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

Abstract

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.

Brassinosteroids participate in the control of basal and acquired freezing tolerance of plants

“…This result supports earlier findings of increased basal CBF expression in BRI1oe plants (8)(9)(10) and increased COR15A levels in plants overexpressing the BR biosynthesis gene DWF4 (DWARF4) (24).…”

“…In the early days of BR research, it became apparent that in addition to promoting growth, BR application increases plant resistance against different types of abiotic stress, including chilling stress (9). In this study we provide evidence that BRs control freezing tolerance and that this ability is conferred by an effect of BRs on the expression of different classes of COR genes, including the CBF regulon.…”

“…In contrast, a semiquantitative analysis had indicated that in the BRI1 allele bri1-9 CBF expression is increased (9). Our study has now quantitatively assessed CBF and downstream COR gene expression in several BR mutants and provides evidence that BR signaling promotes basal expression of CBFs.…”

“…Previously, it was shown that BR treatment improves the chilling tolerance of Arabidopsis plants (8)(9)(10). In addition, we found that application of the BR epibrassinolide (epiBL) improved Arabidopsis survival following subzero temperature exposure (SI Appendix, Fig.…”

“…When externally applied, BRs increased chilling tolerance (8,9). However, the molecular modes of this control remain unclear (10).…”