A wheat (Triticum aestivum) cDNA clone encoding a plastid-localized heat-shock protein.

Higher plants produce both high mo1 wt (in the range of 70,000-110,000) and LMW (in the range of 15,000-30,000) HSPs in response to heat-stress conditions. During periods of heat stress, plant cells produce an abundance of LMW HSPs that are encoded by gene families (Vierling, 1991). The physiological function of these proteins remains obscure, but the evolutionary conservation of this response in higher plants suggests that they perform some important function in protection of plants from heat. LMW HSP genes have been isolated from a variety of crop plants such as soybean, pea, carrot, wheat (Triticum aestivum), and corn. Recently, Vierling (1991) classified these proteins into four families, two of these encode cytoplasmic proteins, one represents endomembrane proteins, and one includes the plastid-localized HSP genes. The question of nonfortuitous association of LMW HSPs with plastids during and after heat shock has been addressed by severa1 research groups who concurred that HSPs with mo1 wts ranging from 21,000 to 28,000 are specifically localized in chloroplasts during and after heat stress (for review, see Vierling, 1991). These nuclear-encoded HSPs consist of an amino-terminal transit peptide region and a carboxylterminal mature peptide that has significant similarity with other cytoplasmic LMW HSPs. The translation products of these mRNAs are transported to plastids where they are associated with either the soluble stroma portion or the grana region of the thylakoid membranes. Comparison of the amino acid sequences of plastid-localized HSPs from petunia, Arabidopsis, pea, soybean, and corn has led to identification of three conserved regions designated I, 11, and 111 (Chen and Vierling, 1991). Regions I and I1 are also shared by cytoplasmic HSPs, but region 111 is exclusively present in plastidlocalized HSPs. The first monocotyledonous cDNA encoding a plastidlocalized HSP26 was isolated from corn by Nieto-Sotelo et al. (1990). Using this cDNA clone as a probe, we previously isolated a cDNA clone from wheat, which encodes an HSP of mo1 wt of 26,600 (Tahsp26.6a) (Weng et al., 1991). We