Exogenously supplied auxin (1-naphthaleneacetic acid) inhibited light-induced activity increase of polyamine oxidase (PAO), a hydrogen peroxide-producing enzyme, in the outer tissues of maize (Zea mays) mesocotyl. The same phenomenon operates at PAO protein and mRNA accumulation levels. The wall-bound to extractable PAO activity ratio was unaffected by auxin treatment, either in the dark or after light exposure. Ethylene treatment did not affect PAO activity, thus excluding an effect of auxin via increased ethylene biosynthesis. The auxin polar transport inhibitors N 1 -naphthylphthalamic acid or 2,3,5-triiodobenzoic acid caused a further increase of PAO expression in outer tissues after light treatment. The small increase of PAO expression, normally occurring in the mesocotyl epidermis during plant development in the dark, was also inhibited by auxin, although to a lesser extent with respect to light-exposed tissue, and was stimulated by N 1 -naphthylphthalamic acid or 2,3,5-triiodobenzoic acid, thus suggesting a complex regulation of PAO expression. Immunogold ultrastructural analysis in epidermal cells revealed the association of PAO with the secretory pathway and the cell walls. The presence of the enzyme in the cell walls of this tissue greatly increased in response to light treatment. Consistent with auxin effects on light-induced PAO expression, the hormone treatment inhibited the increase in immunogold staining both intraprotoplasmically and in the cell wall. These results suggest that both light and auxin finely tune PAO expression during the light-induced differentiation of the cell wall in the maize mesocotyl epidermal tissues.Dark-light transitions dramatically affect organ architecture and growth rate during the first stages of plant development. In particular, for a young seedling buried under the soil surface, rapid extension growth of hypogean organs occurs in the dark to reach sunlight. The mesocotyl is devoted to accomplish this important function in maize (Zea mays) and other Gramineae, the growth of this organ being strongly stimulated in the dark, whereas it is inhibited by light as soon as the coleoptile sprouts from the soil surface. This complex photomorphogenic event, mediated by different classes of photoreceptors (Vanderhoef and Briggs, 1978), is thought to be linked to the reduction of indole-3-acetic acid (IAA) supply from the coleoptile to the mesocotyl (Iino, 1982), particularly in its epidermis (Barker-Bridgers et al., 1998). This process causes a tension increase in the tissue, which constrains the growth of the whole organ, the greatest tensile force loading on the outer walls of epidermal cells (Masuda and Yamamoto, 1972; Kutschera and Briggs, 1987; Bret-Harte et al., 1991; Kutschera, 1992).Cell wall yielding properties depend on a finely regulated balance between wall-loosening and -stiffening events. Wall loosening is thought to be mediated by either enzymatic (Cosgrove, 2000; Darley et al., 2001) or chemical agents (Miller, 1986; Fry, 1998; Schopfer, 2001). To this re...
Plant polyamine oxidases (PAOs; EC 1.5.3.11) are hydrogen peroxide-producing enzymes supposedly involved in cell-wall differentiation processes and defence responses. Maize (Zea mays L.) PAO (MPAO) is a 53 kDa secretory glycoprotein, abundant in primary and secondary cell walls of several tissues. Using biochemical, histochemical, ultrastructural and immunocytochemical techniques, the distribution and sub-cellular compartmentalisation of MPAO in the primary root and mesocotyl of seedlings at different maturation stages or after growth under varying light conditions were analysed. In apical root tissues, MPAO immunoreactivity was mainly detected in the cytoplasmic compartment, while a lower immunoreactivity was observed in the cell walls. In the more mature, basal part of the root, intense immunogold labelling was found in the primary and secondary walls of protoxylem precursors and vessels, while endodermal cells and living metaxylem precursors were immunopositive both in their walls and in their thin cytoplasmic compartments. A re-distribution of MPAO protein from the cytoplasm toward the primary and secondary walls was also recognised when immunoreactivity of basal root tissues from 3-day-old seedlings was compared with that detected in 11-day-old tissues. Accordingly, biochemical analyses revealed MPAO entrapment in the extracellular matrix of mature tissues. In the mesocotyl, an enrichment of MPAO immunolabelling in the cell wall of protoxylem, metaxylem and epidermal tissues, as a function of light exposure, was observed. Taken together, these data support the hypothesised role of PAOs in cell-wall maturation. Moreover, the relevant intraprotoplasmic MPAO localisation observed mainly in differentiating root tissues suggests an additional role in intracellular production of hydrogen peroxide.
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