Correlation of Xylem Sap Cytokinin Levels with Monocarpic Senescence in Soybean

Abstract

Cytokinins (CKs) coming from the roots via the xylem are known to delay leaf senescence, and their decline may be important in the senescence of soybean (Glycine max) plants during pod development (monocarpic senescence). Therefore, using radioimmunoassay of highly purified CKs, we quantified the zeatin (Z), zeatin riboside (ZR), the dihydro derivatives (DZ, DZR), the 0-glucosides, and DZ nucleotide in xylem sap collected from root stocks under pressure at various stages of pod development. Z, ZR, DZ, and DZR dropped sharply during early pod development to levels below those expected to retard senescence. Pod removal at full extension, which delayed leaf senescence, caused an increase in xylem sap CKs (particularly ZR and DZR), while depodding at late podfill, which did not delay senescence, likewise did not increase the CK levels greatly. The levels of the 0-glucosides and the DZ nucleotide were relatively low, and they showed less change with senescence or depodding. The differences in the responses of individual CKs to senescence and depodding suggest differences in their metabolism. Judging from their activity, concentrations and response to depodding, DZR and ZR may be the most important senescence retardants in soybean xylem sap. These data also suggest that the pods can depress CK production by the roots at an early stage and this decrease in CK production is required for monocarpic senescence in soybean.CK2 appears to be the major senescence-retarding hormone in plants, and its role in leaves is particularly important (30). Nonetheless, there is little integrated information on the CK hormone systems regulating senescence or other processes (21). A wide variety of studies have shown that leaf senescence is usually correlated with a decrease in CK activity levels in the leaves and have implicated roots as the major sources of CKs in mature leaves (30). These root-produced CKs are carried through the xylem into the leaves with the transpiration stream.In soybean, the developing pods, specifically the seeds, cause the plant to degenerate (monocarpic senescence) and die (14,15,19,20). Removal of the pods before, but not during, late podfill can prevent the dramatic yellowing and death of the plant (15, 19). How does CK fit into this correlative control picture? Early in reproductive development, the foliar CK-like activity (16) declines. This decrease is due neither to diversion of the flux from the leaves to the pods (22, 23) nor to an increase in the metabolism of CKs (Z and ZR and their metabolites), which is quite rapid anyhow in mature leaves (22,23,27). Thus, a decline in CK production by the roots could account for the decrease in foliar CK levels. In order to test further the connection between CKs and leaf senescence and to fill in a gap in our understanding of the role of CK in the control of senescence, this study examines the xylem sap levels of CK as a relative index of CK flux in plants allowed to develop fruit and senesce normally as well as in depodded plants. MATERIALS AND METHODS Pl...

Changes in Cytokinin Activities and Mass Spectrometric Analysis of Cytokinins in Root Exudates of Rice Plant (Oryza sativa L.)

“…Nooden et al (16) reported that RZ and dihydrozeatin riboside were the predominant cytokinins in the xylem sap of soybean, whereas the contents of 0-glucosides and ribotides were low. Davey and van Staden (4,5) reported that Z and RZ were predominant in the exudates of white lupine plants.…”

“…Cytokinins transported from the roots via the xylem are known to delay leaf senescence (6,16,18). In the exudate of rice plants, several cytokinins were identified (15).…”

“…This fact suggests that physiological activity in the roots of Akenohoshi was higher than that in Nipponbare. It is known that a decline in the physiological activity of roots causes leaf senescence and death of the leaves at the lower positions (10), but the mechanism by which this occurs is not understood well.Cytokinins transported from the roots via the xylem are known to delay leaf senescence (6,16,18). In the exudate of rice plants, several cytokinins were identified (15).…”

Possible Involvement of Leaf Gibberellins in the Clock-Controlled Expression of XSP30, a Gene Encoding a Xylem Sap Lectin, in Cucumber Roots

“…Interestingly, the roots control aspects of the development of aerial organs, possibly acting via growth-related compounds in the xylem sap (Kinet et al, 1993;Satoh, 1996). For example, cytokinin, abscisic acid, and other growth-related compounds that are synthesized in root tissues are involved in stomatal responses (Else et al, 1995;Liang et al, 1997), leaf senescence (Nooden et al, 1990;Soejima et al, 1992), lateral bud development (Bangerth, 1994; Beveridge et al, 1997), flower bud formation (Kinet et al, 1993), leaf greening , and adventitious root formation (Kuroha et al, 2002).Recently, macromolecules have been found in xylem sap, including oligo-and polysaccharides (Satoh et al, 1992; Campbell et al, 1995) and proteins such as peroxidase (Biles and Abeles, 1991), chitinase (Masuda et al, 2001), a cucumber (Cucumis sativus) rootspecific Gly-rich protein (CRGRP; Sakuta et al, 1998;Sakuta and Satoh, 2000), pathogenesis-related (PR) proteins (Rep et al, 2002), a Cys-rich protein (Rep et al, 2003), and a novel 30-kD protein (XSP30; Masuda et al, 1999). However, the regulation of the production of these xylem sap proteins and their physiological functions are not well understood.…”

“…Interestingly, the roots control aspects of the development of aerial organs, possibly acting via growth-related compounds in the xylem sap (Kinet et al, 1993;Satoh, 1996). For example, cytokinin, abscisic acid, and other growth-related compounds that are synthesized in root tissues are involved in stomatal responses (Else et al, 1995;Liang et al, 1997), leaf senescence (Nooden et al, 1990;Soejima et al, 1992), lateral bud development (Bangerth, 1994; Beveridge et al, 1997), flower bud formation (Kinet et al, 1993), leaf greening , and adventitious root formation (Kuroha et al, 2002).…”