Nitrate reductase activity during desiccation and rehydration of the desiccation-tolerant moss Tortula ruralis

Abstract

Water stress reduces the activity of many plant enzymes. When the stress is alleviated there is generally a delay in the recovery of metabolism to unstressed levels. The enzyme nitrate reductase (EC 1.6.6.1) declines in plants experiencing water deficits and upon the alleviation of water stress its activity recovers to control levels in 1–7 days. In desiccation-tolerant plants, metabolic recovery can occur within hours following rehydration. It has been proposed that metabolic characteristics of desiccation-tolerant plants are a source of information that can be used to improve the performance of crop plants under water deficits. In this study, the effect of desiccation and hydration on nitrate reductase activity in the desiccation-tolerant moss Tortula ruralis has been investigated. It was expected that the activity of nitrate reductase would decline during desiccation and recover rapidly following rehydration. Inclusion of nitrate in the hydration medium increased the activity in the tissue. Nitrate reductase activity increased with nitrate concentration in the bathing medium up to levels of 200 mM. The highest activity measured in rehydrated moss samples was 5.23 nkat g −1 dry weight (dw). The activity declined rapidly during dehydration and was not detectable in dried moss samples after 24 h of dehydration. The recovery of activity following rehydration was dependant on the rate of the preceding dehydration. In slowly dried moss, the activity recovered to control levels in less than 8 h while rapidly dried samples required 24 h for full recovery. Nitrite accumulated during slow dehydration but did not accumulate when desiccation was rapid. Following rehydration of slowly dried moss, the amount of nitrite declined and reached a control level within 1 h. It was proposed that nitrite accumulation might provide a source of nitrogen for metabolism during the time required for nitrate reduction to resume following rehydration. Attempts to measure immunologically reactive nitrate reductase protein levels or nitrate reductase mRNA levels using heterologous DNA probes were unsuccessful, suggesting that the Tortula nitrate reductase may be significantly different from that found in algae and higher plants.