequence deduced from the cDNA exhibits homology to tomato CP, and barley and rice type I CPs. Southern blot results indicated that the CP gene is probably a member of a small gene family. Tomato CP mRNA was detected within 3 h after wounding, or treatment with systemin or methyl jasmonate. Employing Western blot analysis, CP protein was shown to increase 12 h after the treatments. Using the tomato def1 mutant, we have demonstrated that a functional octadecanoid pathway is necessary for CP transcription in response to wounding. Carboxypeptidase protein was immunolocalized as protein aggregates within the central vacuoles of palisade mesophyll cells as well as in vascular parenchyma where it had previously been found. Double labeling using antibodies specific for CP and inhibitor II indicated that the two proteins are colocalized in the vacuolar aggregates. Tomato CP is a member of the "late wound-inducible genes" whose mRNAs increase 4-12 h following wounding, in contrast to several "early wound-inducible genes", whose mRNAs appear within 30 min. The data support a role for the enzyme in protein turnover that occurs systemically in leaf cells in response to wounding.