Linoleic acid-induced expression of defense genes and enzymes in tobacco.

Linoleic acid (LA) is a naturally occurring fatty acid (FA) found to elicit induced systemic resistance (ISR) of tobacco against the bacterial soft rot pathogen, Pectobacterium carotovorum subsp. carotovorum (PCC). In this study, we examined effects of six doses of exogenous LA on the induction of defense genes and enzymes. The optimum ISR activity was observed in plants treated with 0.1mM LA where the effect of LA on membrane permeability was minimal. The application of LA as a root drench enhanced the activity of defense enzymes such as phenylalanine ammonia-lyase (PAL), peroxidase (POD), and polyphenol oxidase (PPO) and induced the expression of ß-glucuronidase (GUS). PAL and POD activities were increased in a concentration dependent manner while the maximum PPO activity was observed after treatment with 0.01mM LA. An RT-PCR analysis of the defense-related genes, Coi1, NPR1, PR-1a and PR-1b, of tobacco plants treated with 0.1mM LA revealed an association of LA with elicitation of ISR in tobacco.

Adverse effects of nitrate on stem nodules of Sesbania rostrata Brem.

When the stems of Sesbania rostrata, growing vigorously in nutrient solution, are inoculated with Azorhizobium caulinodans ORS 571 and deprived of nitrate, hundreds of nodules with active N2 fixation (acetylene reduction) develop on each stem within 10 d. The numbers of nodules that develop are not affected by increasing the nitrate concentration in the medium from 0 to 9 mM. However, as the nitrate concentration is raised above about 2 mM the growth of the nodules is strongly inhibited and all aspects of acetylene reduction activity (ARA) are progressively curtailed as the nitrate concentration is increased from zero. In 6 mM nitrate, effects are seen at the earliest possible time of measurement (6 d after inoculation) with maximum inhibition of growth and ARA (about 80%) at 10 d. In plants with stem nodules that have developed in the absence of nitrate since inoculation, the addition of 9 mM nitrate leads to inhibition of further nodule growth and to prompt decrease in specific ARA. The 'growth' and 'activity' responses of the stem nodules of Sesbania are similar in degree and sensitivity to those of root nodules of other legumes and thus they can no longer be considered as resistant to fixed N.