EXTENSIN GENE-EXPRESSION IS INDUCED BY MECHANICAL STIMULI LEADING TO LOCAL CELL-WALL STRENGTHENING IN NICOTIANA-PLUMBAGINIFOLIA

Nicotiana plumbaginifolia Viv. harbors a single extensin gene, althoug h related hydroxyproline-rich sequences are present in the genome. Nor thern analysis showed that the gene is highly expressed in roots and t o a lesser extent in stems. Expression in leaves is low but mRNA level s are increased upon infection with the incompatible bacterium Pseudom onas syringae. Extensin transcript levels in leaves were slightly enha nced after wounding and salicylic acid treatment. In-situ hybridizatio n experiments showed high accumulation of extensin mRNA in cells which , at certain stages of development, require reinforcement of their cel l walls. The cortical cells in stem nodes and roots, which are put und er severe mechanical stress by adjacent developing tissues, tend to ex press the gene to high levels. Immunolocalization of the extensin prot ein in stems and roots demonstrated a close association of the protein with lignin deposition. Mature tissues contained more extensin than y ounger tissues. The extensin promoter was fused to the beta-glucuronid ase gene.