Nicotiana alata Defensin 1 (NaD1) is a member of the large defensin protein group, found in many organisms among plant and animal kingdoms. While animal defensins, found in insects and mammals are mainly against bacteria, plant defensins rarely have antibacterial activity. Alternatively, plant defensins are involved in fungi defense, and even in plant development modifications. NaD1 is one defensin found in Nicotiana alata (Solanaceae), more specifically, in its flowering tissues.
Plant defensins
Defensis belong to a major family of cationic host defence peptides (HDP), and are characterized by a small molecular size (~5 kDa), basic nature and cysteine-rich. Althoug defensins may have a broad variety of primary and secondary structures, they all have a conserved tertiary structure: a triple-stranded antiparallel and one , stabilized by disulfide bridges, forming a cysteine-stabilized α-helix β-sheet motif.
The major role of plant defensins are for sure in defence against pathogens, specially phytopathogenic fungi, such as Fusarium culmorum and Botrytis cinerea. However, some works have demonstrated its role in plant tolerance to environmental stress, and development, altering root growth in Arabidopsis thaliana and organs development in Solanum lycopersicum. Additionally, some types of defensins are associated with antibacterial activity and inhibition of protein synthesis, proteases or ion channels.
Defensins are widespread through the plant's structure, and they have been already described in leaves, tubers, flowers, and even seeds. Anatomicaly, they are found in several cell types, such as xylem, stomata, parenchyma and peripheral cells.
Nicotiana alata Defensin 1 (NaD1)
Nicotiana alata is one of the most studied species in Solanaceae family, specially regarding the plant's chemical defense. In most solanaceous plants we can find two different classes of defensins proteins:
Class I defensins encode a precursor with an signal peptide and a mature defensin domain.
Class II defensins encode a precursor with an additional C-terminal propeptide. Althought, some Class II defensins, including NaD1, remove the C-terminal propeptide in the mature biologically active form.
NaD1 is exclusively found in flowers of N. alata, and therefore have been associated with protection of reproductive tissues against potential fungi pathogens. Recently, it has been demonstrated that NaD1 interacts with the fungi call wall, penetrating the plasma membrane, and finally leading to the fungi death.
Molecular mechanisms
The antifungical activity of NaD1 seems to be related to its positively charged surface, only when in the dimeric configuration. The dimerization creates a positive charged surface, that mediates interactions with the fungi cell surface, allowing cell permeabilization. Once NaD1 enters the fungi cell, it interacts with intracellular targets and leads to fungi cell death.
Structural highlights
Biological activity of NaD1 is correlated with its structure, as any other protein or peptide. In NaD1, the loop region between β2 and β3 has residues of positively-charged amino acid, which functions have already been detailed previously.
