Systemic Induction of Salicylic Acid Accumulation in Cucumber after Inoculation with Pseudomonas syringae pv syringae

Abstract

Inoculation of one true leaf of cucumber (Cucumis sativus L.) plants with Pseudomonas syringae pathovar syringae results in the systemic appearance of salicylic acid in the phloem exudates from petioles above, below, and at the site of inoculation. Analysis of phloem exudates from the petioles of leaves 1 and 2 demonstrated that the earliest increases in salicylic acid occurred 8 hours after inoculation of leaf 1 in leaf 1 and 12 hours after inoculation of leaf 1 in leaf 2. Detaching leaf I at intervals after inoculation demonstrated that leaf I must remain attached for only 4 hours after inoculation to result in the systemic accumulation of salicylic acid. Because the levels of salicylic acid in phloem exudates from leaf I did not increase to detectable levels until at least 8 hours after inoculation with P. s. pathovar syringae, the induction of increased levels of salicylic acid throughout the plant are presumably the result of another chemical signal generated from leaf I within 4 hours after inoculation. Injection of salicylic acid into tissues at concentrations found in the exudates induced resistance to disease and increased peroxidase activity. Our results support a role for salicylic acid as an endogenous inducer of resistance, but our data also suggest that salicylic acid is not the primary systemic signal of induced resistance in cucumber.Inoculation of one leaf of cucumber plants and other cucurbits with necrotic lesion-inducing pathogens (7-9) or necrosis/chlorosis-inducing chemicals (3, 4) results in the expression of systemic resistance against disease caused by a number of pathogens. The onset of resistance has been correlated with the initial appearance of necrotic lesions and generally begins to develop 3 to 4 d after the resistance-inducing inoculation (7-9).We have recently demonstrated that systemic resistance can be induced in cucumber within 24 h by inoculating leaf with the HR2-inducing bacterium Pseudomonas syringae pv syringae (17 demonstrated that this leaf must remain attached for only 6 h to result in the systemic expression of enhanced peroxidase activity and a small, but detectable, increase in the level of systemic disease resistance. Allowing the first leaf to remain on the plant for up to 12 h after inoculation with P. s. pv syringae resulted in a further increase in the level of systemic resistance as compared with plants that had the inoculated first leaf detached 6 h after inoculation. TnS mutants of P. s. pv syringae that had lost the ability to induce the HR were also unable to induce systemic resistance and peroxidase activity.Dean and Kuc (1, 2) have provided strong evidence that the systemic signal(s) for induced resistance was generated in and mobilized out of the leaves that were initially inoculated ("source" leaves) with resistance-inducing pathogens. Metraux et al. ( 12) recently reported that cucumber plants inoculated with either Colletotrichum lagenarium or tobacco necrosis virus on one leaf had higher levels of salicylic acid (an exogenous inducer ofres...

Regulation of the Wound‐Induced Myrosinase‐Associated Protein Transcript in Brassica Napus Plants

“…However, the concentration of SA used in this study is not necessarily unphysiological if compared to the concentration of several 100 pM SA reported in petiole exudates from Pseudomonas-syringae-infected cucumber (Rasmussen et al, 1991) or the 8-pg total SNfresh mass reported from l? syringae infected A. thaliana (Lawton et al, 1995).…”

Accumulation of Salicylic Acid and 4-Hydroxybenzoic Acid in Phloem Fluids of Cucumber during Systemic Acquired Resistance Is Preceded by a Transient Increase in Phenylalanine Ammonia-Lyase Activity in Petioles and Stems1

“…In addition, cucumber plants inoculated on one leaf with the HR-causing pathogen Pseudomonas syringae pv. syringae accumulated high levels of SA in phloem exudates even when the inoculated leaf remained on the plant for only 6 h (Rasmussen et al, 1991). Maximal levels of SA were measured 18 h after inoculation in this system, so the majority of systemically accumulating SA was not synthesized by the inoculated leaf.…”

“…Although the advantages of Arabidopsis as a genetic tool are well documented, the cucumber system offers several advantages for the biochemical study of SAR. First and most importantly, the mobile signal for SAR is produced by cucumber leaves within 6 h after inoculation with incompatible bacteria, providing a narrow time frame within which to look for an active compound (Rasmussen et al, 1991;Smith et al, 1991). Second, the large leaves of cucumber are easy to saturate with inoculum and produce correspondingly large amounts of mobile signal.…”

Compromising early salicylic acid accumulation delays the hypersensitive response and increases viral dispersal during lesion establishment in TMV‐infected tobacco

“…Research into the possible systemic signals responsible for initiating acquired resistance has suggested salicylic acid (SA) to be a likely candidate. Exogenous application of SA induces PR protein gene expression in tobacco (Ward et aL, 1991), cucumber (Metraux et aL, 1990) and Arabidopsis thafiana (Uknes et al, 1993b); whilst endogenous SA levels rise locally around lesions following pathogen attack in tobacco (Malamy et al, 1990;Enyedi et aL, 1992), Arabidopsis thaliana (Uknes et al, 1993b) and also in cucumber, where SA levels were observed to rise in phloem exudates (Metraux et aL, 1990;Rasmussen et al, 1991).…”