Two-dimensional gels of in vitro translation products of mRNAs isolated from quiescent wheat (Triticum aestivum) embryos demonstrate the presence of mRNAs encoding heat shock proteins (hsps). There were no detectable differences in the mRNAs found in mature embryos from field grown, from 250C growth chamber cultivated, or from plants given 38°C heat stresses at different stages of seed development. The mRNAs encoding several developmentally dependent (dd) hsps were among those found in the dry embryos. Stained two-dimensional gels of proteins extracted from 250C growth chamber cultivated wheat embryos demonstrated the presence of hsps, including dd hsps. A study of the relationship of preexisting hsp mRNAs and the heat shock response during early imbibition was undertaken. Heat shocks (420C, 90 minutes) were administered following 1.5, 16, and 24 hours of 250C imbibition. While the mRNAs encoding the low molecular weight hsps decayed rapidly upon imbibition, the mRNAs for dd hsps persisted longer and were still detectable following 16 hours of imbibition. After 1.5 hours of imbibition, the mRNAs for the dd hsps did not accumulate in response to heat shock, even though the synthesis of the proteins was enhanced. Thus, an applied heat shock appeared to lead to the preferential translation of preexisting dd hsp mRNAs. The mRNAs for the other hsps, except hsp 70, were newly transcribed at all of the imbibition times examined. The behavior of the hsp 70 group of proteins during early imbibition was examined by RNA gel blot analysis. The mRNAs for the hsp 70 group were detectable at moderate levels in the quiescent embryo. The relative level of hsp 70 mRNA increased after the onset of imbibition at 250C and remained high through 25.5 hours of prior imbibition. The maximal levels of these mRNAs at 250C was reached at 17. stresses are endured without leaves and a root system, which help moderate extremes of temperature and water status, respectively, in postemergent plants. The study of heat stress responses during early germination has proven interesting in regard to both of these problems. We found that wheat (Triticum aestivum L.) embryos are able to synthesize a full set of hsps3 from the earliest times of imbibition. Early imbibing embryos also synthesize several hsps which cannot be produced after 12 h of imbibition (1 1). Further, embryos imbibed less than 8 h are resistant to heat stresses that are lethal to embryos imbibed 12 or more h (1). These findings suggest that stress proteins and/or their mRNAs may be present in dry embryos. If present, these proteins or mRNAs might contribute both to the strong hs response and the high thermal tolerance found during early imbibition.In order to learn more about the developmental nature of several wheat hsps, as well as the high thermal tolerance of early imbibed wheat, we studied the protein and mRNA composition of quiescent and early imbibed embryos, as well as the way in which the embryos responded to heat stress during early imbibition. Further, we examined...

During the initial 9 to 12 hours of imbibition, the imbibing wheat (Trfticum aestivum L.) seed was found to exhibit substantial tolerance to high temperature relative to later times of imbibition. Tolerance was assessed by seed viability and seedling growth. This initial high temperature tolerance gradually declines with increasing time of seed imbibition. A range of 2 hour heat pretreatments (38-420C) prior to imposition of a 2 hour heat shock (51-530C) during this same 9 to 12 hour interval was unable to increase survival or seedling growth over that of seed that did not receive a pretreatment. However, after 9 to 12 hours of imbibition the pretreatment provided both increased survival and increased seedling growth, measured 120 hours later, i.e., classical thermotolerance could be acquired. This response is called a 'thermotolerance transition.' Isolated embryos responded in a similar manner using a 2,3,5-triphenyltetrazolium chloride assay for viability determination following heat treatments. The high temperature tolerance during early imbibition indicates that the thermotolerance transition involves the loss of an existing thermotolerance coincident with acquiring the ability to become thermotolerant following heat pretreatment. Despite the inability to acquire thermotolerance, heat shock protein synthesis was induced by heat shock immediately upon imbibition of wheat seed or isolated embryos. Developmentally regulated heat shock proteins of 58 to 60, 46, 40, and 14 kilodaltons were detected at 1.5 hours of imbibition following heat shock, but were absent or greatly reduced by 12 hours. Constitutive synthesis of 70 and 90 kilodalton hsp groups appeared to be greater at 1.5 hours of imbibition than at 12 hours of imbibition.

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For comparison of seedling growth competitive responsa~ in a controlled environment, monocultures (intraspecific) and 2 species mixtures (interspecific) of mountain rye (Secuk mou&mum), crested wheatgrass (Agropyron cristatum x deseftorlmr 'Hycrest~, and downy brome (Bromus tectorum) were established. Seedling dry root and sboot weights, shoot area, and maximum root length were compared at 1,2,4, and 6 weeks of growth in shoot roots boxes under a growth chamber environment (16 hr @ 14" C, 1,060 PE m-2 set-1; 8 hr @ 100 C, dark). Soil moisture depletion was monitored gavimetrically. Dry root and shoot weight, sboot area, and root length of mountain rye was greater tban that of both downy brome and Hycrest crested wheatgrass at every sampiing period over the 6-week study when grown in two-species mixtures. No difference was obtained for these seedling growth characters between downy brome and Hycrest mixtures, except for a 6.4 cm vs. 4.8 cm maximum root length at 1 week of growth. Similarly, in monoculture, mountain rye generally produced greater seediing growth than the other 2 species, although exceptions occurred for root weight, shoot area, and root length by 6 weeks of growth. Mountain rye depleted soil moisture in tire growth boxes more rapidly and to a lower potentiai than tire other 2 species. The results of this study indicate mountain rye provide vigorous competition asaseedllhg.

Seeds frequently face a hostile environment during early germination. In order to determine whether seeds have evolved unique mechanisms to deal with such environments, a survey of the heat shock response in isolated embryos of wheat (Triticum aestivum L.) was undertaken. Embryos simultaneously heat shocked and labeled following several different periods of prior imbibition up to 12 hours synthesized many groups of heat shock proteins (hsps) typical of other plant and animal systems. Also, five developmentally dependent hsps, present only in treatments imbibed less than 6 hours prior to heat shock, were detected. These proteins have relative molecular masses of 14, 40, 46, 58, and 60 kilodaltons. One of the developmentally dependent hsps is among the most highly labeled hsps found in eardy imbibed embryos. The possibility that this protein is the Em protein is discussed. The hypothesis that the capacity for hsp synthesis is affected by seed vigor was also tested. The heat shock responses of embryos from two high and two low vigor seed lots were compared using one-and two-dimensional electrophoresis of labelled protein extracts. The results indicate that both of the low vigor lots tested had weaker heat shock responses than their high vigor counterparts overall. Not all hsps were relatively less abundant in low vigor embryos. The developmentally dependent hsps showed little relationship to vigor. Some of the developmentally dependent hsps were actually made in greater amounts, relative to other proteins, in the low vigor seed lots. The results presented here demonstrate that imbibing embryos are capable of expressing an enhanced heat shock response, and that this response is related to seed vigor. Upon imbibition, the quiescent seed embryo faces a hostile environment. Extremes of temperature and moisture may confront the young plant simultaneously or in rapid succession. The imbibing embryo must survive this environment in order to germinate. Since the embryo has no developed leaf or root system, it is unable to regulate its temperature and water status through control of transpiration as do mature plants. In addition to a potentially hostile environment, the embryo must deal with the problem of germination itself. Mem-' Research was supported in part by National Science Foundation grant RII-8610680 (R. H. A.) and United States Department of Agriculture Hatch Funds to the Wyoming Agricultural Experiment Station. This paper is Wyoming AES number JA-1582. branes must go from a disorganized state to the ordered bilayer found in functional cells. Mitochondria, owing in large measure to the disorganization of their membranes, are inactive in the quiescent embryo and must be repaired or replaced in order for germination to occur. Polysomes must be assembled, and new protein and nucleic acid synthesis are initiated immediately upon hydration (6). Osborne (18) has demonstrated that the DNA of quiescent rye embryos accumulates nicks which must also be repaired in order for normal development of the young plant to proceed....