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Abstract:
The discovery in tomato of systemin, the first plant peptide hormone1,2, was a fundamental change for the concept of plant
hormones. Numerous other peptides have since been shown
to play regulatory roles in many aspects of the plant life,
including growth, development, fertilization and interactions
with symbiotic organisms3–6. Systemin, an 18 amino acid peptide
derived from a larger precursor protein7, was proposed
to act as the spreading signal that triggers systemic defence
responses observed in plants after wounding or attack by herbivores1,7,8.
Further work culminated in the identification of a
leucine-rich repeat receptor kinase (LRR-RK) as the systemin
receptor 160 (SR160)9,10. SR160 is a tomato homologue
of Brassinosteroid Insensitive 1 (BRI1), which mediates the
regulation of growth and development in response to the steroid
hormone brassinolide11–13. However, a role of SR160/BRI1
as systemin receptor could not be corroborated by others14–16.
Here, we demonstrate that perception of systemin depends
on a pair of distinct LRR-RKs termed SYR1 and SYR2. SYR1
acts as a genuine systemin receptor that binds systemin with
high affinity and specificity. Further, we show that presence
of SYR1, although not decisive for local and systemic wound responses, is important for defence against insect herbivory.
