| Potato tubers respond to environmental stress by the induction and accumulation of distinct stress-response RNAs. Messages that are newly synthesized in response to wounding such as phenylalanine ammonialyase (PAL) and extensin (EXT) are polysome-associated in wounded tubers. The translation products of these mRNAs are known to be an integral part of the tuber wound response. By contrast, the sucrose-induced tuberization messages (patatin and proteinase inhibitor II) that are present prior to wounding are quickly released from the ribosomes and immediately degraded upon wounding. Further, a third condition, hypoxia, exhibits a different response. Newly synthesized, hypoxically induced mRNAs such as those encoding alcohol dehydrogenase (ADH) and aldolase (ALD) accumulated within the cell and are polysome associated. When wounded tubers are subjected to hypoxia, the wound-response mRNAs are not translated. However, these nontranslated messages remain bound in polyribosomal complexes and are not degraded. The cell therefore exhibits a hierarchal preference for the translation of specific messages under different environmental conditions. These data indicate that there are at least two distinct mechanisms for regulating this selective translation.; Two proteins have been detected by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of isolated polysomes that may be involved in the selection process. A 32 kilodalton (kDa) protein is associated with polysomes isolated from wounded tissue, but not unwounded tissue. This protein is phosphorylated immediately after wounding. The putative kinase activity that is responsible for phosphorylation of this protein is diminished by 24 hours after wounding, although the 32 kDa protein remains bound, presumably in its phosphorylated form, for at least 48 hours after wounding. This protein is not associated with polysomes isolated from hypoxic, nonwounded tubers. Instead, these polysomes display a 48 kDa protein that is not phosphorylated. Polysomes isolated from wounded, hypoxic tubers display both proteins, although the 32 kDa protein is not phosphorylates under hypoxic conditions. These two proteins may mediate the incorporation of specific stress-response mRNA into polyribosomal complexes in a highly coordinated manner. By understanding how this system of regulation works, we shall be better able to engineer more resistant varieties of crop plants. |
