Desiccation-tolerant plants can be grouped into two categories: the 1) desiccation-tolerant plants whose internal water content rapidly equilibrates to the water potential of the environment and 2) the modified desiccation-tolerant plants that all employ mechanisms to retard and control the rate of water loss. Desiccation tolerance can be achieved by mechanisms that incorporate one of two alternatives, viz. cellular protection or cellular recovery (repair). The majority of plants probably utilize aspects of both. Desiccation-tolerant species, in particular the moss Tortula ruralis, appear to utilize a tolerance strategy that combines a constitutive protection system and a rehydration-inducible recovery mechanism. The rehydration-induced recovery mechanism of Tortula ruralis relies heavily upon a change in gene expression that is mediated by posttranscriptional events rather than the slower reacting transcriptional controls. Findings indicate that it takes a certain amount of prior water loss to fully activate the protein-based portion of the recovery mechanisms upon rehydration. Utilizing cDNAs representing individual hydrins (proteins whose synthesis is hydration specific) and rehydrins (proteins whose synthesis is rehydration specific), it was determined that if drying rates were slow rehydrin transcripts selectively accumulate in the dried gametophytes. Studies revealed that this storage involves the formation of mRNPs (messenger ribonucleoprotein particles). The identity and possible functions of the rehydrins of Tortula ruralis are also under investigation, in particular Tr155, a small rehydrin (24kD) appears to be involved in antioxidant production during rehydration.
Stress response genes including heat shock proteins are induced under a variety of conditions to confer cellular protection. This study investigated the role of calcium signaling in the induction of two stress response genes, heme oxygenase-1/hsp32 and hsp70, in isolated rat hepatocytes. Both genes were induced by cellular glutathione depletion. This induction could be inhibited by BAPTA-AM. Culturing in a calcium-free medium prevented the induction of hsp70 gene expression after glutathione depletion without affecting heme oxygenase-1 gene expression. Thapsigargin increased the gene expression of heme oxygenase-1 but not that of hsp70. Thapsigargin-induced heme oxygenase-1 induction was completely inhibited by BAPTA-AM. Incubation with the Ca2+-ionophore A23187 augmented heme oxygenase-1 (two-fold) and hsp70 (5.2-fold) mRNA levels. Our data suggests a significant role of Ca2+-dependent pathways in the induction of the two stress genes. An increase in the cytoplasmic Ca2+ activity seems to play a key role in the cascade of signaling leading to the induction of the two genes. However, the source of Ca2+ that fluxes into the cytoplasm seems to be different. Our data provides evidence for a compartmentalization of calcium fluxes, i.e. the Ca2+ flux from intracellular stores (e.g. the endoplasmic reticulum) plays a major role in the induction of heme oxygenase-1. By contrast, Ca2+ flux from the extracellular medium seems to be a mechanism initiating the cellular signaling cascade leading to hsp70 gene induction.