46 research outputs found
Nitrogen recycling from the xylem in rice leaves: dependence upon metabolism and associated changes in xylem hydraulics
Measurements of amino acids in the guttation fluid and in the xylem exudates of cut leaves from intact plants provide
evidence of the remarkable efficiency with which these nitrogenous compounds are reabsorbed from the xylem sap.
This could be achieved by mechanisms involving intercellular transport and/or metabolism. Developmental changes
in transcripts and protein showed that transcripts for phosphoenolpyruvate carboxykinase (PEPCK) increased from
the base to the leaf tip, and were markedly increased by supplying asparagine. Supplying amino acids also increased
the amounts of protein of PEPCK and, to a lesser extent, of pyruvate, Pi dikinase. PEPCK is present in the hydathodes,
stomata and vascular parenchyma of rice leaves. Evidence for the role of PEPCK was obtained by using 3-mercaptopicolinic
acid (MPA), a specific inhibitor of PEPCK, and by using an activation-tagged rice line that had an increase
in PEPCK activity, to show that activation of PEPCK resulted in a decrease in N in the guttation fluid and that treatment
by MPA resulted in an increase in amino acids in the guttation fluid and xylem sap towards the leaf tip. Furthermore,
increasing PEPCK activity decreased the amount of guttation fluid, whereas decreasing PEPCK activity increased the
amount of xylem sap or guttation fluid towards the leaf tip. The findings suggest the following hypotheses: (i) both
metabolism and transport are involved in xylem recycling and (ii) excess N is the signal involved in modulating xylem
hydraulics, perhaps via nutrient regulation of water-transporting aquaporins. Water relations and vascular metabolism
and transport are thus intimately linked
Photorespiration: metabolic pathways and their role in stress protection
Photorespiration results from the oxygenase reaction catalysed by ribulose-1,5-bisphosphate carboxylase/
oxygenase. In this reaction glycollate-2-phosphate is produced and subsequently metabolized in the
photorespiratory pathway to form the Calvin cycle intermediate glycerate-3-phosphate. During this metabolic
process, CO2 and NH3 are produced and ATP and reducing equivalents are consumed, thus
making photorespiration a wasteful process. However, precisely because of this ine¤ciency, photorespiration
could serve as an energy sink preventing the overreduction of the photosynthetic electron transport
chain and photoinhibition, especially under stress conditions that lead to reduced rates of photosynthetic
CO2 assimilation. Furthermore, photorespiration provides metabolites for other metabolic processes, e.g.
glycine for the synthesis of glutathione, which is also involved in stress protection. In this review, we
describe the use of photorespiratory mutants to study the control and regulation of photorespiratory pathways.
In addition, we discuss the possible role of photorespiration under stress conditions, such as
drought, high salt concentrations and high light intensities encountered by alpine plants
Oxygen requirement and inhibition of C4 photosynthesis
The basis for O2 sensitivity of C4 photosynthesis was evaluated
using a C4-cycle-limited mutant of Amaranthus edulis (a phosphoenolpyruvate
carboxylase-deficient mutant), and a C3-cyclelimited
transformant of Flaveria bidentis (an antisense ribulose-1,5-
bisphosphate carboxylase/oxygenase [Rubisco] small subunit
transformant). Data obtained with the C4-cycle-limited mutant
showed that atmospheric levels of O2 (20 kPa) caused increased
inhibition of photosynthesis as a result of higher levels of photorespiration.
The optimal O2 partial pressure for photosynthesis was
reduced from approximately 5 kPa O2 to 1 to 2 kPa O2, becoming
similar to that of C3 plants. Therefore, the higher O2 requirement
for optimal C4 photosynthesis is specifically associated with the C4
function. With the Rubisco-limited F. bidentis, there was less inhibition
of photosynthesis by supraoptimal levels of O2 than in the
wild type. When CO2 fixation by Rubisco is limited, an increase in
the CO2 concentration in bundle-sheath cells via the C4 cycle may
further reduce the oxygenase activity of Rubisco and decrease the
inhibition of photosynthesis by high partial pressures of O2 while
increasing CO2 leakage and overcycling of the C4 pathway. These
results indicate that in C4 plants the investment in the C3 and C4
cycles must be balanced for maximum efficiency