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Can Mitogens (Auxin and Cytokinin in particular) act as Mitogenic Poisons if present in higher concentration?
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Answers
Answer:
yes they are highly toxic if they are present in higher concentrations
harmuful to plants
Explanation:
Auxins are toxic to plants in large concentrations; they are most toxic to dicots and less so to monocots. ... Used in high doses, auxin stimulates the production of ethylene. Excess ethylene (also native plant hormone) can inhibit elongation growth, cause leaves to fall (abscission), and even kill the plant.
Answer:
Auxin and cytokinin play fairly important roles in many aspects of plant growth and development. The interaction between auxin and cytokinin is particularly important to control a few developmental processes, such as the formation and maintenance of meristems that are essential to establish the whole plant body. For example, the shoot meristems give rise to the above-ground parts of a plant, whereas the root meristems produce the below-ground parts. Many recent studies have provided important information for the understanding of the molecular mechanisms of auxin–cytokinin interaction in the regulation of meristem development.
Maintenance of the cellular optimum auxin concentration can be controlled at multiple levels, such as biosynthesis, transport, perception, and signaling. These multiple regulation pathways contribute to the differential auxin distribution within tissues at different developmental stages. Thus far, one tryptophan (trp)-independent pathway and four trp-dependent pathways for the biosynthesis of auxin/IAA have been proposed in Arabidopsis (Zhao, 2010). These four trp-dependent pathways include the indole-3-acetamide (IAM) pathway, the indole-3-acetaldoxime (IAQx) pathway, the tryptamine (TAM) pathway, and the indole-3-pyruvic acid (IPA) pathway. Two of these pathways—the TAM pathway, considered to be rate-limited through the YUCCA family, and the IPA pathway—have also been highlighted (Vanneste and Friml, 2009). Auxin polar transport is required to direct auxin flows and to form auxin gradients in plants, which are critical for developmental pattern formation. In Arabidopsis, three protein families are required to mediate auxin transport between cells: auxin efflux PINFORMD (PIN) proteins, MULTIDRUG RESISTANCE (MDR)-p-glycoprotein (PGP) proteins, and auxin influx AUXIN RESISTANT 1 (AUX1)/LIKE AUX1 (LAX) proteins (Benjamins and Scheres, 2008; Gao et al., 2008; Zazímalová et al., 2010).
Besides the biosynthesis and transport of auxin, auxin signaling through receptors and downstream signaling components has also been suggested to be the regulating mechanism for many developmental processes. One of the important auxin receptors in Arabidopsis has been identified as the TRANSPORT INHIBITOR REPONSE 1 (TIR1) protein. TIR1 protein is an F-box protein, a component of an SCFTIR1 ubiquitination E3 complex. This E3 complex is involved in proteasome-mediated protein degradation (Ruegger et al., 1998; Gray et al., 2001; Quint and Gray, 2006). Analysis of quadruple tir1-related mutants highlights the role of TIR1 and at least three other TIR1-related AUXIN BINDING F-BOX PROTEINs (AFB1-3) in the auxin signaling for plant development (Dharmasiri et al., 2005). Thus far, two classes of transcriptional regulators represent the core of auxin signaling: the AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) proteins and AUXIN RESPONSE FACTOR (ARF) proteins (Liscum and Reed, 2002; Quint and Gray, 2006). Aux/IAA proteins are known as a family of transcriptional repressors to negatively regulate auxin signaling (Ulmasov et al., 1997b). Recent studies have provided more extensive evidence that Aux/IAA proteins are the targets of SCFTIR1 complex (Paciorek and Friml, 2006; Benjamins and Scheres, 2008; Vanneste and Friml, 2009). Aux/IAAs interact with ARF proteins, a class of transcription factors that mediate auxin-dependent transcriptional regulation (Paciorek and Friml, 2006; Ulmasov et al., 1997b). The ARFs could function as either activators or repressors in the regulation of auxin-induced gene expression (Ulmasov et al., 1997a, 1999).
Like auxin, cytokinin is also a key regulator for various aspects of plant growth and development. Cytokinin homeostasis is spatially and temporally regulated by a fine balance between synthesis and catabolism. The first enzyme identified in the Arabidopsis cytokinin biosynthetic pathway is adenosine phosphate-isopentenyltransferases (IPTs). The IPTs are believed to catalyze the transfer of an isopentenyl group from dimethylallyl diphosphate to an adenine nucleotide (ATP, ADP, or AMP) (Kakimoto, 2001; Takei et al., 2001). Another landmark is the identification of two cytochrome P450 monooxygenases, CYP735A1 and CYP735A2, which catalyze the hydroxylation at the prenyl side chain of the iP-nucleotides to synthesize tZ-nucleotides (Takei et al., 2004). Furthermore, a cytokinin-activating enzyme in rice, LONELY GUY (LOG), has been recently identified to catalyze the last step of cytokinin biosynthesis. This step is involved in converting cytokinin-nucleotides produced by IPTs and CYP735As to the free-base form (Kurakawa et al., 2007). Besides biosynthesis, cytokinin homeostasis is also controlled by its catabolism process through CYTOKININ OXIDASE/DEHYDROGENASEs (CKXs) (Werner et al., 2003).