RNase Activity Decreases following a Heat Shock in Wheat Leaves and Correlates with Its Posttranslational Modification

Abstract

Heat shock results in a coordinate loss of translational efficiency and an increase in mRNA stability in plants. The thermally mediated increase in mRNA half-life could be a result of decreased expression and/or regulation of intracellular RNase enzyme activity. We have examined the fate of both acidic and neutral RNases in wheat seedlings that were subjected to a thermal stress. We observed that the activity of all detectable RNases decreased following a heat shock, which was a function of both the temperature and length of the heat shock. In contrast, no reduction in nuclease activity was observed following any heat-shock treatment. Antibodies raised against one of the major RNases was used in western analysis to demonstrate that the RNase protein level did not decrease following a heat shock, and the data suggest that the observed decrease in RNase activity in heat-shocked leaves may be doe to modification of the protein. Two-dimensional gel/western analysis of this RNase revealed three isoforms. The most acidic isoform predominated in control leaves, whereas the most basic isoform predominated in leaves following a heat shock and correlated with the heat-shockinduced reduction in RNase activity and increase in mRNA half-life. These data suggest that RNase activity may be regulated posttranslationally following heat shock as a means to reduce RNA turnover until recovery ensues.Following exposure to heat shock, gene expression undergoes substantial reprogramming, involving changes in transcription, translation, and RNA turnover (for review, see Lindquist, 1986). In addition to the well-studied alterations in transcriptional activity, there is a rapid disassembly of polyribosomes in many species that results in a reduction in the translation of nonheat-shock mRNAs (Storti et al., 1980;Lin et al., 1984; Gallie et al., 1995). Non-hsp mRNAs are not destroyed but are maintained in HSGs that include two major HSPs, HSP70 and HSP17 (Nover et al., 1983(Nover et al., ,1989) and the mRNAs are subsequently recruited for translation upon recovery (Storti et al., 1980). The HSGs associate with the cytoskeleton, forming perinuclear complexes in plants (Nover et al., 1989; Apuya and Zimmerman, 1992), in invertebrates (Leicht et al., 1986; Arrigo, 1987), and in vertebrates (Collier and Schlesinger, 1986).We have previously shown that heat shock causes a loss in the translational competence of nonheat-shock mRNAs, which is a consequence of the loss in the functional code-' This work was supported by a grant from the U.S. Department * Corresponding author; e-mail drgallie@citrus.ucr.edu; fax of Agriculture (NRICGP 95-37100-1618).

Regulation of Methylbenzoate Emission after Pollination in Snapdragon and Petunia Flowers

“…The involvement of post-translational modifications in the regulation of enzyme activities has been shown for several other plant enzymes (Khayat et al, 1993;Chang and Gallie, 1997;Rudrabhatla and Rajasekharan, 2002;Smith et al, 2002). Protein modifications, such as phosphorylation, methylation, carboxylation, glucosylation, acetylation, and prenylation, often contribute to the regulation of enzyme activities (Lillo et al, 1997;Siddiqui et al, 1998;Tanase et al, 2002).…”

Suppression of LX Ribonuclease in Tomato Results in a Delay of Leaf Senescence and Abscission

“…An effect on the activity of RNases has also been observed in plants, following abiotic stresses such as ozone and salinity (Gomes-Filho and Sodek, 1988;Booker, 2004). An observed decrease in RNase activity in wheat (Triticum aestivum) following heat shock was hypothesized to have a role in the observed increase in mRNA stability (Chang and Gallie, 1997), whereas reduction in activity of specific RNases in response to low-O 2 stress in maize (Zea mays) roots was suggested to facilitate conservation of nontranslating ribosomes and poorly translated mRNA (Fennoy et al, 1997).…”

RNase Activities Are Reduced Concomitantly with Conservation of Total Cellular RNA and Ribosomes in O2-Deprived Seedling Roots of Maize

“…There is also evidence that increased activity of certain RNases is involved in programmed cell death processes, such as xylem formation and endosperm utilization, during germination (Farkas, 1982;Green, 1994). In contrast, a reduction in activity of a 27-kD RNase of wheat leaves occurs in response to heat shock, concomitantly with a change in electrophoretic mobility that is possibly due to dephosphorylation (Chang and Gallie, 1997). These authors suggest that reduced RNase activity in response to heat shock may be involved in stabilization of non-heat-shock protein mRNAs.…”

“…Fennoy and J. Bailey-Serres, unpublished data). Organ-specific differences in RNase activities were also detected in Arabidopsis (Yen and Green, 1991) and wheat (Chang and Gallie, 1997). The activity levels of eight RNases of maize roots decreased in response to flooding (Table I).…”

“…The decrease in abundance of certain RNases after 6 h of O, deprivation may indicate that these proteins have a short half-life. Finally, reduced RNase activity could result from the inhibition of a specific posttranscriptional modification, as postulated for a 27-kD RNase of wheat leaves following heat shock (Chang and Gallie, 1997 Vol. 11 5, 1997 The reduction i n activity of RNases may be involved in reducing the overall turnover of R N A a n d preservation of ribosomes during O, deprivation.…”