ZAVALETA-MANCERA, HILDA A.1,2*, IAN SCOTT2, and THOMAS HOWARD3. 1Universidad Autonoma del Estado de Mexico, UAEMex, Facultad de Ciencias, Unidad Academica del Cerrillo, 50200 Edo. Mex. Mexico; 2University of Wales, Institute of Biological Sciences, Aberystwyth Dyfed, SY23 3DA, UK.; 3Institute of Grassland and Environmental Research IGER, Aberystwyth Dyfed, SY23 3EB, UK.. - Re-differentiation of senescent Nicotiana rustica chloroplasts by cytokinins treatment.
Leaf senescence represents a genetically programmed sequence of events
highly coordinated at the cell and tissue levels that happens at the
final stage of development. The main symptom of leaf senescence is the
yellowing of the blade, due to the loss in the chloroplasts of
chlorophylls and membrane structure. It is a common observation that
greening and senescence occur at opposite ends of the life span of
plant tissues. Greening generally implies differentiation of
etioplasts or proplastids into chloroplasts in the presence of light,
characterized by the formation of thylakoids. In contrast, during
chloroplast senescence, the structure changes markedly: the
chloroplast polyribosome population decreases, the organized granal
stacks are lost, the plastoglobuli appear larger and more abundant,
and finally the plastid envelope ruptures. The present work gives
evidence for re-differentiation of senescent chloroplasts
(gerontoplasts) in cytokinin-induced regreening of senescent
Nicotiana leaves. During regreening the chlorophyll content,
chloroplast structure and photosynthetic activity recovered. Ultrathin
sections of leaf tissue were examined at several times during
senescence and regreening. It was apparent that granal stacks were
rebuilding in the senescent chloroplasts, which were still
recognizable by the presence of some plastoglobuli. No ultrastructural
evidence of proplastids or chloroplast division was observed at any of
the developmental stages examined. Trends in plastid numbers per cell
also supported the gerontoplast-re-differentiation route. Plastid
numbers did not increase during regreening, and when many plastids
were lost during advanced senescence, regreening capacity
deteriorated. Re-differentiation of gerontoplasts was associated with
increased levels of light-harvesting chlorophyll a/b-binding protein
(LHCP-2), cytochrome f , small and large subunits of Rubisco, and the
appearance of NADP-protochlorophyllide oxidoreductase (POR) detected
by Western blotting. Differentiation, de-differentiation, and
re-differentiation concepts are discussed.
Key words: chloroplasts, cytokinins., Nicotiana, re-differentiation, senescence