Overexpression of PaNAC03, a stress induced NAC gene family transcription factor in Norway spruce leads to reduced flavonol biosynthesis and aberrant embryo development. Issue 1 (December 2017) - Dalman, Kerstin
Abstract Background The NAC family of transcription factors is one of the largest gene families of transcription factors in plants and the conifer NAC gene family is at least as large, or possibly larger, as inArabidopsis . These transcription factors control both developmental and stress induced processes in plants. Yet, conifer NACs controlling stress induced processes has received relatively little attention. This study investigates NAC family transcription factors involved in the responses to the pathogenHeterobasidion annosum (Fr.) Bref. sensu lato. Results The phylogeny and domain structure in the NAC proteins can be used to organize functional specificities, several well characterized stress-related NAC proteins are found in III-3 inArabidopsis (Jensen et al. Biochem J 426:183–196, 2010). The Norway spruce genome contain seven genes with similarity to subgroup III-3 NACs. Based on the expression patternPaNAC03 was selected for detailed analyses. Norway spruce lines overexpressingPaNAC03 exhibited aberrant embryo development in response to maturation initiation and 482 misregulated genes were identified in proliferating cultures. Three key genes in the flavonoid biosynthesis pathway: aCHS, aF3'H andPaLAR3 were consistently down regulated in the overexpression lines. In accordance, the overexpression lines showed reduced levels of specific flavonoids, suggesting thatPaNAC03 act as a repressor of this pathway, possibly by directly interacting with the promoter of the repressed genes. However, transactivation studies ofPa NAC03 andPaLAR3 inNicotiana benthamiana showed thatPa NAC03 activatedPaLAR3A, suggesting that PaNAC03 does not act as an independent negative regulator of flavan-3-ol production through direct interaction with the target flavonoid biosynthetic genes. Conclusions PaNAC03 and its orthologs form a sister group to well characterized stress-related angiosperm NAC genes and at leastPaNAC03 is responsive to biotic stress and appear to act in the control of defence associated secondary metabolite production.