The function of melatonin in plants and animals

Melatonin and serotonin in animal systems Neurohormone melatonin (MEL) is secreted principally by the pineal gland of mammals including humans. The concentration levels exhibits a circadian cycle with a peak during the night. Disturbed MEL secretion causes sleep rhythm disorders, geriatric insomnia, jet lag and seasonal affective disorders (Murch & Saxena 2002). MEL also acts as a potent scavenger of oxygen-centered radicals and can inhibit tumour growth and effects of ageing by augmenting the immune response (Brzezinski 1997). Serotonin (SER), which forms in the same metabolic pathway as MEL, functions as a neurotransmitter in brains of animals. In humans it promotes feeling of well-being and happiness (Young 2007). These are very powerful chemicals, indeed!

MEL and SER in plants MEL was first discovered in plants in two surveys of common fruits and vegetables from the market (Hattori et al 1995; Dubbels et al 1995). MEL was reported in growing plants (Murch et al 1997) and a putative pathway for MEL synthesis in plants was described in 2000 (Murch et al 2000).

(1) Antioxidants Plants produce a wide range of phytochemicals that reduce reactive oxygen species (ROS) and protect plant tissues from the by-products of photosynthesis and photorespiration. More ROS is generated under stress, such as salinity, UV exposure, extreme temperatures, sustained darkness or pollution. MEL exhibits high antioxidant activity in solution and detoxifies ROS in plant tissues (Tan et al 2007a; Brown et al 2012; Wang et al 2012; Zhang et al 2013). MEL antioxidant activity occurs via ROS scavenging cascade through AFMK and AMK (kynuric pathway of MEL metabolism) or indirectly by enhancing the activity of other free radical scavenging antioxidants or associated enzymes (Posmyk & Janas 2009; Brown et al 2012).

(2) Growth hormones The structures of MEL and SER are closely related to an important plant growth hormone indole-3-acetic acid (IAA) (see Fig. 1), suggesting a role in growth and development. In higher plants IAA is synthesised in the shoot and stimulates root production in stem cuttings and tissue cultures, suppresses growth of apical buds and stimulates growth of flower parts. IAA action is balanced by cytokinins, which are synthesised in the roots and stimulate shoot formation in tissue culture. Murch et al (2001) and Murch and Saxena (2002) found that increase in MEL concentration induced root growth in cultured St. John’s wort, while accumulation of SER promoted shoot formation. Similar effects were observed in sweet cherry, Arabidopsis and corn (Tan et al 2011; Pelagio-Flores et al 2011). During flowering, MEL and SER accumulate in whole flowers and anthers at very specific stages of microsporogenesis and flower growth (Murch et al 2009). In wine grapes, the MEL and SER levels fluctuate with development through the veraison (ripening) stages (Murch et al 2010). Thus MEL/SER ratio might act in a similar manner to IAA/cytokinin ratio (Murch et al 2001).

(3) Circadian rhythms Poggeler et al (1991) hypothesized that MEL levels in plants may also be timing mechanisms for circadian rhythm or seasonality with the highest level in dark periods and the lowest level during daylight. A diurnal rhythm in MEL with a maximum in the dark phase and low levels during the day has been reported in the short-day plant Chenopodium rubrum, but increasing the duration of the dark phase did not change the the duration of MEL concentration maximum (Wolf et al 2001). There was a 15 – 30 fold higher level of MEL in etiolated (dark grown) seedlings as compared to light adapted tissues of Hypericum perforatum and Arabidopsis thaliana (Murch 2006). MEL levels were higher in Glycyrrhiza uralensis grown under red light than those plants grown under green or blue light (Afreen et al 2006). In water hyacinths a peak in MEL was found late in the light phase, near sunset (Tan et al 2007), while in field grown Vitis vinifera L. cv Malbec the peak occurred at dawn (Boccalandro et al 2011). If the grapes were shielded from sunlight, the concentration of MEL remained high. On the other hand, Byeon et al (2012) found that increased MEL and activity of the genes associated with MEL biosynthesis were detected in detached rice leaves under constant high light with lower concentrations observed in constant darkness. Consequently, none of the systems studied to date have provided definitive evidence of the role of MEL in the circadian rhythms, plant light/dark responses or the timing mechanisms of plants (Kolar & Machackova 2005; Cao et al 2006a,b; Park 2011).

What are the properties of a chronobiological substance? Circadian regulation synchronises internal biological events with external day-night cycles. The regulator consists of an oscillator, inputs that can change (entrain) the oscillation period and outputs that direct many aspects of plant physiology, such as movement of leaves, opening of stomata, hydraulic conductivity of roots, level of photosynthesis activity, growth and signalling (Webb 2003). Some outputs, such as initiation of flowering, operate on longer periods by matching the changes in circadian oscillator to particular time of year. The oscillator was identified in the model plant Arabidopsis as interplay between morning and evening gene expression (Webb 2003). Recent work has used de novo transcriptome analysis to identify 59,184 unigenes covering the entire life cycle of St. John’s wort plants and described 49 unigenes putatively identified as important in the melatonin biosynthetic pathway (He et al 2012). Further work is required to confirm the identity of the putative genes and to understand their regulation and control. The inputs that affect the clock (zeitgebers – time givers) are mainly light and temperature. The phase (particular point in the cycle) can affect various physiological processes. This is called “gating”. Covington and Harmer (2007) found that while exogenous auxins (including IAA) have only a small effect on the plant clock, the clock controls plant sensitivity to auxins. Dodd et al (2005) showed that matching internal circadian clock with the exogenous day-night cycle resulted in more efficient photosynthesis. The synchronous plants contained more chlorophyll, fixed more carbon and grew faster. If changes of MEL concentration are part of the clock mechanism, the changes should persist for several cycles upon external change and then follow external light/dark cycle change (entrain). The best known example in humans of such entrainment is jet-lag and recovery from it.

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    Paper 1: Thanks to Dusan Lazar's visit, we explored the function of Mel in Characeae as antioxidant.

  • paper 1