Decreased Ethylene Biosynthesis, and Induction of Aerenchyma, by Nitrogen- or Phosphate-Starvation in Adventitious Roots of Zea mays L.

Abstract

Plants of Zea mays L. cv TX5855 were grown in a complete, well oxygenated nutrient solution then subjected to nutrient starvation by omitting either nitrate and ammonium or phosphate from the solution. These treatments induced the formation of aerenchyma close to the apex of the adventitious roots that subsequently emerged from the base of the shoot, a response similar to that shown earlier to be induced by hypoxia. Compared with control plants supplied with all nutrients throughout, N-or Pstarvation consistently depressed the rates of ethylene release by excised, 25 mm apical segments of adventitious roots. Some enzymes and substrates of the ethylene biosynthetic pathway were examined. The content of 1-amino cyclopropane-1-carboxylic acid (ACC) paralleled the differences in ethylene production rates, being depressed by N or P deficiency, while malonyl-ACC showed a similar trend. Activity of ACC synthase and of ethylene forming enzyme (g-1 fresh weight) was also greater in control roots than in nutrient starved ones. These results indicate that much of the ethylene biosynthetic pathway is slowed under conditions of N-or P-starvation. Thus, by contrast to the effects of hypoxia, the induction of aerenchyma in roots of Zea mays by nutrient starvation is not related to an enhanced biosynthesis and/or accumulation of ethylene in the root tips.Many dryland species are exposed to temporary periods of oxygen deficiency during their growth following heavy rain, irrigation, or flooding, when the soil becomes water saturated (10,17). Some species respond to this situation by forming roots with continuous, gas-filled channels in the roots (19). These channels improve the internal supply of oxygen, the oxygen originating from the atmosphere or photosynthesis and passing from leaves to roots down a concentration gradient (1). The flooding resistance of a wide range of monocot and dicotyledonous species, both herbaceous and woody, is often associated, in part, with the capacity to develop such aerenchymatous roots (8,9,16,17,19,23 The gaseous plant hormone, ethylene, is associated with a wide variety of responses in higher plant cells (3). In earlier reports we showed that aerenchyma formation in adventitious (nodal) roots of maize by cell lysis is induced by a partial oxygen deficiency (hypoxia) and is closely related to enhanced endogenous concentrations of ethylene (1 1). Hypoxia clearly stimulates the biosynthesis of ethylene in growing maize roots (2,11,12,20) as in other responsive plant tissues (4, 5, 7). Low exogenous concentrations of ethylene under aerobic conditions (1-5 uL L-' in air) induce aerenchyma (18) that is structurally indistinguishable from that induced by hypoxia (1 1, 12); furthermore, hypoxically induced aerenchyma formation in the apices of growing roots is blocked by inhibitors of ethylene biosynthesis or ethylene action (12,18,20).However, Konings and Verschuren (21) showed that formation of aerenchyma in the seminal roots of maize was stimulated under fully aerobic conditions by omission o...

Induction of Enzymes Associated with Lysigenous Aerenchyma Formation in Roots of Zea mays during Hypoxia or Nitrogen Starvation

“…temporary deprivation of N or P does not stimulate the ethybene biosynthetic pathway (Figs. 5 and 6; see also Drew et al, 1989) but greatly enhances the sensitivity of ethylene-responsive cells of the root cortex, leading to cell lysis and a.erenchyma formation (He et al, 1992). The present resulls implicate ethylene in the induction of cellulase activity, w e n under conditioins where ethylene biosynthesis is slowed.…”

“…Caryopses of maize (Zea mays L. cv TX 5855) were germinated and grown in a complete nutrient solution in a controlled-environment room at 25OC as described earlier (Drew et al, 1989). For N deficiency, plants were transferred 10 to 11 d from the start of imbibition from this nutrient solution to one of similar composition but lacking a N source (NH4+ and NO3-).…”

“…Control plants were always maintained with the complete nutrient solution. In the majority of experiments, plants were grown in groups of four in 2-L volumes of nutrient solution (Drew et al, 1989).…”

“…during exposure to an environment that is partially deficient in dissolved 02, aerenchyma form by a process involving extensive lysis of cells of the cortex, beginning about 10 mm behind the root tip (McPherson, 1939;Drew et al, 1979;Konings, 1982). Temporary exposure of adventitious roots to nutrient solutions deficient in N (nitrate and ammonium) or P also causes aerenchyma formation in the absence of any O2 shortage (Konings and Verschuren, 1980;Drew et al, 1989). However, under both O2 shortage and N or P deficiency, aerenchyma formation appears to be triggered by ethylene.…”

“…90-37264-5523. * Corresponding author; fax 1-409-845-0627. 861 depresses it (Drew et al, 1989); instead, they strongly enhanced the sensitivity of cortical cells to ethylene, causing a more rapid lysis of cells in the presence of very low concentrations of the gas (He et al, 1992). As with hypoxia, inhibitors of ethylene-mediated responses (Ag' or AVG) block aerenchyma formation in roots growing in N-or P-deficient solution, suggesting that ethylene is involved in aerenchyma formation under these conditions as well (He et al, 1992).…”