Transcriptional down-regulation by abscisic acid of pathogenesis-related β-1,3-glucanase genes in tobacco cell cultures

Class I isoforms of beta-1,3-glucanases (beta GLU I) and chitinases (CHN I) are antifungal, vacuolar proteins implicated in plant defense. Tobacco (Nicotiana tabacum L.) beta GLU I and CHN I usually exhibit tightly coordinated developmental, hormonal, and pathogenesis-related regulation. Both enzymes are induced in cultured cells and tissues of cultivar Havana 425 tobacco by ethylene and are down-regulated by combinations of the growth hormones auxin and cytokinin. We report a novel pattern of beta GLU I and CHN I regulation in cultivar Havana 425 tobacco pith-cell suspensions and cultured leaf explants. Abscisic acid (ABA) at a concentration of 10 mu M markedly inhibited the induction of beta GLU I but not of CHN I. RNA-blot hybridization and immunoblot analysis 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 accumulation. Comparable inhibition of p-glucuronidase expression by ABA was observed for cells transformed with a tobacco beta GLU I gene promoter/beta-glucuronidase reporter gene fusion. Taken together, the results 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.