Overexpression of PaNAC03, a stress induced NAC gene family transcription factor in Norway spruce leads to reduced flavonol biosynthesis and aberrant embryo development

摘要

BACKGROUND : The NAC family of transcription factors is one of the largest gene families of transcription factors inplants and the conifer NAC gene family is at least as large, or possibly larger, as in Arabidopsis. These transcriptionfactors control both developmental and stress induced processes in plants. Yet, conifer NACs controlling stressinduced processes has received relatively little attention. This study investigates NAC family transcription factorsinvolved in the responses to the pathogen Heterobasidion 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 in Arabidopsis (Jensen et al. Biochem J 426:183–196, 2010). The Norway spruce genome contain seven genes with similarity to subgroup III-3 NACs. Based onthe expression pattern PaNAC03 was selected for detailed analyses. Norway spruce lines overexpressing PaNAC03exhibited aberrant embryo development in response to maturation initiation and 482 misregulated genes wereidentified in proliferating cultures. Three key genes in the flavonoid biosynthesis pathway: a CHS, a F3’H and PaLAR3were consistently down regulated in the overexpression lines. In accordance, the overexpression lines showedreduced levels of specific flavonoids, suggesting that PaNAC03 act as a repressor of this pathway, possibly bydirectly interacting with the promoter of the repressed genes. However, transactivation studies of PaNAC03 andPaLAR3 in Nicotiana benthamiana showed that PaNAC03 activated PaLAR3A, suggesting that PaNAC03 does not actas an independent negative regulator of flavan-3-ol production through direct interaction with the target flavonoidbiosynthetic genes.CONCLUSIONS : PaNAC03 and its orthologs form a sister group to well characterized stress-related angiosperm NACgenes and at least PaNAC03 is responsive to biotic stress and appear to act in the control of defence associatedsecondary metabolite production.