Genetic Evidence for the Reduction of Brassinosteroid Levels by a BAHD Acyltransferase-Like Protein in Arabidopsis    

Abstract

Brassinosteroids (BRs) are a group of steroidal hormones involved in plant development. Although the BR biosynthesis pathways are well characterized, the BR inactivation process, which contributes to BR homeostasis, is less understood. Here, we show that a member of the BAHD (for benzylalcohol O-acetyltransferase, anthocyanin O-hydroxycinnamoyltransferase, anthranilate N-hydroxycinnamoyl/benzoyltransferase, and deacetylvindoline 4-O-acetyltransferase) acyltransferase family may play a role in BR homeostasis in Arabidopsis (Arabidopsis thaliana). We isolated two gain-of-function mutants, brassinosteroid inactivator1-1Dominant (bia1-1D) and bia1-2D, in which a novel BAHD acyltransferase-like protein was transcriptionally activated. Both mutants exhibited dwarfism, reduced male fertility, and deetiolation in darkness, which are typical phenotypes of plants defective in BR biosynthesis. Exogenous BR treatment rescued the phenotypes of the bia1-1D mutant. Endogenous levels of BRs were reduced in the bia1-1D mutant, demonstrating that BIA1 regulates endogenous BR levels. When grown in darkness, the bia1 loss-of-function mutant showed a longer hypocotyl phenotype and was more responsive to exogenous BR treatment than the wild-type plant. BIA1 expression was predominantly observed in the root, where low levels of BRs were detected. These results indicate that the BAHD acyltransferase family member encoded by BIA1 plays a role in controlling BR levels, particularly in the root and hypocotyl in darkness. Taken together, our study provides new insights into a mechanism that maintains BR homeostasis in Arabidopsis, likely via acyl conjugation of BRs.

The Arabidopsis geneATST4ais not a typical brassinosteroids catabolic gene

“…This observation is significant since all the known BR catabolic genes characterized so far result in dramatic BR-deficient phenotypes both at seedling and at adult stages when overexpressed in Arabidopsis. [1][2][3][4][5][6][7][8][9][10] This suggests that ATST4a overexpression did not reduce BR content in the transgenic lines. In addition, quantitative transcript analysis of the feedback regulated genes DWF4 and BAS1 also suggests that BR content is not reduced in the ATST4a-OX lines when compared with wild type (Fig.…”

“…1,3,6,7,9 However, despite the observation that ATST4a is expressed in the seedling hypocotyl (Fig. 2B), atst4a1-1 did not display any defect in light-mediated hypocotyl inhibition (Fig.…”

“…[11][12][13][14] Genetic analyses of knockout or knockdown mutants of most of these BR catabolic genes show a reduction in hypocotyl-elongation inhibition in white light in Arabidopsis seedlings. 1,3,6,7,9 The knockout or knockdown approach may in some cases fail to provide any evidence of involvement in BR catabolism due to genetic redundancy or yet other unknown reasons. 6 In vivo and in vitro biochemical assays of the enzymes encoded by some of these genes were also used to confirm their role in BR catabolism.…”

“…In all cases, in planta overexpression of BR catabolic genes leads to characteristic BR-deficient phenotypes at both seedling and adult stages of Arabidopsis. [1][2][3][4][5][6][7][8][9][10] These BR-deficient phenotypes typified by a BR biosynthetic mutant de-etiolated 2-1 (det2-1), include small organ size, small round and dark green leaves, and delayed flowering. [11][12][13][14] Genetic analyses of knockout or knockdown mutants of most of these BR catabolic genes show a reduction in hypocotyl-elongation inhibition in white light in Arabidopsis seedlings.…”

“…The discovery of multiple independent BR-catabolic pathways and their enzymes in Arabidopsis demonstrates the importance of this process in plant growth and development. [1][2][3][4][5][6][7][8][9][10] These BR catabolic pathways were mainly discovered and latter characterized using genetic manipulation of the genes involved. In all cases, in planta overexpression of BR catabolic genes leads to characteristic BR-deficient phenotypes at both seedling and adult stages of Arabidopsis.…”