Repair-Based Mechanism of Desiccation Tolerance

Desiccation tolerance of the moss Tortula ruralis is characterized by a desiccation-induced change in gene expression that becomes evident upon rehydration. As reported earlier, this change in gene expression is apparently brought about by a change in the control of translation and does not include a major shift in mRNA abundance. A full qualitative and quantitative analysis of the alteration in gene expression, which is characterized by the loss of (or greater than fivefold decrease in) the synthesis of 25 hydration (h) proteins and initiation (or greater than fivefold increase) of the synthesis of 74 rehydration (r) proteins, is given in this report. Exposure to a desiccating atmosphere, for times that result in varying levels of water loss, enabled the determination that the control of synthesis of r proteins is different from the control of synthesis of h proteins. The r and h protein synthesis responses are intemally coordinate, however. Similarly, the return to normal levels of h protein synthesis differs from that of the r proteins. The return to normal synthetic levels for all h proteins is synchronous, but the rate of loss of r protein synthesis varies with each individual r protein. Run-off translation of polysomes isolated from gametophytes during the drying phase demonstrates that there are no novel mRNAs recruited and no particular mRNA is favored for translation during desiccation. These findings add credence to the argument that translational control is the major component of the desiccation-induced alteration in gene expression in this plant, as discussed. Aspects of the response of protein synthesis to desiccation are consistent with the hypothesis that T. ruralis exhibits a repair-based mechanism of desiccation tolerance. Although many vascular plants are capable of producing certain specialized structures that are capable of withstanding desiccation (e.g. seeds, spores, or pollen), few can survive this stress if it is applied to their vegetative tissues. The vast majority of desiccation-tolerant plants belong to the less complex groups of the plant kingdom: algae, lichens, and bryophytes. All are poikilohydric and thus exhibit a rapid equilibration of their internal water content with the water poten1 Supported by the National Science Foundation grant DCB8819019. tial of the environment. A few ferns, and even fewer angiosperms, are tolerant of desiccation (6, 8). These plants exhibit modified poikilohydry, however, because most have features that permit them to reduce their rate of water loss and many cannot survive rapid desiccation (1, 12). Poikilohydric plants have been postulated to utilize a repair-based mechanism of desiccation tolerance (7) in contrast to a protection-based mechanism implied for desiccation-tolerant angiosperms (1, 24) and seeds (1 1). Our understanding of desiccation tolerance mechanisms in poikilohydric plants comes from studies involving the desiccation-tolerant bryophyte Tortula ruralis (as reviewed in ref. 7). The majority of these studies focus on the response ofgene expression to desiccation and rehydration. Gametophytes of T. ruralis rapidly lose the ability to synthesize proteins during the drying phase of a desiccation event (2, 3) as do gametophytes of other bryophytes (16, 27). Desiccation can occur so rapidly that polysomes are trapped on preexisting mRNAs (10), but the moss still survives this treatment. Synthesis of protective proteins during drying seems unlikely under these circumstances. Constitutive synthesis of protective proteins cannot be ruled out, however. T. ruralis and other desiccationtolerant mosses are not resistant to the damage to cellular integrity that is wrought by a desiccation-rehydration event, but unlike desiccation-intolerant mosses they repair that damage quickly upon rehydration (20). These observations led to the hypothesis of the presence of a desiccation tolerance mechanism based upon cellular repair in these poikilohydric plants. Further evidence for such a mechanism derives from work in which the events occurring during rewetting of dried moss were investigated. Protein synthesis recovers rapidly upon rehydration of desiccation-tolerant mosses, e.g. T. ruralis (3, 4, 14), Tortula norvegica, and Tortula caninervis (M. J. Oliver, unpublished data), Polytrichium commune (27), and Neckera crispa (16). RNA synthesis also rapidly recovers upon rehydration of dried T. ruralis (21). Ribosomes and rRNAs are stable during desiccation, and both conserved and newly synthesized ribosomes and rRNAs are quickly utilized in the formation of new polysomes upon rehydration of T. ruralis (13, 21, 22). mRNAs are also stable to desiccation and are rapidly utilized in protein synthesis upon rehydration (22,