Temperature-dependent changes of cell shape during heterophyllous leaf formation in Ludwigia arcuata (Onagraceae).

Although elongation of epidermal cells in submerged leaves is thought to be a common feature of heterophyllous aquatic plants, such elongation has not been observed in Ludwigia arcuata Walt. (Onagraceae). In this study we found that reduced culture temperature induced the elongation of epidermal cells of submerged leaves in L. arcuata. Since such submerged leaves also showed a reduction in the number of epidermal cells aligned across the leaf transverse axis, these data indicate that heterophyllous leaf formation in L. arcuata is partially temperature sensitive, i.e., the elongation of epidermal cells was temperature sensitive while the reduction in the number of epidermal cells did not show such temperature sensitivity. To clarify the mechanisms that cause such temperature sensitivity, we examined the effects of ethylene, which induced the formation of submerged-type leaves on aerial shoots at the relatively high culture-temperature of 28 degrees C. At 23 degrees C, ethylene induced both cell elongation and reduction in the number of epidermal cells across the leaf transverse axis, while cell elongation was not observed at 28 degrees C. Moreover, both submergence and ethylene treatment induced a change in the arrangement of cortical microtubules (MTs) in epidermal cells of developing leaves at 23 degrees C. Such changes in the arrangement of MTs was not induced at 28 degrees C. Factors involved in the temperature-sensitive response to ethylene would be critical for temperature-sensitive heterophyllous leaf formation in L. arcuata.

Triiodothyronine but not thyroxine accelerates myofibrillar proteolysis via ATP production in cultured muscle cells.

These experiments were done to clarify that the differential effects of thyroxine (T(4)) and triiodothyronine (T(3)) on skeletal muscle protein turnover are caused by their roles on ATP production. Primary cultured chick muscle cells were treated with a physiological level of T(4) (60 ng/ml), T(3) (12 ng/ml), or ATP (0.5 mM) for 6 days and the protein content, ATP production, proteasome activity, and myofibrillar protein breakdown were measured. The protein content measured as an index of cell growth was not affected by T(4), T(3), or ATP. The cellular ATP level was increased by T(3) and ATP, but not by T(4). Proteasome activity and N(tau)-methylhistidine (MeHis) release measured as an index of myofiblillar protein breakdown was also increased by T(3) and ATP, but not by T(4). These results indicate that T(3) but not T(4) increases ATP production followed by an increase in proteasome activity, and thus stimulates myofibrillar proteolysis.