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Ethylene has been implicated in signaling cell death in the lysigenous formation of gas spaces (aerenchyma) in the cortex of adventitious roots of maize (Zea mays) subjected to hypoxia. Various antagonists that are known to modify particular steps in signal transduction in other plant systems were applied at low concentrations to normoxic and hypoxic roots of maize, and the effect on cell death (aerenchyma formation) and the increase in cellulase activity that precedes the appearance of cell degeneration were measured. 60th cellulase activity and cell death were inhibited in hypoxic roots in the presence of antagonists of inositol phospholipids, CaZ+-calmodulin, and protein kinases. By contrast, there was a parallel promotion of cellulase activity and cell death in hypoxic and normoxic roots by contact with reagents that activate G-proteins, increase cytosolic CaZ+, or inhibit protein phosphatases. Most of these reagents had no effect on ethylene biosynthesis and did not arrest root extension. These results indicate that the transduction of an ethylene signal leading to an increase in intracellular Ca2+ is necessary for cell death and the resulting aerenchyma development in roots of maize subjected to hypoxia.

The Ma, gene is one of six genes that regulate the photoperiodic sensitivity of flowering in sorghum (Sorghum bicolor [L.] Moench).The masR mutation of this gene causes a phenotype that is similar to plants that are known to lack phytochrome B, and masR sorghum lacks a 123-kD phytochrome that predominates in light-grown plants and that is present in non-magR plants. A population segregating for Ma, and masR was created and used to identify two randomly amplified polymorphic DNA markers linked to Ma,. These two markers were cloned and mapped in a recombinant inbred population as restriction fragment length polymorphisms. cDNA clones of PHYA and PHYC were cloned and sequenced from a cDNA library prepared from green sorghum leaves. Using a genome-walking technique, a 7941 -bp partia1 sequence of PHYB was determined from genomic DNA from masR sorghum. PHYA, PHYB, and P H Y C all mapped to the same linkage group. The Ma,-linked markers mapped with PHYB more than 121 centimorgans from PHYA and PHYC. A frameshift mutation resulting in a premature stop codon was found in the PHYB sequence from magR sorghum. Therefore, we conclude that the Ma, locus in sorghum is a PHYB gene that encodes a 123-kD phytochrome.The transition from vegetative to reproductive growth is the result of the activation of genes responsible for inflorescence and floral organ formation. These genes, which control apex identity and floral organ morphogenesis, are strictly regulated, since their improper expression results in abnormal flowers and inflorescences (Okamuro et al., 1993;Veit et al., 1993). The initial activation of these genes is usually the result of environmental cues that indicate an appropriate time to flower. The mechanisms by which environmental factors activate inflorescence and floral organ production are complex and many genes are known to be involved in the transduction of environmental signals that regulate flowering (Bernier et al., 1993; Coupland, 1995).Of all of the environmental factors that are sensed by plants, daylength is probably the most important in inducing flowering. The phenomenon whereby daylength regulates flowering is referred to as photoperiodism. An understanding of the effect of daylength on reproductive development has agronomic importance because the ability to alter flowering time allows the cultivation of a species in environments that differ greatly from the one in which it originally evolved. Our understanding of photoperiodism has historically relied upon a physiological examination of the phenomenon. Recently, genetic analysis of floral induction has provided new insights into this process. In the LD plant Arabidopsis thaliana a series of genes has been recognized that influences flowering time, and these genes have been categorized into six phenotypic groups based on earliness or lateness in flowering in response to short days, long days, and vernalization; (Coupland, 1995). The existence of these separate phenotypic classes suggests the existence of severa1 pathways that regulate photoperiod sens...

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...