Roles of ROS scavenging enzymes and ABA in desiccation tolerance in ferns.

Abstract

The fern species Loxogramme abyssinica and Crepidomanes inopinatum occur widely in KwaZulu Natal Afromontane forests. Although both species are suspected to be desiccation tolerant (DT), little is known about their mechanisms of tolerance. The present study was undertaken to characterize DT in both ferns, specifically the roles of the reactive oxygen species (ROS) scavenging enzymes superoxide dismutase (SOD) and peroxidase (POX), sugar afccumulation, and abscisic acid. Broadly speaking, DT mechanisms are either constitutive (always present) or induced in response to stress. C. inopinatum dries rapidly because it is a “filmy” fern, lacking a cuticle; in contrast L. abyssinica possesses a cuticle, and therefore dries slowly. It was predicted that the fast-drying C. inopinatum would rely mainly on constitutive mechanisms, while the slow-drying L. abyssinica would depend on inducible mechanisms. Plant were collected from the field, transported to the laboratory and then subjected to desiccation. Two methods of desiccation were used. The first one was a relatively mild, slow and short term (48 h) desiccation over calcium acetate, and the second was a harsher, rapid and long term (1 week) desiccation over silica gel. Measurement of chlorophyll fluorescence parameters showed that both species displayed rapid recovery during rehydration after slow or fast desiccation, confirming that both species are genuinely poikilohydric. POX activity remained constant in both species during slow desiccation and subsequent rehydration, suggesting that ROS scavenging by POX is a constitutive DT mechanism. However, the absolute POX activity of C. inopinatum was much higher than in L. abyssinica. Rapid long term desiccation reduced POX activity in both species, but the activity recovered during rehydration. In both species, slow desiccation increased SOD activity, and activity declined to original values during rehydration. Slow and fast drying increased the concentrations of soluble sugars in both species, and concentrations rapidly declined to initial values during rehydration. ABA pretreatment had little effect on DT in either species, although tolerance was slightly increased in L. abyssinica. Results of this study suggested that both species depend largely on inducible DT mechanisms. Counter to the original hypothesis, inducible mechanisms occur even in filmy ferns that desiccate rapidly. The results from this project will contribute to our understanding of how ferns can survive in stressful environments in South Africa, and potentially could help improve abiotic stress tolerance in crop plants.