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

Abstract

Changes in exudation rate and cytokinin activities in the exudates were measured in two varieties of rice (Oryza sativa L.), cv Nipponbare (a Japanese normal cultivar) and cv Akenohoshi (a highyielding cultivar). The exudation rates of Akenohoshi, the leaves of which remained green for a longer time, were higher than those of Nipponbare after the booting stage. Cytokinin activities in the exudates of Akenohoshi were higher than those of Nipponbare during the ripening period. Cytokinins in the exudates collected during the middle of the ripening stage were analyzed with mass spectrometry using deuterium-labeled standards. trans-Zeatin, trans-ribosylzeatin, and N'-isopentenyladenosine were detected as free cytokinins, and zeatin was detected in the hydrolysates of highly polar fractions ("conjugated zeatin") in the exudates of both cultivars. Conjugated zeatin was the predominant cytokinin in both cultivars. Therefore, we suggest that conjugated zeatin is an important form of cytokinin during the ripening stage. The level of each of the cytokinins in Akenohoshi was higher than that in Nipponbare. Also, we discuss the correlation between the leaf senescence and cytokinin content in root exudates.In a previous study to clarify the physiological and ecological characteristics of high-yielding rice varieties (11, 12), it was reported that the yield in Akenohoshi was about 1780 kg/ha higher than that in Nipponbare, a Japanese standard cultivar, and that Akenohoshi had a higher dry matter production capacity compared with Nipponbare. This was attributed to the maintenance of a higher net assimilation rate during the ripening period in the former. 1724 stage were 9.2 ,umol of CO2 m-2 s-' (44.6% of its maximum rate at the panicle formation stage) and 12.7 smol of CO2 m-2 S-1 (62.0%), respectively. In addition, the cause for these differences between the two cultivars was examined by measuring the active and passive water absorption abilities of the roots. The rate of exudation from the main stem and the resistance to water transport from the root surface to the leaves under intense transpiration were higher and lower in Akenohoshi than in Nipponbare, respectively. These differences between the two cultivars were especially large during the ripening stage. 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). However, the constitution of cytokinins in the exudate, the level of these cytokinins, and their effects on leaf photosynthesis and senescence have been examined in only a few studies (20).In this report, we discuss the changes in the exudation rates and cytokinin activities in the exudat...

Effects of Soil Temperature on Growth and Root Function in Rice

“…In addition, increased levels of ABA and decreased levels of cytokinins in shoots were observed when shoot growth was retarded at high soil temperatures in grass (Liu and Huang, 2005). Cytokinin was associated with the maintenance of photosynthetic rate in rice (Ookawa et al, 2004;San-oh et al, 2006;Soejima et al, 1992Soejima et al, , 1995. Then, in this study, the interactive effect of ABA and cytokinins would relate to the decrease in photosynthesis, which contribute to the decline in dry weight of shoot and root under high soil temperature stress.…”

Effects of a Reduction in Soil Moisture from One Month beforeFlowering through Ripening on Dry Matter Production and Ecophysiological Characteristics of Wheat Plants

“…The activity of cytokinins in the xylem sap was higher in the plants in the D plots than in those in the W plots (Table 4). The activities of cytokinins in the xylem sap were high in slow-senescing rice (Soejima et al, 1992), sorghum (Amber et al, 1992), maize (Kondo et al, 2000) and wheat (Nakamura et al, 2003) plants. The activity of cytokinins in the xylem sap might also explain the higher rate of photosynthesis during leaf senescence in the plants in the D plots.…”

“…The activity of the cytokinins in xylem exudates was determined essentially as described by Soejima et al (1992). Forty stems were bundled and cut at a position that was about 20cm above soil level.…”

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).…”