Involvement of NAC transcription factor NaNAC29 in Alternaria alternata resistance and leaf senescence in Nicotiana attenuata

NAC-LIKE, ACTIVATED BY AP3/PI (NAP) is a NAC transcription factor regulating leaf senescence in Arabidopsis thaliana. In wild tobacco Nicotiana attenuata, a nuclear localized NAC transcription factor NaNAC29 was identified to be highly elicited after inoculation of Alternaria alternata, a notorious necrotic fungus on tobacco species. The NaNAC29 possesses similar tertiary structure to NAP with 60% amino acid identity. However, it remains unknown the role of NaNAC29 in plant defense responses to A. alternata and leaf senescence in N. attenuata. In this paper, Defensin-like protein 1 (NaDLP1) was highly induced in N. attenuata after A. alternata inoculation and bigger lesions were developed in NaDLP1-silenced plants. Interestingly, A. alternata-induced NaDLP1 was reduced by 76% in VIGS NaNAC29 plants and by 61% in JA deficient irAOC plants at 3 days post inoculation. The regulation of NaDLP1 expression by NaNAC29 was clearly independent on JA pathway, since exogenous methyl jasmonate treatment could not complement the induction levels of NaDLP1 in NaNAC29-silenced plants to the levels in WT plants. Otherwise, the expression of NaNAC29 was low expressed in young leaves but highly in senescent leaves and dark-treated leaves. NaNAC29-silenced plants, which were generated by virus-induced gene silencing (VIGS NaNAC29), showed delayed senescence phenotype. In addition, constitutive over-expression of NaNAC29 in A. thaliana could rescue the delayed-senescence phenotype of nap and caused precocious leaf senescence of wild-type Col-0 plants. All the data above demonstrate that NaNAC29 is a NAP homolog in N. attenuata participating in the defense responses to A. alternata by regulation of a defensin protein NaDLP1 and promoting leaf senescence.