STIG1 controls exudate secretion in the pistil of petunia and tobacco.

The lipid-rich, sticky exudate covering the stigma of solanaceous species such as tobacco (Nicotiana tabacum) and petunia (Petunia hybrida) contains several proteins, of which only some have been characterized to date. Proteome analysis of the stigmatic exudate in both species revealed the presence of a cysteine-rich, slightly acidic 12-kD protein called stigma-specific protein 1 (STIG1). In both tobacco and petunia, Stig1 is highly expressed at the mRNA level in very young and developing flowers, whereas hardly any Stig1 transcript is detected in mature flowers. This expression pattern coincides with the differentiation of the secretory zone, forming the intercellular spaces into which the exudate is secreted. Using reverse genetics, we show that STIG1 is involved in the secretion and merging of exudate lipids in the intercellular spaces of the secretory zone and that plants lacking STIG1 show an accelerated deposition of exudate onto the stigmatic surface. This phenotype was observed both in a petunia knockout mutant and in tobacco transgenic plants. We therefore propose that STIG1 plays a role in the temporal regulation of the essential exudate secretion onto the stigma.