Defective endosperm ($De^*$)-B30 is a dominant maize mutation in the gene that encodes the storage protein, ${alpha}$-zein protein. The $De^*$-B30 mutation results in a defective signal peptide in a 19-kD ${alpha}$-zein protein, which triggers endoplasmic reticulum (ER) stress, leading to up-regulation of genes associated with the unfolded protein response. To extend our knowledge of the physiological responses to constitutive ER stress in plants, transgenic Arabidopsis plants were constructed, in which $De^*$-B30 transcripts were constitutively expressed under the control of the CaMV 35S promoter. Transgenic plants exhibited pale green leaves and growth retardation during the early vegetative stage. In addition, the growth rate of hypocotyl elongation was depressed in dark-grown transgenic seedlings. However, RNA blot analyses revealed no induction of the ER stress-inducible genes, including AtBiP1, AtCNX1, and AtCRT1 in transgenic Arabidopsis plants. Even though transgenic plants also were revealed to retain wildtype level of tunicamycin sensitivity, they showed an increase in hydrogen peroxide production. Higher levels of AtGST1 gene expression in transgenic plants were revealed. These findings suggest that reactive oxygen species are involved in the response to constitutive ER stress in Arabidopsis.