Developmental control of small heat shock protein expression during pea seed maturation

Plants synthesize four classes of small heat shock proteins (sHSPs); two classes are targeted to the plastid and endoplasmic reticulum, respectively, and two are found in the cytoplasm. In this paper, we describe a new role for the two classes of cytoplasmic HSPs in maturing embryos of developing seeds. The expression of each class of sHSPs was examined in pea seeds grown under non-stress conditions using Western and Northern analysis. Class I and class II cytoplasmic sHSPs are coordinately expressed in the embryo and accumulate to levels seen in moderately heat-stressed leaves. Their induction in cotyledons coincides with the mid-maturation phase of seed development, and induction in axes roughly coincides with abscission of the seed from the ovary wall. Both classes of sHSPs persisted in cotyledons for 4 days after the onset of imbibition, but disappeared from axes shortly after germination. Neither class of cytoplasmic sHSP is expressed in non-embryonic organs associated with the seed. The timing and organ specificity of sHSP expression is paralleled by the expression of the corresponding mRNAs. Neither the plastid nor the endoplasmic reticulum sHSPs were consistently expressed during seed development, but both could be induced by heat-stressing the developing seed. Developmental regulation of the cytoplasmic sHSPs is evidence that these proteins function not only in responding to heat-stress but also during seed development and/or germination.