Utility of leaf senescence-associated gene homologs as developmental markers in common wheat

Senescence is the final stage of development in plant tissues. In the senescence process, many senescence-associated genes (SAGs) are reportedly transcriptionally up-regulated. Here, we reported the isolation of nine wheat SAG cDNA clones named TaSAG1 to TaSAG9, and evaluated the usefulness of the SAG homologs for wheat developmental molecular markers based on their expression patterns. The nine wheat SAGs were identified in wheat EST libraries based on their homology to rice SAGs. All wheat SAG transcripts were up-regulated during natural senescence as well as during dark-induced senescence in seedling leaves. However, the expression patterns of wheat SAGs in the flag leaf did not necessarily correspond to those in seedling leaves during senescence. The nine wheat SAGs also showed variable expression patterns in developing and ripening seeds. The transcript accumulation patterns of TaSAG5 and TaSAG6 increased linearly during the period examined in the flag leaf and seed, and are therefore available as molecular markers to respectively evaluate the degree of wheat flag leaf senescence and seed maturation. Transcript accumulation levels of the six SAGs were increased before apparent necrotic cell death of seeding leaves exhibiting wheat hybrid necrosis. These results suggested that necrotic cell death in wheat hybrid necrosis could be closely related not only to senescence but also to defense responses.