Compartmentation of soluble carbohydrates, of starch and of malate in motor organs (pulvini) and other parts of Phaseolus coccineus L. leaves

Abstract

Quantitative histochemistry was used to investigate the tissue-specific compartmentation of soluble carbohydrates (sucrose, glucose, fructose), starch and malate in the laminar pulvinus, leaf blade and petiole of Phaselous coccineus L. at day and night positions of diurnal leaf movement. Total carbohydrate levels measured in a series of cross sections along individual pulvini of 24-d-old plants showed only small differences between the day and night positions of the respective leaf. In contrast, the level of malate changed during diurnal leaf movement, especially in the central part of a pulvinus. The levels of glucose and fructose in the pulvinus increased towards the transition zones between the pulvinus and lamina, and pulvinus and petiole, and this trend was even more pronounced for starch. By contrast, sucrose levels were highest in the pulvinus proper. The transverse compartmentation of metabolites was studied in distinct, approx. 0.5-mm-thick tissue slices from the central part of a pulvinus. These were dissected further into up to 14 distinct subsamples (bundle, bundle sheath, motor tissues, flanks). Irrespective of the position of the leaf (day or night), the central vascular core and the surrounding bundle sheath had high levels of sucrose (up to 500 mmol-(kg DW)(-1)) and low levels of glucose and fructose (below 100 mmol-(kg DW)(-1)), while in the cortex the situation was reversed. In the night position the level of sucrose decreased by approx. 30% in the bundle sheath and the central vascular core but not in the other sections. We thus suggest that because of the relatively small diurnal changes in their cortical pools, soluble sugars are not involved in the osmotic processes resulting in leaf movement. In contrast, pulvini from 14-d-old plants showed an interesting diurnal change in starch and malate pools in the outermost layer of the extensor. Here starch increased at night while the malate pool was lowered nearly stoichiometrically. Inverse pool sizes were found in the day position of the respective leaves. Although less significant, the opposite diurnal variation occurred in samples taken from the flexor region. We thus were able to locate areas of different carbohydrate activities in the laminar pulvinus of P. coccineus. The central vascular core, including the bundle sheath, is involved in temporary storage of photoassimilates, and the cortical regions are responsible for osmotically driven leaf movement. The results are discussed with respect to guard-cell physiology.

A Tale of Two Sugars: Trehalose 6-Phosphate and Sucrose

“…The pattern of starch turnover in guard cells is different to that in mesophyll cells; guard cell chloroplasts contain high levels of starch in the dark, and this is degraded in the light (Outlaw and Manchester, 1979;Rieger et al, 1992). Recent studies have shown that rapid degradation of starch is needed to drive stomatal opening in the light, in response to phototropinmediated blue light signaling, and that degradation occurs via a distinct pathway involving a-AMYLASE3 and a redox-sensitive b-amylase, BAM1 (Valerio et al, 2011;Prasch et al, 2015;Horrer et al, 2016).…”

Trehalose metabolism in plants

“…phosphate, to the plant in return for carbohydrates. There is evidence that trehalose plays a significant role in ectomycorrhizal relationships (Rieger et al, 1992;Corrê a et al, 2010;Nehls et al, 2010). Trehalose represents a major sink for carbon in Amanita muscaria and Pisolithus microcarpus ectomycorrhizae associated with poplar (Populus tremula x tremuloides) and Eucalyptus globulus roots, respectively (Martin et al, 1998;L opez et al, 2007).…”

Malate Compartmentation-Responses to a Complex Metabolism

“…4. Malate can be used as an osmoticum (42, 85,86,90,118). The best known example are guard cells, in which malate synthesis and degradation play an important role in stomatal movement.…”