Xylem differentiation in plants is under strict hormonal regulation. Auxins and cytokinins, together with brassinosteroids (BRs), appear to be the main hormones controlling vascular differentiation. In this report, we study the effect of these hormones on the basic peroxidase isoenzyme from Zinnia elegans (ZePrx), an enzyme involved in lignin biosynthesis. Results showed that auxins and cytokinins induce ZePrx, similarly to the way in which they induce seedling secondary growth (in particular, metaxylem differentiation). Likewise, the exogenous application of BR reduces the levels of ZePrx, in a similar way to their capacity to inhibit seedling secondary growth. Consistent with this notion, the exogenous application of BR reverses the auxin/cytokinin-induced ZePrx expression, but has no effect on the auxin/cytokinin-induced secondary growth. This differential hormonal response is supported by the analysis of the ZePrx promoter, which contains (a) cis-elements directly responsive to these hormones and (b) cis-elements targets of the plethora of transcription factors, such as NAC, MYB, AP2, MADS and class III HD Zip, which are up-regulated during the auxin- and cytokinin-induced secondary growth. Taken together, these results suggest that ZePrx is directly and indirectly regulated by the plethora of hormones that control xylem differentiation, supporting the role of ZePrx in xylem lignification.

Hypocotyl formation during the epigeal germination of seedlings is under strict hormonal regulation. In a 3 d old Zinnia elegans seedling system, gibberellic acid (GA 3 ) exerts an opposite effect to that exerted by light on hypocotyl photomorphogenesis because GA 3 promotes an etiolated-like growth with an inhibition of radial (secondary) growth. For this reason, the effect of GA 3 on the basic peroxidase isoenzyme from Z. elegans (ZePrx), an enzyme involved in hypocotyl lignin biosynthesis, was studied. The results showed that GA 3 reduces ZePrx activity, similarly to the way in which it reduces seedling secondary growth. This hormonal response is supported by the analysis of the ZePrx promoter, which contains four types of GA 3 -responsive cis-elements: the W Box/O2S; the Pyr Box; the GARE; and the Amy Box. Taken together, these results suggest that ZePrx is directly regulated by GA 3 , with this effect matching the inhibitory effect of GA on the hypocotyl secondary growth.López Núñez-Flores MJ, Gutiérrez J, Gómez-Ros LV, Novo Uzal E, Sottomayor M, Ros Barceló A (2010) Downregulation of the basic peroxidase isoenzyme from Zinnia elegans by gibberellic acid.