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Glucosinolates (GSL) are nitrogen- and sulphur-rich natural plant products that serve as chemoprotective compounds in plant biotic defence reactions against herbivores and pathogens. GS also function as flavour compounds and exhibit strong anticancerogenic properties beneficial to human health. Although considerable progress has been made concerning the biosynthesis of glucosinolates, little is known how plants regulate the synthesis of these metabolites. The MYB28, MYB29 and MYB76 (referred to as HIGH ALIPHATIC GLUCOSINOLATE 1, 3 and 2) transcription factors were identified as novel regulators of glucosinolate biosynthesis. Molecular and biochemical characterization of Arabidopsis gain- and loss-of-function mutants revealed a significant correlation between the MYB28/HAG1, MYB29/HAG3 and MYB76/HAG2 transcript levels and the accumulation of aliphatic Met-derived glucosinolates. MYB28/HAG1, MYB29/HAG3 and MYB76/HAG2 over-expression caused a considerable increase in the level of aliphatic glucosinolates due to the specific activation of genes involved in aliphatic glucosinolate biosynthesis. Disruption of MYB28/HAG1 and MYB29/HAG3 gene functions caused a dramatic decrease in the content of aliphatic glucosinolates, whereas myb76/hag2 loss-of-function mutants showed no changes in glucosinolate profiles except for the slight decrease in the level of 4MSOB glucosinolate. Analysis of the ProHAG:GUS activity revealed similar expression patterns in generative organs and rosette leaves of Arabidopsis plants, covering the main sites of aliphatic glucosinolate accumulation and overlapping with the expression of glucosinolate biosynthetic genes. Mechanical stimuli transiently induced MYB/HAG expression demonstrating their role in early plant responses to biotic stresses. Expression of MYB28/HAG1 was clearly induced by glucose, indicating a novel signaling mechanism for the integration of carbohydrate availability in glucosinolates production, whereas MYB29/HAG3 was shown to be involved in MeJa-induced glucosinolate biosynthesis. Notably, MYB76/HAG2 expression was independent from plant elicitors and seems to play an accessory role in glucosinolate biosynthesis. Besides, MYB28/HAG1 over-expression reduced performance of the generalist lepidopteran herbivore Spodoptera exigua in weight-gain experiments. Finally, MYB28/HAG1, MYB76/HAG2 and MYB29/HAG3 reciprocally trans-activate each other and comprise a complex regulatory network in concert with other regulators (MYB51, MYB34, MYB122, WRKY25 and SLIM1) to control glucosinolate biosynthesis in response to different environmental stimuli. |
