Changes in endogenous cytokinin levels in cotyledons of Cucurbita pepo (zucchini) during natural and dark‐induced senescence

Abstract

Cytokinin (CK) levels in cotyledons of Cucurbita pepo L. (zucchini) were investigated through the processes of postgermination, greening, natural senescence and subsequent rejuvenation. The concentrations of the physiologically active CK bases, ribosides and nucleotides, as well as the cis-isomers of zeatin derivatives, decreased between the first and fifth weeks of cultivation under controlled light conditions. At the same time, the levels of storage CK O-glucosides and physiologically inactive CK 7-and 9-glucosides increased with senescence. With plant decapitation and subsequent cotyledon rejuvenation, not only the chlorophyll content but also the levels of physiologically active CKs, nucleotides and cis-zeatin derivatives increased. The levels of O-glucosides, however, decreased. When 1-week-old seedlings were transferred to the dark, there was a progressive reduction in cotyledon chlorophyll content, deterioration of chloroplast ultrastructure and a decrease in physiologically active CKs and their nucleotides. In contrast with natural senescence, the storage CK O-glucosides decreased under dark conditions, suggesting different metabolic regulation of endogenous CK levels during natural and dark-induced senescence of zucchini cotyledons. The chlorophyll loss of dark-treated cotyledons could be partially reversed, even after 5 days, with return to light conditions. During this recovery, physiologically active CKs and their nucleotides again increased, whereas the storage CK O-glucosides and cis-zeatins decreased. The present results suggest that dark-induced destruction and subsequent restoration of chloroplasts during light shifts are controlled by changes in the levels of physiologically active CKs and their nucleotides.Abbreviations -CK, cytokinin; DHZ, dihydrozeatin; DHZ7G, dihydrozeatin 7-glucoside; DHZ9G, dihydrozeatin 9-glucoside; DHZR, dihydrozeatin 9-riboside; DHZRMP, dihydrozeatin 9-riboside-5 0 -monophosphate; DHZROG, dihydrozeatin 9-riboside O-glucoside; FW, fresh weight; iP, N 6 -(D 2 -isopentenyl)adenine; iP7G, N 6 -(D 2 -isopentenyl)adenine 7-glucoside; iP9G, N 6 -(D 2 -isopentenyl)adenine 9-glucoside; iPR, N 6 -(D 2 -isopentenyl)adenine 9-riboside; iPRMP, N 6 -(D 2 -isopentenyl)adenine 9-riboside-5 0

Different effects of dark treatment on pigment composition and photosystem I and II activities in intact cotyledons and primary leaves of Cucurbita pepo (zucchini)

“…On the other hand, dark treatment caused a decrease in total chloroplast RNA synthesis and in the content of biologically active cytokinins. Prolongation of the dark treatment of zucchini seedlings (5 days) resulted in promotion of cotyledon senescence as judged by the progressive reduction in chlorophyll content and deterioration of chloroplast ultrastructure (Ananieva et al 2004). These changes correlated with a reduction in the content of bioactive CKs which are the major senescence-inhibiting hormones (van Staden et al 1988;Kim et al 2006).…”

Control by cytokinins of the cellular behavior in the plate meristem of zucchini cotyledons

“…So, to differentiate the effect on cell enlargement from that on cell division, the other constituent of organ growth, a thorough cytological study is required. The endogenous pool of cytokinins responsible for the growth of nontreated cotyledons has a complex and dynamic nature (Ananieva et al 2004). The differences in action between cytokinins differing in their chemical nature are not well understood, although various exogenous cytokinins have been applied in experimental studies.…”

Methyl jasmonate-dependent senescence of cotyledons in Ipomoea nil

“…These substances cause chlorophyll loss, degradation of Rubisco and inhibition of its biosynthesis. They can also induce the disorganization of the thylakoid membrane system in chloroplasts, affect the functional activity of PSII, decrease the rates of transpiration and photosynthesis, as well as cell up-regulates several SAGs shown in different plants (Ananieva et al 2004b;Beltrano et al 1998;He and Gan 2001;He et al 2002;Liu et al 2015;Weaver et al 1998;Weidhase et al 1987a, b;Woo et al 2001;Oh et al 1996). Jasmonates have been widely shown to be powerful promoters of leaf senescence (Chen and Kao 1998;Chou and Kao 1992;Hung and Kao 1996;Tsai et al 1996;Ueda and Kato 1981;Weidhase et al 1987a), but their participation in cotyledon senescence is still poorly understood (Ananieva et al 2004a(Ananieva et al , 2007.…”

“…JAs, i.e., both JA and JAMe, are promoters of leaf senescence upon their exogenous application (Creelman and Mullet 1997;Wasternack and Hause 2002). JAMe is the most effective stimulator of leaf senescence in the leaves of wild-type and mutant Arabidopsis (Oh et al 1996;Weaver et al 1998;Woo et al 2001), leaf segments of barley (Reinbothe et al 1993(Reinbothe et al , 1997Weidhase et al 1987a) and intact cotyledons of Cucurbita pepo (zucchini) (Ananieva et al 2004b).…”