Local and Systemic Changes in Photosynthetic Parameters and Antioxidant Activity in Cucumber Challenged with Pseudomonas syringae pv lachrymans

Abstract

We studied changes in gas exchange, photochemical activity and the antioxidant system in cucumber leaves locally infected with Pseudomonas syringae pv lachrymans and in uninfected systemic ones. Infection-induced declined net photosynthesis rate and the related changes in transpiration rate, the intracellular CO2 concentration, and prolonged reduction in maximal PSII quantum yield (Fv/Fm), accompanied by an increase in non-photochemical quenching (NPQ), were observed only in the infected leaves, along with full disease symptom development. Infection severely affected the ROS/redox homeostasis at the cellular level and in chloroplasts. Superoxide dismutase, ascorbate, and tocopherol were preferentially induced at the early stage of pathogenesis, whereas catalase, glutathione, and the ascorbate–glutathione cycle enzymes were activated later. Systemic leaves retained their net photosynthesis rate and the changes in the antioxidant system were partly like those in the infected leaves, although they occurred later and were less intense. Re-balancing of ascorbate and glutathione in systemic leaves generated a specific redox signature in chloroplasts. We suggest that it could be a regulatory element playing a role in integrating photosynthesis and redox regulation of stress, aimed at increasing the defense capacity and maintaining the growth of the infected plant.

Alterations in Primary Carbon Metabolism in Cucumber Infected with Pseudomonas syringae pv lachrymans: Local and Systemic Responses

“…In our experimental system, the Psl infection-induced angular leaf spot disease symptoms, in the form of angular necrotic lesions with extensive chlorosis ( Figure A1 ), were restricted to the inoculated third leaf (L3 + Psl ) and never spread to the other leaves and the stem [ 1 ].…”

“…The unchanged expression of 3PGAD in the early phase of pathogenesis (1–2 day), accompanied by downregulation of RuBisCO LU , allowed redistribution of the reducing power in the chloroplasts by decreasing the NAD(P)H consumption by 3PGAD; therefore, favoring activation of defense responses in L3 + Psl . In the systemic leaves, however, its increased transcript accumulation at 2 day, concomitant with RuBisCO LU , was associated with the maintenance of high photosynthetic activity [ 1 ]. Interestingly, the chloroplast 3PGAD, apart from participating in photosynthetic CO 2 fixation, is involved in plant innate immune response.…”

“…The plant immune system comprises both constitutive and induced defense responses. The constitutive defense is based on preformed structural barriers, such as a waxy epidermal cuticle, rigid cell walls, and antimicrobial secondary metabolites [ 1 , 2 , 3 ]. The induced defense responses, activated by recognition of pathogen-associated molecular patterns (PAMPs) through plant cell surface-anchored pattern recognition receptors (PRRs), form the pattern-triggered immunity (PTI).…”

“…These metabolic adjustments are usually related to the pathogen-induced alterations in the source-sink relationships in the infected plant. In interactions with several pathogens, the inhibition of photosynthesis is often confined to the infection sites, whereas in plant tissues surrounding the infected areas and in uninfected distal leaves, it is upregulated or unchanged [ 1 , 12 , 15 ].…”

“…syringae pv lachrymans ( Psl ) pathosystem—we found a pathogen-induced decline in the photochemical activity of PSII, stomatal conductance, and photosynthetic gas exchange at the advanced stage of pathogenesis. These changes occurred in the infected leaves, along with full disease symptom development, and not in the pathogen-free systemic leaves and were also related to redox regulations [ 1 ].…”