). † These authors contributed equally to this work. SUMMARYLOV KELCH PROTEIN2 (LKP2), ZEITLUPE (ZTL)/LOV KELCH PROTEIN1 (LKP1) and FLAVIN-BINDING KELCH REPEAT F-BOX1 (FKF1) constitute a family of Arabidopsis F-box proteins that regulate the circadian clock. Over-expression of LKP2 or ZTL causes arrhythmicity of multiple clock outputs under constant light and in constant darkness. Here, we show the significance of LKP2 and ZTL in the photoperiodic control of flowering time in Arabidopsis. In plants over-expressing LKP2, CO and FT expression was down-regulated under longday conditions. LKP2 and ZTL physically interacted with FKF1, which was recruited from the nucleus into cytosolic speckles. LKP2 and ZTL inhibited the interaction of FKF1 with CYCLING DOF FACTOR 1, a ubiquitination substrate for FKF1 that is localized in the nucleus. The Kelch repeat regions of LKP2 and ZTL were sufficient for their physical interaction with FKF1 and translocation of FKF1 to the cytoplasm. Overexpression of LKP2 Kelch repeats induced late flowering under long-day conditions. lkp2 ztl double mutant plants flowered earlier than wild-type plants under short-day (non-inductive) conditions, and both CO and FT expression levels were up-regulated in the double mutant plants. The early flowering of lkp2 ztl was dependent on FKF1. LKP2, ZTL or both affected the accumulation of FKF1 protein during the early light period. These results indicate that an important role of LKP2 and ZTL in the photoperiodic pathway is repression of flowering under non-inductive conditions, and this is dependent on FKF1.
The levels of endogenous brassinosteroids (BRs) and the expression of the biosynthesis/metabolism/perception genes involved have been investigated during the development and germination of pea (Pisum sativum) seeds. When seeds were rapidly growing, the level of biologically active BRs (brassinolide [BL] and castasterone [CS]) and the transcript levels of two BR C-6 oxidases (CYP85A1 and CYP85A6) reached a maximum, suggesting the significance of BL and CS in seed development. In the early stages of germination, CS, but not BL, appeared and its level increased in the growing tissues in which the transcript level of CYP85A1 and CYP85A6 was high, suggesting the significance of CS in seed germination and early seedling growth of pea. 6-Deoxocathasterone (6-deoxoCT) was the quantitatively major BR in mature seeds. At the early stage of germination, the level of 6-deoxoCT was specifically decreased, whereas the levels of downstream intermediates were increased. It seems that 6-deoxoCT is the major storage BR and is utilized during germination and early growth stages. The level of the mRNAs of BR biosynthesis and perception genes fluctuated during seed development. In mature seeds, most of mRNAs were present, but the level was generally lower compared with immature seeds. However, CYP90A9 mRNA rapidly increased during seed development and reached the maximum in mature seeds. The mRNAs stored in mature pea seeds seem to be utilized when seeds germinate. However, it was found that de novo transcription of mRNAs also starts as early as during seed imbibition.
C-6 oxidation genes play a key role in the regulation of biologically active brassinosteroid (BR) levels in the plant. They control BR activation, which involves the C-6 oxidation of 6-deoxocastasterone (6-DeoxoCS) to castasterone (CS) and in some cases the further conversion of CS to brassinolide (BL). C-6 oxidation is controlled by the CYP85A family of cytochrome P450s, and to date, two CYP85As have been isolated in tomato (Solanum lycopersicum), two in Arabidopsis (Arabidopsis thaliana), one in rice (Oryza sativa), and one in grape (Vitis vinifera). We have now isolated two CYP85As (CYP85A1 and CYP85A6) from pea (Pisum sativum). However, unlike Arabidopsis and tomato, which both contain one BR C-6 oxidase that converts 6-DeoxoCS to CS and one BR C-6 Baeyer-Villiger oxidase that converts 6-DeoxoCS right through to BL, the two BR C-6 oxidases in pea both act principally to convert 6-DeoxoCS to CS. The isolation of these two BR C-6 oxidation genes in pea highlights the species-specific differences associated with C-6 oxidation. In addition, we have isolated a novel BR-deficient mutant, lke, which blocks the function of one of these two BR C-6 oxidases (CYP85A6). The lke mutant exhibits a phenotype intermediate between wild-type plants and previously characterized pea BR mutants (lk, lka, and lkb) and contains reduced levels of CS and increased levels of 6-DeoxoCS. To date, lke is the only mutant identified in pea that blocks the latter steps of BR biosynthesis and it will therefore provide an excellent tool to further examine the regulation of BR biosynthesis and the relative biological activities of CS and BL in pea.
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