Hormone and seed-specific regulation of pea fruit growth

Abstract

Growth of young pea (Pisum sativum) fruit (pericarp) requires developing seeds or, in the absence of seeds, treatment with gibberellin (GA) or auxin (4-chloroindole-3-acetic acid). This study examined the role of seeds and hormones in the regulation of cell division and elongation in early pea fruit development. Profiling histone H2A and γ-tonoplast intrinsic protein (TIP) gene expression during early fruit development identified the relative contributions of cell division and elongation to fruit growth, whereas histological studies identified specific zones of cell division and elongation in exocarp, mesocarp, and endocarp tissues. Molecular and histological studies showed that maximal cell division was from -2 to 2 d after anthesis (DAA) and elongation from 2 to 5 DAA in pea pericarp. Maximal increase in pericarp γ-TIP message level preceded the maximal rate of fruit growth and, in general, γ-TIP mRNA level was useful as a qualitative marker for expanding tissue, but not as a quantitative marker for cell expansion. Seed removal resulted in rapid decreases in pericarp growth and in γ-TIP and histone H2A message levels. In general, GA and 4-chloroindole-3-acetic acid maintained these processes in deseeded pericarp similarly to pericarps with seeds, and both hormones were required to obtain mesocarp cell sizes equivalent to intact fruit. However, GA treatment to deseeded pericarps resulted in elevated levels of γ-TIP mRNA (6 and 7 DAA) when pericarp growth and cell enlargement were minimal. Our data support the theory that cell division and elongation are developmentally regulated during early pea fruit growth and are maintained by the hormonal interaction of GA and auxin.