Brassinosteroid/Sterol Synthesis and Plant Growth as Affected bylka and lkb Mutations of Pea1

Abstract

The dwarf pea (Pisum sativum) mutants lka and lkb are brassinosteroid (BR) insensitive and deficient, respectively. The dwarf phenotype of the lkb mutant was rescued to wild type by exogenous application of brassinolide and its biosynthetic precursors. Gas chromatography-mass spectrometry analysis of the endogenous sterols in this mutant revealed that it accumulates 24-methylenecholesterol and isofucosterol but is deficient in their hydrogenated products, campesterol and sitosterol. Feeding experiments using 2 H-labeled 24-methylenecholesterol indicated that the lkb mutant is unable to isomerize and/or reduce the ⌬ 24(28) double bond. Dwarfism of the lkb mutant is, therefore, due to BR deficiency caused by blocked synthesis of campesterol from 24-methylenecholesterol. The lkb mutation also disrupted sterol composition of the membranes, which, in contrast to those of the wild type, contained isofucosterol as the major sterol and lacked stigmasterol. The lka mutant was not BR deficient, because it accumulated castasterone. Like some gibberellin-insensitive dwarf mutants, overproduction of castasterone in the lka mutant may be ascribed to the lack of a feedback control mechanism due to impaired perception/signal transduction of BRs. The possibility that castasterone is a biologically active BR is discussed.

RAV-Like1Maintains Brassinosteroid Homeostasis via the Coordinated Activation ofBRI1and Biosynthetic Genes in Rice

“…This is in contrast with what is observed in BR biosynthetic mutants, in which the blockage of a specific reaction leads to the significant accumulation of intermediate species prior to the reaction step (Hong et al, 2002(Hong et al, , 2003Tanabe et al, 2005). Previous studies showed that BR signaling mutants, which included bri1 d61, accumulate high levels of all the BR intermediates Nomura et al, 1999;Yamamuro et al, 2000); however, endogenous BR levels were lower in ravl1-1 mutants when compared with wild-type plants, even if the expression of BRI1 was severely repressed. This may be a consequence of the inactivation of BR biosynthetic genes in ravl1-1 plants.…”

Brassinosteroid Levels Increase Drastically Prior to Morphogenesis of Tracheary Elements

“…In Arabidopsis, 6-DeoxoBRs are more active than 6-OxoBRs in the light, whereas in the dark 6-DeoxoBRs are less active (Fujioka et al, 1997;Choe et al, 1998). In tomato, however, 6-DeoxoBRs are more active than 6-OxoBRs both in the light and in the dark (Koka et al, 2000), and in pea, 6-OxoBRs are more active than 6-DeoxoBRs in the light (Nomura et al, 1999). These results have suggested that activities of exogenously supplied BRs are modulated by intracellular BR synthetic activity, which varies among tissues and under light conditions.…”

“…However, the predominance of either pathway varies among plant species and among tissues. For instance, both the late and early C6-oxidation pathways occur in pea (Nomura et al, 1999) and Arabidopsis (Fujioka et al, 1997), whereas the late C6-oxidation pathway is predominant in tomato (Bishop et al, 1999).…”

Brassinosteroid-6-Oxidases from Arabidopsis and Tomato Catalyze Multiple C-6 Oxidations in Brassinosteroid Biosynthesis

“…Such regulation seems to be cancelled in BR-insensitive mutants because CS accumulates at aberrant levels in pea lka (Nomura et al, , 1999 and tomato curl-3 (T. Nomura, T. Yokota, and G.J. Bishop, unpublished data).…”

“…Many of these mutants also have darkgreen leaves, reduced fertility, a prolonged lifespan, and display abnormal skotomorphogenesis. BRinsensitive mutants have been identified in Arabidopsis, pea, tomato, and rice (Oryza sativa; Clouse et al, 1996;Kauschmann et al, 1996;Li and Chory 1997;Nomura et al, 1999;Koka et al, 2000;Yamamuro et al, 2000).…”