Required for correct xylem specification through regulation of the lifetime of the xylem elements. Prevents premature death of the xylem vessel elements. S-adenosyl 3-(methylsulfanyl)propylamine + spermidine = H(+) + S-methyl-5'-thioadenosine + thermospermine Highly expressed in stem internodes and roots. Lower levels in young seedlings before flowering and rosette leaves. Expressed in the vascular tissues. Restricted to procambial and/or provascular cells during primary root development and early leaves development. Expressed throughout early globular-staged embryos and during embryogenesis. Up-regulated by auxin (IAA), but not by abscisic acid (ABA), brassinolid (BR), gibberelic acid (GA3)or benzyl aminopurine (BA). Down-regulated by salt stress and thermospermine. Severe reduction in the length of stem internodes. Increased thickness of veins in leaves and inflorescence stems. Altered morphology of xylem vessel elements. Belongs to the spermidine/spermine synthase family.