Senescence progression in a single darkened cotyledon depends on the light status of the other cotyledon in Cucurbita pepo (zucchini) seedlings: potential involvement of cytokinins and cytokinin oxidase/dehydrogenase activity

Abstract

Darkness mediates different senescence-related responses depending on the targeting of dark treatment (whole plants or individual leaves) and on the organs that perceive the signal (leaves or cotyledons). As no data are available on the potential role of darkness to promote senescence when applied to individual cotyledons, we have investigated how darkness affects the progression of senescence in either a single or both individually darkened cotyledons of young 10-day-old Cucurbita pepo (zucchini) seedlings. Strong acceleration of senescence was observed when both cotyledons were darkened as judged by the damage in their anatomical structure, deterioration of chloroplast ultrastructure in parallel with decreased photosynthetic rate and photochemical quantum efficiency of PSII. In addition, the endogenous levels of cytokinins (CKs) and IAA were strongly reduced. In a single individually darkened cotyledon, the structure and function of the photosynthetic apparatus as well as the contents of endogenous CKs and IAA were much less affected by darkness, thus suggesting inhibitory effect of the illuminated cotyledon on the senescence of the darkened one. Apparently, the effect of darkness to accelerate/delay senescence in a single darkened cotyledon depends on the light status of the other cotyledon from the pair. The close positive correlation between CK content and the activity of CK oxidase/dehydrogenase (CKX; EC 1.4.3.18/1.5.99.12) suggested that CKX was essentially involved in the mechanisms of downregulation of endogenous CK levels. Our results indicated that CKX-regulated CK signaling could be a possible regulatory mechanism controlling senescence in individually darkened cotyledons.

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

“…In contrast to developmental cotyledon senescence, the storage cytokinin O-glucosides were decreased only in the cotyledons of whole darkened plants, suggesting different metabolic regulation of endogenous CK levels during natural and dark-induced senescence. Besides, involvement of cytokinin oxidase/dehydrogenase in cytokinin signaling during senescence has been recently demonstrated in individually darkened cotyledons (Ananieva et al 2008b).…”

Local induction of senescence by darkness in Cucurbita pepo (zucchini) cotyledons or the primary leaf induces opposite effects in the adjacent illuminated organ

“…The CKX activity was determined by in vitro assays based on the conversion of [2-3 H]iP (prepared by Isotope Laboratory, Institute of Experimental Botany AS CR, Prague, Czech Republic) to [ 3 H]adenine, as described previously (Ananieva et al 2008). The assay mixture (50 lL final volume) included 100 mM MOPS-NaOH buffer containing 75 lM 2,6-dichloroindophenol (pH 7.0), 2 lM substrate ([2-3 H]iP, 7.4 TBq mol -1 ) and enzyme preparation equivalent to 175 mg or 35 mg tissue fresh weight (corresponding to 0.10-0.21 or 0.31-0.41 mg protein g -1 FW) for cotyledons and Fig.…”

“…Detection and quantitation of cytokinins were carried out using HPLC/MS system, as described elsewhere (Ananieva et al 2008). Internal deuterated CKs standards were added to the samples prior to extraction.…”

“…Our recent results on the potential role of darkness to affect senescence progression when applied to individual Cucurbita pepo (zucchini) cotyledons have shown a strong acceleration of senescence when both cotyledons were darkened for 2 days as determined by deterioration of chloroplast ultrastructure in parallel with a decrease in photosynthetic rate, photochemical quantum efficiency of photosystem II (PSII), endogenous levels of CKs and strong inhibition of chloroplast transcription based mainly on the activity of plasted-encoded RNA polymerse (PEP) (Ananieva et al 2008;Mishev et al 2009Mishev et al , 2011. In contrast, when a single cotyledon was individually darkened, senescence was delayed suggesting the inhibitory effect of the illuminated cotyledon on the senescence of the darkened one (Ananieva et al 2008). Thus, recognition of mechanisms controlling the communication between the two cotyledons in their response to the locally applied dark treatment provokes the question if similar communication exists between the cotyledons and the primary leaf.…”

Methyl jasmonate-dependent senescence of cotyledons in Ipomoea nil

“…Many SAGs are similar to genes probably involved in the breakdown and mobilization of nutrients, such as proteases, RNases, or glutamine synthetases (Lin et al 2015). An essential role in coordinating anatomical, physiological and biochemical changes accompanying senescence is attributed to plant hormones, such as cytokinin, ethylene, abscisic acid, auxin and jasmonates (JAs) (Ananieva et al 2008a;Ellis et al 2005;Hung and Kao 2004;Jing et al 2008;Okushima et al 2005). Hormonal pathways are involved in cotyledon senescence-process that may be age-controlled or stimulated by plant responses to stresses.…”