Silencing the Hydroxyproline-Rich Glycopeptide Systemin Precursor in Two Accessions of Nicotiana attenuata Alters Flower Morphology and Rates of Self-Pollination

Systemins and their hydroxyproline-rich glycopeptide systemin (ppHS) subfamily members are known to mediate antiherbivore defenses in some solanaceous taxa but not others; functions other than in defense remain largely unexplored. Nicotiana attenuata's ppHS is known not to function in herbivore defense. NappHS transcripts are abundant in flowers, particularly in pistils, and when two N. attenuata accessions from Utah and Arizona were transformed to silence NappHS by RNAi (IRsys), seed capsule production and seed number per capsule were reduced in both accessions. These reductions in reproductive performance could not be attributed to impaired pollen or ovule viability; hand-pollination of all IRsys lines of both accessions restored seed production per capsule to levels found in wild-type plants. Rather, changes in flower morphology that decreased the efficiency of self-pollination are likely responsible: IRsys plants of both accessions have flowers with pistils that protrude beyond their anthers. Because these changes in flower morphology are reminiscent of CORONATINE-INSENSITIVE1-silenced N. attenuata plants, we measured jasmonates (JAs) and their biosynthetic transcripts in different floral developmental stages, and found levels of JA-isoleucine (Ile)/leucine and threonine deaminase transcripts, which are abundant in wild-type pistils, to be significantly reduced in IRsys buds and flowers. Threonine deaminase supplies Ile for JA-Ile biosynthesis, and we propose that ppHS mediates JA signaling during flower development and thereby changes flower morphology. These results suggest that the function of ppHS family members in N. attenuata may have diversified to modulate flower morphology and thereby outcrossing rates in response to biotic or abiotic stresses.