PASTICCINO (PAS) genes are required for coordinated cell division and differentiation during plant development. In loss-of-function pas mutants, plant aerial tissues showed ectopic cell division that was specifically enhanced by
cytokinins, leading to disorganized
tumor-like tissue. To determine the role of the PAS genes in controlling cell proliferation, we first analyzed the expression profiles of several genes involved in cell division and meristem function. Differentiated and meristematic cells of the pas mutants were more competent for cell division as illustrated by the ectopic and enlarged expression profiles of
CYCLIN-DEPENDENT KINASE A and
CYCLIN B1. The expression of meristematic homeobox genes KNOTTED-LIKE IN ARABIDOPSIS (KNAT2, KNAT6), and SHOOT MERISTEMLESS was also increased in pas mutants. Moreover, the loss of meristem function caused by shoot meristemless mutation can be suppressed by pas2. The KNAT2 expression pattern defines an enlarged meristematic zone in pas mutants that can be mimicked in wild type by
cytokinin treatment.
Cytokinin induction of the primary
cytokinin response markers, ARABIDOPSIS RESPONSE REGULATOR (ARR5 and ARR6), was enhanced and lasted longer in pas mutants, suggesting that PAS genes in wild type repress
cytokinin responses. The expression of the
cytokinin-regulated
cyclin D,
cyclin D3.1, was nonetheless not modified in pas mutants. However, primary
auxin response genes were down-regulated in pas mutants, as shown by a lower
auxin induction of IAA4 and IAA1 genes, demonstrating that the
auxin response was also modified. Altogether, our results suggest that PAS genes are involved in the hormonal control of cell division and differentiation.