Metabolic Response of Maize Roots to Hyperosmotic Shock

Abstract

31P nuclear magnetic resonance spectroscopy was used to study the response of maize (Zea mays L.) root tips to hyperosmotic shock. The aim was to identify changes in metabolism that might be relevant to the perception of low soil water potential and the subsequent adaptation of the tissue to these conditions. Osmotic shock was found to result in two different types of response: changes in metabolite levels and changes in intracellular pH. The most notable metabolic changes, which were produced by all the osmotica tested, were increases in phosphocholine and vacuolar phosphate, with a transient increase in cytoplasmic phosphate. It was observed that treatment with ionic and nonionic osmotica produced different effects on the concentrations of bioenergetically important metabolites. It is postulated that these changes are the result of hydrolysis of phosphatidylcholine and other membrane phospholipids, due to differential activation of specific membrane-associated phospholipases by changes in the surface tension of the plasmalemma. These events may be important in the detection of osmotic shock and subsequent acclimatization. A cytoplasmic alkalinization was also observed during hyperosmotic treatment, and this response, which is consistent with the activation of the plasmalemma H -ATPase, together with the other metabolic changes, may suggest the existence of a complex and integrated mechanism of osmoregulation.

Intracellular pH stability in the aquatic resurrection plant Chamaegigas intrepidus in the extreme environmental conditions that characterize its natural habitat

“…These results contrast with those of Spickett et al (1992), who found that hyperosmotic shock caused a small alkalinization of both the cytoplasm and the vacuole in maize root tips. Suspending maize root tips in solutions with a water potential of k1n35 MPa caused an increase in pH cyt of between 0n05 and 0n1 pH units within 3 h of reducing the water potential of the suspending medium to k1n35 MPa with several non-ionic osmotica, and a similar effect was observed in response to salt stress (Spickett et al, 1993).…”

“…The contaminant was present in a range of PEG compounds with different molecular weights and in principle it could confuse the interpretation of $"P NMR analyses of the response to hyperosmotic shock. In maize root tips (Spickett et al, 1992), the large vacuolar P i signal would have masked any contribution from a contaminating phosphodiester ; whereas in Prasiola crispa (Bock et al, 1996), the absence of a vacuole allowed the detection of an endogenous phosphodiester signal with an intensity that increased during PEG treatment. Bock et al (1996) speculated that the increase in the unidentified phosphodiester could be responsible for stimulating the hydrolysis of polyphosphate during the response to hyperosmotic shock, but the credibility of this suggestion now depends on showing that the PEG solution was not contaminated.…”

“…For example, although the cytoplasmic pH (pH cyt ) is often only weakly dependent on the external pH, this generalization tends to fail as the external pH reaches more extreme values that threaten the survival of the plant (Felle, 1988 ;Fox & Ratcliffe, 1990 ;Gout, Bligny & Douce, 1992). Hyperosmotic shock can also lead to pH changes, with maize root tips showing a small increase in both pH cyt and the vacuolar pH (pH vac ) (Spickett, Smirnoff & Ratcliffe, 1992) and Valerianella locusta leaf discs showing a decrease in pH cyt (Daeter & Hartung, 1990). By contrast, the desiccation-resistant Antarctic alga Prasiola crispa showed no change in pH cyt in response to a hyperosmotic shock (Bock et al, 1996) and root tips of the salt-tolerant grass Spartina anglica maintained a constant pH cyt during salt stress (Spickett, Smirnoff & Ratcliffe, 1993).…”

“…normally be expected in the spectra of roots (Spickett et al, 1992(Spickett et al, , 1993 and leaves (Loughman, Ratcliffe & Southon, 1989). A 4-h acquisition time was usually required to define the position of the cytoplasmic inorganic phosphate (P i ) signal in the whole plant spectra, which is much longer than the 5-30 min that is commonly used for the spectra of root tips, and this was attributed to the low cytoplasmic volume of the mature tissues of the C. intrepidus plant and the difficulty in maximizing the packing of the sample in the detectable volume of the NMR tube.…”

“…Hyperosmotic shock with non-ionic osmotica caused increases in the phosphocholine and vacuolar P i signals in the $"P NMR spectra of maize root tips, as well as a transient increase in the cytoplasmic P i pool, and these metabolic changes were considered to be a consequence of the hydrolysis of membrane constituents triggered during the detection and response to osmotic shock (Spickett et al, 1992). By contrast, PEG treatment had no effect on the phosphocholine, cytoplasmic P i or vacuolar P i pools in the C. intrepidus leaves, and this suggests that the membranes of C. intrepidus are well protected against dehydration.…”