At the Edges of Photosynthetic Metabolic Plasticity—On the Rapidity and Extent of Changes Accompanying Salinity Stress-Induced CAM Photosynthesis Withdrawal

Abstract

The common ice plant (Mesembryanthemum crystallinum L.) is a facultative crassulacean acid metabolism (CAM) plant, and its ability to recover from stress-induced CAM has been confirmed. We analysed the photosynthetic metabolism of this plant during the 72-h response period following salinity stress removal from three perspectives. In plants under salinity stress (CAM) we found a decline of the quantum efficiencies of PSII (Y(II)) and PSI (Y(I)) by 17% and 15%, respectively, and an increase in nonphotochemical quenching (NPQ) by almost 25% in comparison to untreated control. However, 48 h after salinity stress removal, the PSII and PSI efficiencies, specifically Y(II) and Y(I), elevated nonphotochemical quenching (NPQ) and donor side limitation of PSI (YND), were restored to the level observed in control (C3 plants). Swelling of the thylakoid membranes, as well as changes in starch grain quantity and size, have been found to be components of the salinity stress response in CAM plants. Salinity stress induced an over 3-fold increase in average starch area and over 50% decline of average seed number in comparison to untreated control. However, in plants withdrawn from salinity stress, during the first 24 h of recovery, we observed chloroplast ultrastructures closely resembling those found in intact (control) ice plants. Rapid changes in photosystem functionality and chloroplast ultrastructure were accompanied by the induction of the expression (within 24 h) of structural genes related to the PSI and PSII reaction centres, including PSAA, PSAB, PSBA (D1), PSBD (D2) and cp43. Our findings describe one of the most flexible photosynthetic metabolic pathways among facultative CAM plants and reveal the extent of the plasticity of the photosynthetic metabolism and related structures in the common ice plant.

Plant Responses and Tolerance to Salt Stress: Physiological and Molecular Interventions

“…The rapid shutdown of the energy-demanding functional CAM seems to be one small component of the flexibility features. As we showed here, the metabolic flexibility of the common ice plant includes rapid and far-reaching changes [8].…”

Overexpression of McHB7 Transcription Factor from Mesembryanthemum crystallinum Improves Plant Salt Tolerance

“…However, in the McHB7-OE plants, not only were SOD and POD elevated under salt stress conditions, but the stress-related proteins, such as DHAR1 and P5SCB, were significantly induced by salt stress that would help plants remove extra ROS from chloroplasts and keep the photosynthesis stable. Moreover, the salinity-induced degradation of starch that was required for the synthesis of phosphoenolpyruvate (PEP) used in the primary CO 2 fixation was found and confirmed in chloroplasts of the ice plant [ 50 ]. Among the increased metabolites in OE lines after salt stress treatment, some metabolites were found to be involved in starch metabolism, thus providing a new insight into the function of Mc HB7 in plant photosynthesis by regulating starch metabolism.…”

How the Ethylene Biosynthesis Pathway of Semi-Halophytes Is Modified with Prolonged Salinity Stress Occurrence?

“…Moreover, the common ice plant is a well-recognized CAM facultative plant model with an extremely high plasticity of photosynthetic apparatus. Recent reports confirmed that the common ice plant's mesophyll cells rapidly (up to 24 h) undergo the process of functional CAM withdrawal, involving broad rearrangements of PSI (photosystem I) and PSII (photosystem II) protein expression, chloroplast ultrastructure, and photochemistry performance in response to salinity stress removal [35][36][37][38][39]. The features mentioned above of the plant model allow the study of another important aspect determining the regulation of ET biosynthesis, and consequently also affecting the ET regulatory system, parallel to changes in the intracellular concentration of CO 2 .…”