Regulation of chloroplast development and function by cytokinin

Abstract

A role of the plant hormone cytokinin in regulating the development and activity of chloroplasts was described soon after its discovery as a plant growth regulator more than 50 years ago. Its promoting action on chloroplast ultrastructure and chlorophyll synthesis has been reported repeatedly, especially during etioplast-to-chloroplast transition. Recently, a protective role of the hormone for the photosynthetic apparatus during high light stress was shown. Details about the molecular mechanisms of cytokinin action on plastids are accumulating from genetic and transcriptomic studies. The cytokinin receptors AHK2 and AHK3 are mainly responsible for the transduction of the cytokinin signal to B-type response regulators, in particular ARR1, ARR10, and ARR12, which are transcription factors of the two-component system mediating cytokinin functions. Additional transcription factors linking cytokinin and chloroplast development include CGA1, GNC, HY5, GLK2, and CRF2. In this review, we summarize early and more recent findings of the long-known relationship between the hormone and the organelle and describe crosstalk between cytokinin, light, and other hormones during chloroplast development.

Enhanced Secondary- and Hormone Metabolism in Leaves of Arbuscular Mycorrhizal Medicago truncatula

“…CKs are known to promote cell division and branching as well as to support the development of chloroplasts (Cortleven and Schmülling, 2015). Hence, the observed increase of most CK metabolites may be the factor involved in remodeling the shoot architecture in AM and P i plants, whereas the specific increase in DHZ may affect the chloroplast number.…”

Cytokinin Regulates the Etioplast-Chloroplast Transition through the Two-Component Signaling System and Activation of Chloroplast-Related Genes

“…Cytokinin, in particular, can partially replicate the effects of light on plant growth and development. Indeed, soon after the discovery of cytokinin as a plant hormone, it was found to regulate the development and function of chloroplasts in a variety of plant species (Miller et al, 1956; for a recent review, see Cortleven and Schmülling, 2015). In 1994, Chory and coworkers reported that cytokinin could induce a deetiolation response in plastids of Arabidopsis (Arabidopsis thaliana; Chory et al, 1994); in the absence of cytokinin, dark-grown seedlings had small etioplasts with a prolamellar body but, in presence of cytokinin, the seedlings had large lens-shaped plastids that lacked a prolamellar body and contained some bithylakoid membranes.…”

Phytochrome and phytohormone interplay in tomato: impacts on fruit physiology and quality traits

“…In contrast, the higher chlorophyll content observed in SlPHYB2 RNAi immature fruits was not accompanied by changes in plastid abundance but instead was linked to the up-regulation of the master regulator of chloroplast development and maintenance SlGLK2 (Figure 3.2C). Given the key role played by cytokinins in regulating plastid division and maturation in plants and the widely reported crosstalk between this hormonal class and PHY signaling (Okazaki et al, 2009;Cortleven and Schmülling, 2015), a transcriptional profiling of type-A TOMATO RESPONSE REGULATOR (TRR) was performed. Four out of the five type-A TRR analyzed were significantly down-regulated in immature fruits of SlPHYA RNAi compared to the WT genotype (Figure 3.3B).…”

“…SlARC3 and SlARC6), as well as those encoding chloroplast division-related proteins specific to land plants, such as SlPDV2. In Arabidopsis, PDV2 determines the rate of chloroplast division and is positively regulated by cytokinins, being strongly promoted in transgenic plants overexpressing the cytokinin signaling-related transcription factor CRF2 (Okazaki et al, 2009;Cortleven and Schmülling, 2015). SlCRF2, along with other SlCRF and TRR genes, were drastically repressed in PHYA-downregulated fruits, implying that changes in cytokinin signaling mediate the PHYA-dependent regulation of plastid division during early stages of tomato development.…”

“…fruit photosynthesis and carbon importing from the leaves) are modulated by both light and hormonal signals (Sagar et al, 2013;Albacete et al, 2014;Cocaliadis et al, 2014). Among plant hormones, auxins and cytokinins are known to control chloroplast biogenesis and differentiation in vegetative and reproductive tissues (Chory et al, 1994;Ullanat and Jayabaskaran, 2002;Sagar et al, 2013;Cortleven and Schmülling, 2015) and also transcriptionally and post-transcriptionally regulate key proteins related to carbohydrate metabolism and sink activity (Roitsch and Ehneß, 2000;Sagar et al, 2013;Albacete et al, 2014). Despite the massive advances in clarifying the intricate phytochrome-hormonal signaling network controlling various developmental responses in vegetative tissues (Halliday and Fankhauser, 2003;Lau and Deng, 2010), the phytochrome and hormonal signaling crosstalk responsible for controlling sugar metabolism in developing fruits remain elusive.…”