Takahito Nomura

Cloning the Tomato Curl3 Gene Highlights the Putative Dual Role of the Leucine-Rich Repeat Receptor Kinase tBRI1/SR160 in Plant Steroid Hormone and Peptide Hormone Signaling

Brassinosteroids (BRs) are plant steroid hormones that are essential for normal plant development. To gain better understanding of the conservation of BR signaling, the partially BR-insensitive tomato mutant altered brassinolide sensitivity1 (abs1) was identified and found to be a weak allele at the curl3 (cu3) locus. BR content is increased in both of these mutants and is associated with increased expression of Dwarf. The tomato homolog of the Arabidopsis Brassinosteroid Insensitive1 Leu-rich repeat (LRR) receptor-like kinase, named tBri1, was isolated using degenerate primers. Sequence analysis of tBRI1 in the mutants cu3 and abs1 revealed that cu3 is a nonsense mutant and that abs1 is a missense mutant. A comparison of BRI1 homolog sequences highlights conserved features of BRI1 sequences, with the LRRs in close proximity to the island domain showing more conservation than N-terminal LRRs. The most homologous sequences were found in the kinase and transmembrane regions. tBRI1 (SR160) also has been isolated as the putative receptor for systemin, a plant peptide hormone. This finding suggests a possible dual role for tBRI1 in steroid hormone and peptide hormone signaling.

Regulation of transcript levels of the Arabidopsis cytochrome P450 genes involved in brassinosteroid biosynthesis

Cytochrome P450 enzymes of the closely related CYP90 and CYP85 families catalyze essential oxidative reactions in the biosynthesis of brassinosteroid (BR) hormones. Arabidopsis CYP90B1/DWF4 and CYP90A1/CPD are responsible for respective C-22 and C-23 hydroxylation of the steroid side chain and CYP85A1 catalyzes C-6 oxidation of 6-deoxo intermediates, whereas the functions of CYP90C1/ROT3, CYP90D1, and CYP85A2 are still unknown. Semiquantitative reverse transcriptase-polymerase chain reaction analyses show that transcript levels of CYP85 and CYP90genes are down-regulated by brassinolide, the end product of the BR biosynthesis pathway. Feedback control of the CYP90C1,CYP90D1, and CYP85A2 genes by brassinolide suggests that the corresponding enzymes might also participate in BR synthesis. CYP85 andCYP90 mRNAs show strong and transient accumulation during the 1st week of seedling development, as well as characteristic organ-specific distribution. Transcripts of CYP90A1 andCYP85A2 are preferentially represented in shoots andCYP90C1, CYP90D1, andCYP85A1 mRNAs are more abundant in roots, whereasCYP90B1 is ubiquitously expressed. Remarkably, the spatial pattern of CYP90A1 expression is maintained in the BR-insensitive cbb2 mutant, indicating the independence of organ-specific and BR-dependent regulation. Quantitative gas chromatography-mass spectrometry analysis of endogenous BRs in shoots and roots of Arabidopsis, pea (Pisum sativum), and tomato (Lycopersicon esculentum) reveal similar partitioning patterns of BR intermediates in these species. Inverse correlation between CYP90A1/CPDtranscript levels and the amounts of the CYP90A1 substrate 6-deoxocathasterone in shoots and roots suggests that transcriptional regulation plays an important role in controlling BR biosynthesis.