Thellungiella halophila(salt cress), a halophyte, is a genetic model system with a short life cycle, small plant size, small genome, and copious seed production. Its genes are characterized by a very high sequence similarity of about 90-95 percent at cDNA level, compared to its close relative, Arabidopsis (Lorenzen et.al. 2004). Thellungiella halophila is more salt tolerant than Arabidopsis thalina.
The difference is brought about by several genetic and physical characteristics. Thellungiella halophila is a halophyte while Arabidopsis is a glycophyte. Unlike Arabidopsis, Thellungiella grows well in adverse conditions of cold, drought and nitrogen limitation.Thellungiella halphila growing in in the Glasshouse of the Vrije University of Amsterdam.Courtesy of University of Glasgow.(The two photos show the plants grown in Low-Salt Hydroponic Culture)To explain these differences, scientists investigate metabolic, physiological and molecular mechanisms in the two species. Thellungiella can grow and reproduce under extreme conditions unlike Arabidopsis. Thellungiella plants germinate and set seeds over an extended period of time, and they flower later.
In addition, the plants require vernalizatiion to boost flowering. Though the two plants are very similar in shape and form, leaves of Thellungiella are serrated, succulent and waxier than that of Arabidopsis.Laboratory experiments reveal that Thellungiella has a very high concentration and different composition of spectacular waxes in the leaves as compared to Arabidopsis. At the tissue level, halophila leaves have an additional layer of palisade mesophyll cells. The roots develop an extra cortex cell and endodermis compared to Arabidopsis.
In addition to this, Thellungilla’s stomata are present at a higher density, and less open than in Arabidopsis (Kruk et.al. 2013). This quality makes the former’s stomata to respond to salt stress by closing more tightly. However, other laboratory studies indicate that stomatal conductance and transpiration is less affected by salt in Thellungiella than in Arabidopsis.Tolerance of Thellugiella to extreme salinity has been proved in laboratories. It has been found out that Thellungiella halophila accumulates less Na+ in the shoots than Arabidopsis.
This finding suggests that transport processes in the two plants are very important for salt tolerance. The former’s lower net intake of Na+ is brought about by a lower net Na+ uptake into the root, which is either though a higher unidirectional influx or a lower unidirectional efflux. This brings about a high phloem loading (Na+ recycling from the shoots to the roots)A study by (Villanueva, 2007) confirms that the Na+ uptake across the plasma membranes of root cells produce an electric current that changes when a voltage step is applied. The study also found out that Na+ inward current is significantly smaller in Thellungiella than in Arabidopsis. This difference is brought about by a higher selectivity of the former’s voltage-independent Channel (VIC) for K+ over Na+ than the latter’s. The difference between in Na+ permeability between the two plants is reflected in membrane potential. Arabidopsis shows a larger depolarization after addition of Na+ to the eternal medium while Thellungiella shows only a transient and small depolarization. This observation is very important because membrane depolarization decrease uptake of K+ via inward rectifying K+ selective medium.
This shows higher levels of K+ content and lower Na+ in Thellungiella compared to Arabidopsis.Moreover, studies indicate the expression of the high-affinity uptake system HAK5 relies on the membrane capability, with HAK-5 transcript levels increasing at the most negative membrane potentials. The difference in membrane potentials of the two species can explain) high affinity K+-uptake)-higher HAK5 under salt stress in Thellungiella as compared to Arabidopsis (Wang.et.al, 2005).
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