Interactions of abscisic acid and sugar signalling in the regulation of leaf senescence.

Leaf senescence can be triggered by a high availability of carbon relative to nitrogen or by external application of abscisic acid (ABA). Most Arabidopsis mutants with decreased sugar sensitivity during early plant development are either ABA insensitive (abi mutants) or ABA deficient (aba mutants). To analyse the interactions of carbon, nitrogen and ABA in the regulation of senescence, wild-type Arabidopsis thaliana (L.) Heynh. and aba and abi mutants were grown on medium with varied glucose and nitrogen supply. On medium containing glucose in combination with low, but not in combination with high nitrogen supply, senescence was accelerated and sucrose, glucose and fructose accumulated strongly. In abi mutants that are not affected in sugar responses during early development (abi1-1 and abi2-1), we observed no difference in the sugar-dependent regulation of senescence compared to wild-type plants. Similarly, senescence was not affected in the sugar-insensitive abi4-1 mutant. In contrast, the abi5-1 mutant did exhibit a delay in senescence compared to its wild type. As ABA has been reported to induce senescence and ABA deficiency results in sugar insensitivity during early development, we expected senescence to be delayed in aba mutants. However, the aba1-1 and aba2-1 mutants showed accelerated senescence compared to their wild types on glucose-containing medium. Our results show that, in contrast to sugar signalling in seedlings, ABA is not required for the sugar-dependent induction of leaf senescence. Instead, increased sensitivity to osmotic stress could have triggered early senescence in the aba mutants.

Spatial patterns and metabolic regulation of photosynthetic parameters during leaf senescence.

• To prevent premature cell death and to allow efficient nutrient mobilization from senescing leaves, the photosynthetic apparatus has to be dismantled systematically. This requires temporal, spatial and metabolic regulation of photosynthetic function and photoprotection. • Conventional pulse-modulated fluorometry and chlorophyll fluorescence imaging were used to study age- and nutrient-dependent senescence patterns in Arabidopsis thaliana. • Nonphotochemical quenching (NPQ) rose during leaf maturation, indicating increased energy dissipation. During later stages of senescence, overall plant NPQ declined, but NPQ remained high in the base of rosette leaves. Other fluorescence parameters also showed spatial patterns, for example minimum fluorescence (F0 ) was temporarily increased in the tips of inner rosette leaves from where high F0 spread to the base, in a zone preceding cell death. Senescence-dependent changes in chlorophyll fluorescence characteristics were accelerated by growth on glucose-containing medium in combination with low, but not with high, nitrogen supply. • Our experiments revealed distinct spatial patterns of photosynthetic and photoprotective processes in senescing leaves and induction of these processes by high sugar-to-nitrogen ratios.