Nitrate uptake and metabolism by roots of soybean plants under oxygen deficiency.

Abstract

Nitrate is reported to improve tolerance of plants towards oxygen deficiency imposed by waterlogging of the root system, but little is known of the mechanism underlying the phenomenon. We studied the metabolism of nitrate in roots under hypoxia, using soybean plants growing in a hydroponic system after suspending aeration and covering the surface of the nutrient solution with mineral oil. Nitrate depletion from the medium was greater under hypoxia than normoxia, but in the presence of chloramphenicol, consumption under hypoxia was significantly reduced. Nitrite accumulated in the medium under hypoxia and this was partially eliminated by chloramphenicol. Nitrate consumption sensitive to chloramphenicol was attributed to bacterial activity. Endogenous root nitrate was strongly reduced under hypoxia indicating mobilization. Although the transport of nitrate to the shoot via the xylem was also reduced under hypoxia, the severity of this reduction was dependent on the concentration of nitrate in the medium, suggesting that at least some of the nitrate in the xylem came from the medium. Root nitrate reductase was also strongly reduced under hypoxia, but recovered rapidly on return to normoxia. Overall, the data are consistent with two main metabolic fates for chloramphenicolinsensitive nitrate depletion under hypoxia: the reduction of some nitrate to nitrite (despite the reduced nitrate reductase activity) followed by its release to the medium (at least one-third of the nitrate consumed followed this route), and the transport of nitrate to the shoot. Nevertheless, it is highly unlikely that these metabolic routes account for all the nitrate consumed.Key words: Glycine max, hypoxia, nitrate reductase, nitrite, waterlogging. absorção e metabolismo do nitrato em raízes de soja sob deficiência de oxigênio. É conhecido que o nitrato melhora a tolerância de plantas à deficiência de oxigênio provocada pela inundação do sistema radicular. Porém, pouco se sabe sobre as causas deste fenômeno. Realizamos um estudo do metabolismo do nitrato em raízes de soja sob hipóxia, empregando plantas cultivadas em sistema hidropônico, após suspender a aeração e cobrir a superfície da solução nutritiva com óleo mineral. O consumo de nitrato no meio foi maior sob hipóxia do que normóxia. Porém, na presença de cloranfenicol o consumo em hipóxia foi reduzido de forma significativa. Houve acúmulo do nitrito no meio sob hipóxia e este acúmulo foi parcialmente eliminado por cloranfenicol. A redução do consumo de nitrato na presença de cloranfenicol foi atribuída à atividade bacteriana. O nitrato endógeno da raiz foi bastante reduzido sob hipóxia, indicando mobilização do mesmo. Apesar do transporte do nitrato até a parte aérea, via xilema, ter sido também reduzido sob hipóxia, a intensidade dessa redução foi dependente da concentração de nitrato no meio, o que indica que pelo menos uma porção do nitrato encontrado no xilema teve origem no meio. A atividade da redutase do nitrato da raiz também foi fortemente reduzida sob hipóxia, porém rec...

Utilization of 15NO3 − by nodulated soybean plants under conditions of root hypoxia

“…Although our own studies with soybean also demonstrate the operation of the nitrate assimilatory pathway during hypoxia, our conclusion is that the process is quite limited under these conditions. This conclusion is supported by other studies where nitrite was shown to accumulate under hypoxia (Lee 1979;Morard et al 2004;Libourel et al 2006;Brandão and Sodek 2009;Horchani et al 2010). Some of the accumulated nitrite can be reduced to NO by mitochondrial activity where nitrite substitutes oxygen as terminal electron acceptor under oxygen deficiency (Stoimenova et al 2007;Gupta et al 2011;Oliveira et al 2013b, c), thereby producing ATP and regenerating NAD + which may underlie the beneficial effect of nitrate on plant tolerance to hypoxia.…”

“…Several studies indicate induction of the enzyme nitrate reductase under hypoxia (GarciaNovo and Crawford 1973;Glaab and Kaiser 1993;Botrel et al 1996;Morard et al 2004;Horchani et al 2010) but in other cases a decline in activity was observed (Brandão and Sodek 2009). Nevertheless, the reduction of nitrite, the product of nitrate reductase, appears to be strongly impaired under hypoxia (Lee 1978;Botrel et al 1996;Morard et al 2004) leading to the accumulation of nitrite (Lee 1979;Libourel et al 2006;Brandão and Sodek 2009;Horchani et al 2010). Part of this nitrite can be reduced to nitric oxide, a process that may be important for NAD + regeneration (Igamberdiev and Hill 2004;Stoimenova et al 2007;Oliveira et al 2013b).…”

“…This beneficial effect has stimulated research into the uptake and metabolism of nitrate under hypoxia. There is evidence that hypoxia impairs both the uptake and assimilation of nitrate (Lee 1978;Trought and Drew 1981;Buwalda and Greenway 1989;Morard et al 2004;Brandão and Sodek 2009;Oliveira et al 2013a) although information on the metabolism of nitrate under such conditions is still inconclusive. Several studies indicate induction of the enzyme nitrate reductase under hypoxia (GarciaNovo and Crawford 1973;Glaab and Kaiser 1993;Botrel et al 1996;Morard et al 2004;Horchani et al 2010) but in other cases a decline in activity was observed (Brandão and Sodek 2009).…”

“…There is evidence that hypoxia impairs both the uptake and assimilation of nitrate (Lee 1978;Trought and Drew 1981;Buwalda and Greenway 1989;Morard et al 2004;Brandão and Sodek 2009;Oliveira et al 2013a) although information on the metabolism of nitrate under such conditions is still inconclusive. Several studies indicate induction of the enzyme nitrate reductase under hypoxia (GarciaNovo and Crawford 1973;Glaab and Kaiser 1993;Botrel et al 1996;Morard et al 2004;Horchani et al 2010) but in other cases a decline in activity was observed (Brandão and Sodek 2009). Nevertheless, the reduction of nitrite, the product of nitrate reductase, appears to be strongly impaired under hypoxia (Lee 1978;Botrel et al 1996;Morard et al 2004) leading to the accumulation of nitrite (Lee 1979;Libourel et al 2006;Brandão and Sodek 2009;Horchani et al 2010).…”

“…During hypoxia this endogenous nitrate is mobilized and metabolized (Sousa and Sodek 2002;Brandão and Sodek 2009) even when nitrate is present in the medium. A similar phenomenon occurs under normoxia when plants are transferred to a medium free of mineral N (Sprent et al 1987;Lima and Sodek 2003;Oliveira et al 2013a), a further indication that the capacity for nitrate uptake during hypoxia is limited.…”