PLANT HETEROPHYLLY - O.M. NEDUKHA - 2011

CHAPTER TWO. MECHANISMS OF HETEROPHYLLY MANIFESTATION IN PLANTS

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2.2. ENDOGENOUS MECHANISMS OF HETEROPHYLLY MANIFESTATION IN PLANTS

2.2.1. Phytohormonal Regulation

It is well known that phytohormones operate at various levels of plant Organization: they can regulate Cell growth, activate Chromatin, interact with receptor Proteins, and enhance the synthesis of all RNA types, thereby controlling the synthesis of nuclear and cytoplasmic proteins (Medvedev, 2004). Experimental studies have proven the vital role of phytohormones in the functioning of submerged and aerial leaves in plants exhibiting heterophylly. Experiments involving the application of exogenous Abscisic acid (ABA) and gibberellic acid (GA) have revealed their direct impact on leaf Morphology. These studies were conducted on Hippuris vulgaris and Potamogeton palustris L. plants growing at depths of up to 3 meters (Bodkin et al., 1980). At such depths, far-red light intensity is severely reduced, and the red-to-far-red light ratio changes with increasing depth. It has been shown that the plant's response to submergence is specifically mediated by phytohormone action (Table 2.2.1.1).

TABLE 2.2.1.1. Effect of ABA on Hippuris vulgaris leaves grown underwater (Wells, Pigliucci, 2000)

Under experimental conditions, when submerged leaves emerge onto the Water surface, The phenomenon of heterophylly is observed, with abscisic acid playing a key role. T.E. Goliber and L.J. Feldman (Goliber, Feldman, 1989) demonstrated that ABA content increases in the aerial leaves of Hippuris vulgaris compared to its submerged leaves. Heterophylly in higher aquatic plants can also be induced in laboratory conditions through Treatment with phytohormones. This was proven using laboratory-cultivated leaves of Ludwigia arcuata (Asuka Kuwabara et al., 2000). The authors suggest that changes in leaf shape may be triggered by specific signals originating from ABA.

Upon treating Potamogeton nodosus tubers with a 10^-5 M ABA solution, floating leaves developed instead of submerged ones; these leaves bore Stomata on their upper surface, and The ratio of leaf width to length increased compared to that of submerged leaves. These effects were partially or completely replicated when ABA was used in combination with gibberellic acid, kinetin, or benzyladenine (Anderson, 1978; 1982).

In heterophyllous aquatic plants, much like in terrestrial plants, METABOLISM/18.html">The Influence of Gibberellins is quite substantial. Under the action of phytohormones, submerged leaves in Callitriche heterophylla (Deschamp, Cook 1984) and C. platycarpa Kutz. (Musgrave et al., 1972) become heavily dissected; however, stomatal density decreased in the former species and increased in the latter compared to control plants. In Proserpinaca palustris (Kane, 1984; Kane, Albert, 1999), leaves emerging from the water adopt a dissected shape, with stomatal density increasing relative to their submerged counterparts.

ABA acts as a mediator inducing The formation of specific leaf shapes in plants characterized by heterophylly (Lin, 2002). It has been established that in Hippuris vulgaris and Marsilea quadrifolia, an increase in endogenous ABA levels correlates with morphological changes during the onset of heterophylly (Goliber, Feldman, 1989; Lin, Yang, 1999).

It has been shown that ABA content increases in response to far-red light (Goliber, 1989). ABA is involved in Phytochrome signaling during changes in light intensity, as discovered in studies on Hippuris vulgaris. Meanwhile, in another species, Marsilea quadrifolia, blue light induced heterophylly without de novo ABA synthesis (Lin, Yang, 1999). Furthermore, these authors found that ABA content in the aerial leaves of M. quadrifolia was comparable to that in its submerged leaves. The authors believe that independent signaling pathways exist within the plant for regulating heterophylly, some of which are not mediated by ABA action (Lin, Yang, 1999). In H. vulgaris, fluctuations in ABA levels have been shown to correlate with increased illumination and phytochrome suppression in aerial-type leaves (Goliber, 1989). Consequently, ABA serves as a signal for phytomorphogenesis in H. vulgaris.

The Effect of ABA is not universal (Deschamp, Cook, 1984; Bruni et al., 1996; Lin, Yang 1999). It is believed that this phytohormone acts exclusively on developing or newly forming leaves. Conversely, ABA treatment has no effect on fully mature leaves (Yang et al., 1995; Gee, Anderson 1998; Hsu et al., 2001). The latter authors (Hsu et al., 2001) suggest that the tissue-level target for ABA is the SHOOT apex, from which either the submerged or aerial leaf form develops. ABA influences the genetic program encoding a specific blade morphology type (aerial) during heterophylly.

Gibberellic acid (GA) affects leaf morphology in a manner distinct from ABA (Deschamp, Cooke, 1984). A species-specific dependence in plant heterophylly responses to gibberellic acid has been established. The water fern Marsilea drummondii Braun., along with the hydrophytes Potamogeton nodosus and Callitriche heterophylla, respond to GA by producing leaves whose shape resembles that of submerged leaves, whereas the water hyacinth (Eichhornia crassipes), conversely, develops typical aerial leaves (Watson et al., 1982). In Eichhornia crassipes, GA promotes the formation of terrestrial-type leaves (Watson et al., 1982). C.N. Wells and M. Pigliucci suggested that GA in this case may induce the Formation of the submerged leaf type, much like the effect of a short photoperiod (Wells, Pigliucci, 2000).

Recently, A. Kuwabara and co-workers (Kuwabara et al., 2003), investigating the effects of Ethylene and ABA on heterophylly in Ludwigia arcuata leaves, demonstrated that ethylene treatment induced the formation of submerged-type leaves on the aerial stems of this species. Ethylene concentration measurements in submerged stems revealed higher levels than those in emergent stems (Kuwabara et al., 2003). Additionally, ethylene has been proven to promote submerged elongation in both rice (Oryza sativa) and marsh yellowcress (Rumex palustris) leaves (Jackson, 2007).

It has been shown that the phytohormone ethylene acts as a signal for rapid stem elongation underwater (Malone, Ridge, 1983; Ridge, 1987; Voesenek et al., 2003; 2004). A direct correlation has been established between The rate of Biosynthesis and its tissue content: the activation of this phytohormone's biosynthesis depends on its diffusion rate away from the plant. The endogenous concentration of synthesized ethylene in submerged leaves progressively increases because its diffusion from the leaves into the surrounding water layer is restricted by water density. Ethylene synthesis involves the enzyme ACS synthase, whereas its oxidation involves ACC oxidase (Vriezen et al., 1999; Van der Straeten et al., 2001; Rieu et al., 2005), and The activity of these Enzymes presumably changes significantly in submerged leaves.