Systemins: a functionally defined family of peptide signals that regulate defensive genes in Solanaceae species

Abstract

Numerous plant species have been known for decades that respond to herbivore attacks by systemically synthesizing defensive chemicals to protect themselves from predators. The nature of systemic wound signals remained obscure until 1991, when an 18-aa peptide called systemin was isolated from tomato leaves and shown to be a primary signal for systemic defense. More recently, two new hydroxyproline-rich, glycosylated peptide defense signals have been isolated from tobacco leaves, and three from tomato leaves. Because of their origins in plants, small sizes, hydroxyproline contents (tomato systemin is proline-rich), and defense-signaling activities, the new peptides are included in a functionally defined family of signals collectively called systemins. Here, we review structural and biological properties of the systemin family, and discuss their possible roles in systemic wound signaling.

Fig. 1.

A simplified diagram of the systemin signaling pathway. The pathway shows several key steps of the signaling pathway, and in particular the steps leading to the blockage of a proton ATPase that leads to the alkalinization of the extracellular medium, which is the basis of the assay developed to identify signaling peptides.

Fig. 2.

The amino acid sequences of tobacco hydroxyproline-rich systemins, TobHypSys I and TobHypSys II, are shown compared with the sequence of tomato systemin. Hydroxyproline (O), proline (P), threonine (T), and serine (S) residues are in red, charged amino acids are in black, and neutral amino acids are in blue. The ranges of pentose units attached to each peptide, determined by mass spectrometry, are shown at the right.

Fig. 3.

The amino acid sequences of tomato hydroxyproline-rich systemins, TomHypSys I, proline (P), TomHypSys II, and TomHypSys III, are shown. Hydroxyproline (O), threonine (T), and serine (S) residues are colored as in Fig. 2. The number of pentose units associated to each peptide are shown in the right column.

Fig. 4.

Box diagrams of the precursors of tobacco and tomato systemin peptides. The open boxes represent the leader sequences of the newly translated proteins. The HypSys peptides are shown as hatched boxes, with their numeral identities above each peptide. The systemin peptide is shown as Sys. The length of each preproprecursor is shown in parentheses.