Auxin Gradients Determine Reproductive Development in Pea (Pisum sativum)

Auxins are involved in the regulation of fruit set and development; however, the role of IAA is unclear in pea (Pisum sativum) since the endogenous auxin 4-Cl-IAA appears to be the auxin stimulating ovary (pericarp) growth. To further understand the role of auxins during fruit development, auxin localization, quantitation, transport, and gene expression activity were assessed in this model legume species. IAA levels and auxin activity (DR5::β-Glucuronidase [GUS] staining and enzyme activity) were substantially reduced in the pericarp vascular tissues, pedicels, and peduncles of fruit upon seed removal, reflecting auxin transport streams derived from the seeds through these tissues. Seed removal modified auxin response factor PsARF7/19, PsARF8, and PsARF5 transcript levels in the pericarp and attachment tissues in a manner suggesting tissue-specific regulation of their expression by auxin and ethylene. Pericarp application of polar auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) increased auxin (DR5::GUS staining/enzyme) activity within pericarps of seeded, but not deseeded fruits, and NPA application to the peduncle modified IAA levels and DR5::GUS staining/enzyme activity, suggesting polar auxin transport from the seeds to surrounding tissues. However, the NPA application did not induce parthenocarpic fruit growth as in other model species. These data support that in pea, auxin is transported from the seeds to adjacent tissues at least partially through NPA-sensitive pathways, that seed-derived IAA plays a role in maintaining auxin gradients through the pericarp and attachment tissues likely for establishing the seed as a major sink, and that auxin and ethylene pathways interact to determine the fate of fruit development.