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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.

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