PIRIN2 suppresses S-type lignin accumulation in a noncell-autonomous manner in Arabidopsis xylem elements.

PIRIN (PRN) genes encode cupin domain-containing proteins that function as transcriptional co-regulators in humans but that are poorly described in plants. A previous study in xylogenic cell cultures of Zinnia elegans suggested a role for a PRN protein in lignification. This study aimed to identify the function of Arabidopsis (Arabidopsis thaliana) PRN proteins in lignification of xylem tissues. Chemical composition of the secondary cell walls was analysed in Arabidopsis stems and/or hypocotyls by pyrolysis-gas chromatography/mass spectrometry, 2D-nuclear magnetic resonance and phenolic profiling. Secondary cell walls of individual xylem elements were chemotyped by Fourier transform infrared and Raman microspectroscopy. Arabidopsis PRN2 suppressed accumulation of S-type lignin in Arabidopsis stems and hypocotyls. PRN2 promoter activity and PRN2:GFP fusion protein were localised specifically in cells next to the vessel elements, suggesting a role for PRN2 in noncell-autonomous lignification of xylem vessels. Accordingly, PRN2 modulated lignin chemistry in the secondary cell walls of the neighbouring vessel elements. These results indicate that PRN2 suppresses S-type lignin accumulation in the neighbourhood of xylem vessels to bestow G-type enriched lignin composition on the secondary cell walls of the vessel elements. Gene expression analyses suggested that PRN2 function is mediated by regulation of the expression of the lignin-biosynthetic genes.