Contribution of Arginine to Proline Accumulation in Water-stressed Barley Leaves

Abstract

Barley (Hordeum vulgare cv Prior) leaves converted L-U_'4C-arginine to labeled proline. Accumulation of radioactivity in proline was greater in wilted leaves, but only after 9 hours of incubation. As the increase in free proline was detectable after only 3 to 6 hours, it is likely that the observed stimulation of proline labeling represents a result rather than a cause of proline accumulation. Furthermore, the loss of total arginine during water stress was only 10 to 15% of the increase in proline. We condude that arginine probably contributes less than 1% of the carbon in the expanding proline pool of wilted barley leaves.Wilted leaves of certain plants accumulate quantities of free proline considerably greater than could be released through net protein breakdown (5). Most of the accumulated proline must, therefore, be synthesized from other compounds during the period of water stress. Glutamic acid and arginine are likely precursors for water stress-induced proline synthesis, as either may be converted to P5C (Al-pyrroline-5-carboxylic acid) which is the immediate precursor of proline. While glutamate is clearly involved in proline synthesis during water stress (1,3,6), the contribution of arginine has not been evaluated. Although leaf tissues typically contain little free arginine, the inexplicably low protein-arginine content of wilted Bermuda grass leaves (1) suggests that in at least some cases, arginine might contribute significantly to the build up of free proline that occurs during water stress. Experiments reported here were designed to clarify the involvement of arginine metabolism in the accumulation of proline by water-stressed barley leaves. MATERIALS AND METHODSExperimental procedures were as previously described (3). Briefly, second leaves of 2-week-old barley plants (Hordeum vulgare cv Prior) were excised and allowed to take up radioactive precursor (L-U-_4C-arginine, 50 mCi/mol, obtained from Amersham/Searle and purified by elution from Dowex 50 in the ammonium form) in 4 ,1l aqueous solution. Stressed leaves were previously excised and wilted to 75 % of initial fresh weight, and not rewatered after uptake of radioactive material; turgid leaves were kept in vials of distilled H20 for the rest of the experiment.

Overexpression of [delta]-Pyrroline-5-Carboxylate Synthetase Increases Proline Production and Confers Osmotolerance in Transgenic Plants

“…108, 1995 glutamate via P5CS under stress conditions . This is consistent with the radiolabeled isotope studies that have revealed that glutamate rather than Orn is the principal precursor for Pro biosynthesis in osmotically stressed plants (Boggess and Stewart, 1976;Rhodes et al, 1986).…”

“…Many eubacteria, algae, and higher plants accumulate free Pro in response to osmotic stresses (Schobert, 1977; Csonka enon first observed by Kemble andMacPherson (1954) in andHanson, 1991;Delauney and Verma, 19931, a phenom-have cloned c~~~s encoding p5CR and p5cs, a bifuncwilted rYe grass. Accumulation Of due PrimarilY to de novo synthesis (Boggess et 1976;Stewart and Hansonf l9B0; 1986; 'Oetbe% and Sharpr 1991)~ although a reduced rate Of catabolism has been Observed genase activities and catalyzes the first two steps in Pro biosynthesis (Delauney and Verma, 1990;Hu et al, 1992).…”

NITROGEN METABOLISM IN POTERIUM SANGUISORBA DURING WATER STRESS

“…Although proline accumulation in water-stressed tissues may be attributed to increased synthesis and/or reduced breakdown (see Stewart and Larher, 1980), radioactive isotope experiments have indicated that stimulation of proline biosynthesis from glutamate is the major factor (Barnett and Naylor, 1966;Boggess and Stewart, 1976;Stewart and Boggess, 1977). As the enzymic data presented here suggested a decreased rate of nitrogen assimilation during drought, proline accumulation may therefore occur at the expense of other nitrogenous compounds.…”

Role of Carbohydrates in Proline Accumulation in Wilted Barley Leaves

“…These comparisons add to the previously discussed differences in the mechanism causing proline to accumulate in bean and barley (15). Those experiments showed that in wilted bean leaves, ornithine and arginine serve as precursors to proline in addition to glutamate (15) but in wilted barley a stimulation of the conversion of ornithine and arginine to proline is only a consequence of wilting (3). The water stress-induced stimulation of the conversion of glutamate to other metabolic products in starved leaves suggests that wilting causes more rapid loss of label from glutamate in barley.…”