PLANT HETEROPHYLLY - O.M. NEDUKHA - 2011

CHAPTER TWO. MECHANISMS OF HETEROPHYLLY EXPRESSION IN PLANTS

Conclusions

The existence of heterophlly in higher aquatic plants is driven by both endogenous and exogenous factors, notably: the aqueous environment surrounding submerged leaves, reduced light intensity, and altered CO₂ levels characterized by a low diffusion coefficient in Water; Changes in the light spectrum within the water depending on the depth of SHOOT submergence; Temperature fluctuations; increased soil moisture around the roots of hydrophytes, reduced oxygen content in muddy or sandy soils, and elevated salt concentrations.

Based on comprehensive data examining the mechanisms of heterophlly expression in aquatic and terrestrial plants, The Role of phytohormones, the expression of specific genes, and the synthesis of amorphous and crystalline Cellulose in The Cell walls of morphologically distinct leaves were investigated. Since the leaves of heterophyllous plants undergo all developmental phases—from initiation and maturity to natural senescence—under either aquatic or terrestrial conditions, the identified differences can be attributed to endogenous adaptive mechanisms, specifically:

✵ redistribution of phytohormones (ABA, Ethylene, and GA) in heterophyllous plants: an increase in ABA levels alongside a decrease in GA and ethylene content in aerial leaves, whereas submerged leaves exhibit the exact opposite trend, showing elevated levels of GA and ethylene coupled with reduced ABA. This occurs through The regulation of phytohormone synthesis and their subsequent impact on Gene Expression.

✵ The Effect of gibberellin and ethylene on the expression of genes involved in Cell Cycle regulation and cell elongation. Compared to aerial leaves, submerged leaves show enhanced activation of mRNAs encoding expansins A and B, endotransglucosylases (Hydrolases), cyclins (CYC2Os1) and cyclin-dependent kinase (OsACO), α-amylase (OsAmy3D), among others, which influences changes in Cell Division and growth rates;

✵ a decrease in the relative content of Calcium Ions—one of the secondary messengers in METABOLISM/14.html">Chloroplasts—in submerged leaves compared to aerial leaves. This is most likely achieved through the partial inhibition or alteration of The rate of calcium-dependent processes in chloroplasts, specifically chlorophyll synthesis and the repair of PS II reaction centers, as well as The Influence of Ca²⁺ on the activation of NAD kinase and Calvin cycle Enzymes;

✵ redistribution of cellulose forms in the cell walls of heterophyllous plants: an increase in the amorphous cellulose fraction and a decrease in the crystalline fraction in submerged leaves compared to both the aerial-aquatic and terrestrial plant leaves. The rise in amorphous cellulose content and the increased amorphous-to-crystalline ratio allow this phenomenon to be viewed as a pathway for activating apoplastic water transport in submerged leaves, given that amorphous cellulose molecules possess The ability to absorb water, whereas crystalline cellulose cannot.

Overall, the obtained data lead to the Conclusion that the Structural and functional differences in the leaves and stems of heterophyllous aquatic plants are driven by cellular mechanisms that govern the growth and functioning of these Organs in response to drastic environmental shifts: air ↔ water medium.