Small heat-shock proteins (sHSPs) accumulate in plants in response to high-temperature stress. Specific sHSPs, the cytosolic class I and class II proteins, are also expressed in the absence of stress in maturing seeds of several species, and a role for these proteins in desiccation tolerance, dormancy, or germination has been hypothesized. We demonstrate that class I sHSPs are expressed during Arabidopsis seed development in a pattern similar to that previously observed in other species: they are first detected during mid-maturation, are most abundant in dry seeds, and decline rapidly during germination. Although the class I sHSP family in Arabidopsis appears to consist of four genes, expression of a single gene, Athsp 17.4, accounts for the majority of sHSPs in maturing seeds. sHSP levels were also examined in seeds of several Arabidopsis mutants with reduced sensitivity to abscisic acid inhibition, including aba1, abi1, and abi2, abi3-1, abi3-6, abi4, and abi5-1. The abi3-1 mutant has 10-fold reduced levels of sHSPs; sHSPs are undetectable in the abi3-6 mutant. All other mutants were indistinguishable from wild type. These results suggest that sHSP expression in seeds is regulated by the ABI3 response pathway and wild-type levels of sHSPs are not sufficient for seed dormancy and not necessary for desiccation tolerance. However, roles in either process cannot be ruled out. In total the data indicate that the expression of sHSPs in seeds is part of the normal developmental program of late seed maturation and the presence of sHSPs has adaptive significance for plant reproduction.
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