Constitutive activation of stress-inducible genes in a brassinosteroid-insensitive 1 (bri1) mutant results in higher tolerance to cold.

Many plant hormones are involved in coordinating the growth responses of plants under stress. However, not many mechanistic studies have explored how plants maintain the balance between growth and stress responses. Brassinosteroids (BRs), plant-specific steroid hormones, affect many aspects of plant growth and development over a plant's lifetime. In this study we determined that exogenous treatment of BR helped the plant overcome the cold condition only when pretreated with less than 1 nM, and the brassinosteroid-insensitive 1 (bri1) mutation, which results in defective BR signaling and subsequent dwarfism, generates an increased tolerance to cold. In contrast, BRI1-overexpressing plants were more sensitive to the same stress than wild-type. We found that the bri1 mutant and BRI1-overexpressing transgenic plants contain higher basal level of expression of CBFs/DREB1s than wild-type. However, representative cold stress-related genes were regulated with the same pattern to cold in wild-type, bri1-9 and BRI1 overexpressing plants. To examine the global gene expression and compare the genes that show differential expression pattern in bri1-9 and BRI1-GFP plants other than CBFs/DREB1s, we analyzed differential mRNA expression using the cDNA microarray analysis in the absence of stress. Endogenous expression of both stress-inducible genes as well as genes encoding transcription factors that drive the expression of stress-inducible genes were maintained at higher levels in bri1-9 than either in wild-type or in BRI1 overexpressing plants. This suggests that the bri1-9 mutant could always be alert to stresses that might be exerted at any times by constitutive activation of subsets of defense.