Karen J. Bailey

The signalling peptide EPFL9 is a positive regulator of stomatal development

The putative secretory peptides epidermal patterning factor 1 (EPF1) and EPF2 act as negative regulators of stomatal clustering and density early in Arabidopsis leaf development. Here, we investigated whether the related peptide gene epidermal patterning factor-like 9 (EPFL9), which is coexpressed with EPF1 and stomatal density and distribution 1 (SDD1), also plays a role in controlling stomatal development. Plants manipulated to constitutively overexpress EPFL9 showed increased stomatal density and clustering, and those manipulated to have reduced EPFL9 expression showed reduced stomatal density with no clustering, confirming that EPFL9 is a regulator of stomatal development. Genetic analysis was consistent with EPFL9 acting independently of EPF1 to control stomatal clustering, independently of EPF2 to regulate stomatal density, and independently of SDD1 to control both stomatal clustering and density. These findings demonstrate that at least three secretory peptides independently regulate stomatal development. Surprisingly, EPFL9 acts to increase, rather than decrease, stomatal density and clustering. However, in common with EPF1 and EPF2, EPFL9 is unlikely to be a substrate for proteolysis by SDD1.

Coordinate Regulation of Phosphoenolpyruvate Carboxylase and Phosphoenolpyruvate Carboxykinase by Light and CO2 during C4 Photosynthesis

The aim of this study was to investigate the relationship between the phosphorylation and activation states of phosphoenolpyruvate carboxykinase (PEPCK) and to investigate how the phosphorylation states of PEPCK and phosphoenolpyruvate carboxylase (PEPC) are coordinated in response to light intensity and CO2 concentration during photosynthesis in leaves of the C4 plant Guinea grass (Panicum maximum). There was a linear, reciprocal relationship between the phosphorylation state of PEPCK and its activation state, determined in a selective assay that distinguishes phosphorylated from nonphosphorylated forms of the enzyme. At high photon flux density and high CO2 (750 μL L−1), PEPC was maximally phosphorylated and PEPCK maximally dephosphorylated within 1 h of illumination. The phosphorylation state of both enzymes did not saturate until high light intensities (about 1,400 μmol quanta m−2 s−1) were reached. After illumination at lower light intensities and CO2 concentrations, the overall change in phosphorylation state was smaller and it took longer for the change in phosphorylation state to occur. Phosphorylation states of PEPC and PEPCK showed a strikingly similar, but inverse, pattern in relation to changes in light and CO2. The protein phosphatase inhibitor, okadaic acid, promoted the phosphorylation of both enzymes. The protein synthesis inhibitor, cycloheximide, blocked dark phosphorylation of PEPCK. The data show that PEPC and PEPCK phosphorylation states are closely coordinated in vivo, despite being located in the mesophyll and bundle sheath cells, respectively.

C4 Photosynthesis in the Rice Paddy: Insights from the Noxious Weed Echinochloa glabrescens

Transcriptomic profiling of the paddy weed Echinochloa glabrescens identifies its C4 molecular signature and genes important for paddy growth. The C4 pathway is a highly complex trait that increases photosynthetic efficiency in more than 60 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 phosphoenolpyruvate carboxylase. 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.

Nitrogen recycling from the xylem in rice leaves: dependence upon metabolism and associated changes in xylem hydraulics

Highlight Manipulation of phosphoenolpyruvate carboxykinase activity in rice leaves suggests that metabolism and transport participate in recycling of xylem amino acids and amides and that excess N modulates xylem hydraulics.

Mutants of C4 photosynthesis.

The method of screening for mutants was based on that originally described by Somerville and Ogren, who isolated a range of mutants of Arabidopsis thaliana (1) that lacked key enzymes of the photorespiratory carbon and nitrogen cycle (2). Somerville and Ogren argued that in C3 plants grown at elevated concentrations of CO2 glycollate-2-P formation would be prevented due to the inhibition of the oxygenase function of Rubisco. Thus there would be no flux of carbon and nitrogen through the photorespiratory cycle and any enzyme deficiencies would not be detrimental. Mutant plants would grow normally in elevated CO2 but would exhibit severe stress symptoms when exposed to ambient air. Utilising this principle, a total of seven barley mutants ( plus one double mutant) lacking enzymes of the photorespiratory cycle, have been isolated at Lancaster and Rothamsted (3). More recently heterozygous barley mutants containing enzyme activity (e.g. glutamine synthetase, glutamate synthase and glycine decarboxylase) varying from 40 to 100% have been studied in detail (4,5).