Aliphatic glucosinolates are compounds which occur in high concentrations in Arabidopsis thaliana and other Brassicaceae species. They are important for the resistance of the plant to pest insects. Previously, the biosynthesis of these compounds was shown to be regulated by transcription factors MYB28 and MYB29. We now show that MYB28 and MYB29 are partially redundant, but in the absence of both, the synthesis of all aliphatic glucosinolates is blocked. Untargeted and targeted biochemical analyses of leaf metabolites showed that differences between single and double knock-out mutants and wild type plants were restricted to glucosinolates. Biosynthesis of long-chain aliphatic glucosinolates was blocked by the myb28 mutation, while short-chain aliphatic glucosinolates were reduced by about 50% in both the myb28 and the myb29 single mutants. Most remarkably, all aliphatic glucosinolates were completely absent in the double mutant. Expression of glucosinolate biosynthetic genes was slightly but significantly reduced by the single myb mutations, while the double mutation resulted in a drastic decrease in expression of these genes. Since the myb28myb29 double mutant is the first Arabidopsis genotype without any aliphatic glucosinolates, we used it to establish the relevance of aliphatic glucosinolate biosynthesis to herbivory by larvae of the lepidopteran insect Mamestra brassicae. Plant damage correlated inversely to the levels of aliphatic glucosinolates observed in those plants: Larval weight gain was 2.6 fold higher on the double myb28myb29 mutant completely lacking aliphatic glucosinolates and 1.8 higher on the single mutants with intermediate levels of aliphatic glucosinolates compared to wild type plants.

SUMMARYThe floral transition is the switch from vegetative development to flowering. Proper timing of the floral transition is regulated by different pathways and is critical for the reproductive success of plants. Some of the flowering pathways are controlled by environmental signals such as photoperiod and vernalization, others by autonomous signals such as the developmental state of the plant and hormones, particularly gibberellin. SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1) acts in Arabidopsis as an integrative centre of these pathways, promoting the floral transition. In this work, we show that AGAMOUS-LIKE 42 (AGL42), AGAMOUS-LIKE 71 (AGL71) and AGAMOUS-LIKE 72 (AGL72), which encode MADS-box transcription factors phylogenetically closely related to SOC1, are also involved in the floral transition. They promote flowering at the shoot apical and axillary meristems and seem to act through a gibberellin-dependent pathway. Furthermore SOC1 directly controls the expression of AGL42, AGL71 and AGL72 to balance the expression level of these SOC1-like genes. Our data reveal roles for AGL42, AGL71 and AGL72 in the floral transition, demonstrate their genetic interactions with SOC1 and suggest that their roles differ in the apical and axillary meristems.

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). † These authors contributed equally to this work. SUMMARYThe inflorescence meristem produces floral primordia that remain undifferentiated during the first stages of flower development. Genes controlling floral meristem identity include LEAFY (LFY), APETALA1 (AP1), CAULIFLOWER (CAL), LATE MERISTEM IDENTITY 1 (LMI1), SHORT VEGETATIVE PHASE (SVP) and AGAMOUS-LIKE24 (AGL24). The lfy mutant shows partial reversions of flowers into inflorescence shoot-like structures and this phenotype is enhanced in the lfy ap1 double mutant. Here we show that combining the lfy mutant with agl24 and svp single mutants or with the agl24 svp double mutant enhances the lfy phenotype and that the lfy agl24 svp triple mutant phenocopies the lfy ap1 double mutant. Analysis of the molecular interactions between LFY, AGL24 and SVP showed that LFY is a repressor of AGL24 and SVP, whereas LMI1 is a positive regulator of these genes. Moreover, AGL24 and SVP positively regulate AP1 and LFY by direct binding to their regulatory regions. Since all these genes are important for establishing floral meristem identity, regulatory loops are probably important to maintain the correct relative expression levels of these genes.