We analysed the level of polyamines (PAs) bound to thylakoids and the level and activity of thylakoid transglutaminases throughout barley leaf senescence, retarded by kinetin. The level of PAs bound to thylakoids changed in senescing barley leaves: bound putrescine (PU) and spermidine (SD) increased throughout senescence, whereas bound spermine (SM) decreased. Kinetin diminished the increase in thylakoid-bound PU and SD and almost completely abolished the decrease of the bound SM. These data suggest different roles of PU/SD and SM in thylakoid degradation. Immunodetection of transglutaminases (TGase) in thylakoid fraction revealed three bands of 33, 58 and 78 kDa. During senescence the intensity of all bands increased and it was correlated with an increase in TGase activity. Kinetin down-regulated the accumulation of the 58- and 78-kDa TGases and the TGase activity. We postulate that formation of covalent bonds between PAs and proteins by TGase is involved in chloroplast senescence. The kinetin-mediated preservation of low TGase levels and activity throughout leaf senescence may represent an important component of the mechanism of kinetin action in the retardation of leaf senescence.
The effect of light irradiance on the amount of ATP synthase α-subunit in mesophyll (M) and bundle sheath (BS) chloroplasts of C4 species such as maize (Zea mays L., type NADP-ME), millet (Panicum miliaceum, type NAD-ME) and guinea grass (Panicum maximum, type PEP-CK) was investigated in plants grown under high, moderate and low light intensities equal to 800, 350 and 50 µmol photons m-2 s-1, respectively. The results demonstrate that α-subunit of ATP synthase in both M and BS chloroplasts is altered by light intensity, but differently in the investigated species. Moreover, we identified two isoforms of the CF1 α-subunit, called α and ά. The CF1 α-subunit was the major isoform and was present in all light conditions, whereas ά was the minor isoform in low light. A strong increase in the level of the ά-subunit in maize mesophyll and bundle sheath thylakoids was observed after 50 h of high light treatment. The α and ά-subunits from investigated C4 species displayed apparent molecular masses of 64 and 67 kDa, respectively, on SDS/PAGE. The presence of the ά-subunit of ATPase was confirmed in isolated CF1 complex, where it was recognized by antisera to the α-subunit. The N-terminal sequence of ά-subunit is nearly identical to that of α. Our results indicate that both isoforms coexist in M and BS chloroplasts during plant growth at all irradiances. We suggest the existence in M and BS chloroplasts of C4 plants of a mechanism(s) regulating the ATPase composition in response to light irradiance. Accumulation of the ά isoform may have a protective role under high light stress against over protonation of the thylakoid lumen and photooxidative damage of PSII.
JavaScript is turned off in your web browser. Turn it on to take full advantage of this site, then refresh the page.