Thermospermine levels are controlled by an auxin-dependent feedback loop mechanism in Populus xylem

Polyamines are small polycationic amines that are widespread in living organisms. Thermospermine, synthesized by thermospermine synthase ACAULIS5 (ACL5), was recently shown to be an endogenous plant polyamine. Thermospermine is critical for proper vascular development and xylem cell specification, but it is not known how thermospermine homeostasis is controlled in the xylem. We present data in the Populus model system supporting the existence of a negative feedback control of thermospermine levels in stem xylem tissues, the main site of thermospermine biosynthesis. While over-expression of the ACL5 homologue in Populus, POPACAULIS5, resulted in strong up-regulation of ACL5 expression and thermospermine accumulation in leaves, the corresponding levels in the secondary xylem tissues of the stem were similar or lower than those in the wild-type. POPACAULIS5 over-expression had a negative effect on accumulation of indole-3-acetic acid, while exogenous auxin had a positive effect on POPACAULIS5 expression, thus promoting thermospermine accumulation. Further, over-expression of POPACAULIS5 negatively affected expression of the class III homeodomain leucine zipper (HD-Zip III) transcription factor gene PttHB8, a homologue of AtHB8, while up-regulation of PttHB8 positively affected POPACAULIS5 expression. These results indicate that excessive accumulation of thermospermine is prevented by a negative feedback control of POPACAULIS5 transcript levels through suppression of indole-3-acetic acid levels, and that PttHB8 is involved in the control of POPACAULIS5 expression. We propose that this negative feedback loop functions to maintain steady-state levels of thermospermine, which is required for proper xylem development, and that it is dependent on the presence of high concentrations of endogenous indole-3-acetic acid, such as those present in the secondary xylem tissues.

ACAULIS5 Is Required for Cytokinin Accumulation and Function During Secondary Growth of Populus Trees

In the primary root and young hypocotyl of Arabidopsis, ACAULIS5 promotes translation of SUPPRESSOR OF ACAULIS51 (SAC51) and thereby inhibits cytokinin biosynthesis and vascular cell division. In this study, the relationships between ACAULIS5, SAC51 and cytokinin biosynthesis were investigated during secondary growth of Populus stems. Overexpression of ACAULIS5 from the constitutive 35S promoter in hybrid aspen (Populus tremula × Populus tremuloides) trees suppressed the expression level of ACAULIS5, which resulted in low levels of the physiologically active cytokinin bases as well as their direct riboside precursors in the transgenic lines. Low ACAULIS5 expression and low cytokinin levels of the transgenic trees coincided with low cambial activity of the stem. ACAULIS5 therefore, contrary to its function in young seedlings in Arabidopsis, stimulates cytokinin accumulation and cambial activity during secondary growth of the stem. This function is not derived from maturing secondary xylem tissues as transgenic suppression of ACAULIS5 levels in these tissues did not influence secondary growth. Interestingly, evidence was obtained for increased activity of the anticlinal division of the cambial initials under conditions of low ACAULIS5 expression and low cytokinin accumulation. We propose that ACAULIS5 integrates auxin and cytokinin signaling to promote extensive secondary growth of tree stems.