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

Transcriptomics of C4 photosynthesis in rice paddy

This is the author accepted manuscript. The final version is available from the American Society of Plant Biologists via http://dx.doi.org/​10.​1104/​pp.​15.​00889The C₄ pathway is a highly complex trait that increases photosynthetic efficiency in over sixty plant lineages. Although the majority of C₄ plants occupy disturbed, arid and nutrient-poor habitats, some grow in high-nutrient, waterlogged conditions. One such example is Echinochloa glabrescens, which is an aggressive weed of rice paddies. We generated comprehensive transcriptome datasets for C₄ E. glabrescens and C₃ rice to identify genes associated with adaption to waterlogged, nutrient-replete conditions, but also used the data to better understand how C₄ photosynthesis operates in these conditions. Leaves of E. glabrescens exhibited classical Kranz anatomy with lightly lobed mesophyll cells having low chloroplast coverage. As with rice and other hygrophytic C₃ species, leaves of E. glabrescens accumulated a chloroplastic phosphoenolpyruvate carboxylase protein, albeit at reduced amounts relative to rice. The arid-grown species Setaria italica (C₄) and Brachypodium distachyon (C₃) were also found to accumulate chloroplastic PEPC. We identified a molecular signature associated with C₄ photosynthesis in nutrient-replete, waterlogged conditions that is highly similar to those previously reported from C₄ plants that grow in more arid conditions. We also identified a cohort of genes that have been subjected to a selective sweep associated with growth in paddy conditions. Overall, this approach highlights the value of using wild species such as weeds to identify adaptions to specific conditions associated with high-yielding crops in agriculture

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

Plant biochemistry and molecular biology

xvi, 364 p. : ill. ; 26 cm