A wound-inducible member of the hydroxyproline-rich glycoprotein gene family in sunflower.

HRGPs4 are a diverse family of cell wall proteins including extensins, hydroxyproline-rich lectins of the Solanaceae, and arabinogalactans (7, 8). HRGPs can comprise as much as 15% of the cell wall dry weight. Pathogen-induced accumulation ofextensin-like HRGPs and the correlation ofincreased cell wall HRGP with increased pathogen resistance are documented (3, 4). The precise function of HRGPs in the plant's response to pathogens is unknown, but they may provide a structural matrix for lignification, or they may agglutinate microbial pathogens (5, 9). We have analyzed the structure and expression ofmembers of a gene family that encodes HRGPs in sunflower (Helianthus annuus) (Table I, Fig. 1). Reconstruction experiments indicated approximately seven HRGP genes in the sunflower genome. The DNA sequence ofone family member, HaGX3, revealed a single open reading frame with no introns encoding a proline-rich protein 262 amino acids in length. A hydrophobic amino terminal sequence of 28 amino acids shares significant homology with a carrot extensin signal peptide. The S(P)4 repeat characteristic of the carrot HRGP (1) is present 21 times, usually embedded in larger motifs, VYKS(P)4 or VHLS(P)4. Additionally, degenerate S(P). motifs are also present in the sequence. Expression of mRNA from the sunflower HRGP gene family is complex. HRGP transcripts 1.7 kb in length are primarily expressed in leaf mRNA populations following exposure of sunflowers to ethylene. In contrast, the majority ofHRGP transcripts detected