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

Kerstin Dalman,Julia Johanna Wind,Miguel Nemesio-Gorriz,Malin Elfstrand,Ines Ezcurra,Almuth Hammerbacher,Karl Lundén

doi:10.1186/s12870-016-0952-8

Abstract

BackgroundThe 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 in Arabidopsis. 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 pathogen Heterobasidion annosum (Fr.) Bref. sensu lato.ResultsThe 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 on the expression pattern PaNAC03 was selected for detailed analyses. Norway spruce lines overexpressing PaNAC03 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: a CHS, a F3’H and PaLAR3 were consistently down regulated in the overexpression lines. In accordance, the overexpression lines showed reduced levels of specific flavonoids, suggesting that PaNAC03 act as a repressor of this pathway, possibly by directly interacting with the promoter of the repressed genes. However, transactivation studies of PaNAC03 and PaLAR3 in Nicotiana benthamiana showed that PaNAC03 activated PaLAR3A, 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.ConclusionsPaNAC03 and its orthologs form a sister group to well characterized stress-related angiosperm NAC genes and at least PaNAC03 is responsive to biotic stress and appear to act in the control of defence associated secondary metabolite production.

Highlights

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 in Arabidopsis
Norway spruce contain multiple clade III-3NAC transcription factor gene family members The RNAseq dataset from the time course study of H. annosum s.s. inoculated Norway spruce [9, 26] contained six putatively unique transcripts (PUTs) with similarity to NAC transcription factors (TFs), all PUTs had at least one blastn hit in the P. abies genome v1.0 high confidence gene catalogue
Three of the PUTs, named PaNAC03, PaNAC04 and PaNAC05, all had highly significant blastn hits to unique gene models in the P. abies v1.0 gene catalogue and significant blastx hits to Arabidopsis NACs (Table 1)

Summary

Introduction

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 in Arabidopsis. These transcription factors control both developmental and stress induced processes in plants. Conifer NACs controlling stress induced processes has received relatively little attention. H. annosum s.l. is a complex of five closely related species [11, 12] that have partly overlapping host ranges These results indicated a differential control of defence responses between resistant and susceptible genotypes

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