A heat-tolerant maize (Zea mays L.) line, ZPBL 1304, synthesizes a unique set of five heat-shock polypeptides of 45 kDa. Previous studies suggested that these polypeptides might play a role in the development of thermotolerance in maize (Ristic et al., 1996, J. Plant Physiol. 149:424-432; Ristic et al., 1998, J. Plant Physiol. 153:497-505). In the present study, we isolated these polypeptides, sequenced them, and investigated their subcellular distribution and origin. Of the five polypeptides of 45 kDa, three polypeptides, including the two most abundant ones, yielded amino acid sequences similar to the chloroplast and bacterial protein synthesis elongation factor (EF-Tu). This was further confirmed using an antibody raised against maize EF-Tu, which showed a very strong reaction with the 45-kDa heatshock protein(s). Studies on subcellular distribution and origin revealed that the 45-kDa polypeptides were localized to the chloroplasts, and were likely of nuclear origin. A full-length maize EF-Tu cDNA (Zmeftu1), previously isolated from the B73 line of maize, was used as a probe for northern blot analysis of RNA extracted from the ZPBL 1304 maize line (the nucleotide and deduced amino acid sequences of Zmeftu1 are 88% identical to the rice EF-Tu sequence). Northern blots showed a 1.85-fold increase in steady-state levels of EF-Tu mRNA during heat stress. An increase in EF-Tu transcript levels during heat stress was accompanied by increased levels of the EF-Tu protein. Isolated chloroplasts from heat-stressed plants also had higher levels of EF-Tu as compared to control chloroplasts. The maize EF-Tu polypeptides showed > 80% sequence similarity with the bacterial EF-Tu, which has recently been shown to function as a molecular chaperone and to play a role in the protection of other proteins from thermal denaturation (Caldas et al., 1998, J. Biol. Chem. 273:11478-11482). It is hypothesized that chloroplast EF-Tu of the ZPBL 1304 maize line plays an important role in the development of thermotolerance.

The development of microspores and the associated changes in the tapetum were examined in the normal (+/+) and male-sterile, stamenless-2 (sl-2/sl-2) mutant anthers of tomato (Lycopersicon esculentum). Anthers of eight comparable stages, from the microspore mother cell stage to anthesis, of both lines were processed for light microscopy. Until the formation of tetrads (stage ii), there were no differences in the sporogenous tissue, but the tapetal cells of the mutant were more enlarged than the normal and had, at places, divided to form a bilayer. Later, the tapetal cells in both lines became amoeboid and had sporopollenin-like deposits. At stage iv, whereas the tapetal cells of the normal had started to degenerate, those of the mutant were intact but had large vacuoles. Also at this stage, the deposition of exine was evident in normal microspores, but it was lacking in most mutant microspores, which enlarged considerably and eventually degenerated. From stage v onwards, the normal microspores progressed from the binucleate pollen to pollen containing many vacuoles to mature pollen. In the mutant, tapetum degeneration was delayed until stage v, and later, although some microspores closer to the tapetum appeared normal, most either were empty or had large vacuoles. It is suggested that the delay in tapetum degeneration coupled with the failure of exine deposition, presumably associated with low esterase activity, is responsible for pollen degeneration in the sl-2/sl-2 mutant.

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