TRANSCRIPTIONAL DOWN-REGULATION BY ABSCISIC-ACID OF PATHOGENESIS-RELATED BETA-1,3-GLUCANASE GENES IN TOBACCO CELL-CULTURES

Class I isoforms of beta-1,3-glucanases (beta GLU I) and chitinases (C HN I) are antifungal, vacuolar proteins implicated in plant defense. T obacco (Nicotiana tabacum L.) beta GLU I and CHN I usually exhibit tig htly coordinated developmental, hormonal, and pathogenesis-related reg ulation. Both enzymes are induced in cultured cells and tissues of cul tivar Havana 425 tobacco by ethylene and are down-regulated by combina tions of the growth hormones auxin and cytokinin. We report a novel pa ttern of beta GLU I and CHN I regulation in cultivar Havana 425 tobacc o pith-cell suspensions and cultured leaf explants. Abscisic acid (ABA ) at a concentration of 10 mu M markedly inhibited the induction of be ta GLU I but not of CHN I. RNA-blot hybridization and immunoblot analy sis showed that only class I isoforms of beta GLU and CHN are induced in cell culture and that ABA inhibits steady-state beta GLU I mRNA acc umulation. Comparable inhibition of p-glucuronidase expression by ABA was observed for cells transformed with a tobacco beta GLU I gene prom oter/beta-glucuronidase reporter gene fusion. Taken together, the resu lts strongly suggest that ABA down-regulates transcription of beta GLU I genes. This raises the possibility that some of the ABA effects on plant-defense responses might involve beta GLU I.