ABI5 is a major regulator of late maturation in legume seeds, linking longevity, oligosaccharide accumulation and chloroplast dedifferentiation
Abstract
Abscisic acid insensitive 5 (ABI5) is a bZIP transcription factor that activates ABA- and
stress-responsive genes in seeds and seedlings. Here, we will present evidence that ABI5 is
also an important regulator of late maturation in legumes. Using Tnt1 insertion mutants of
Medicago truncatula and EMS mutants of pea, we discovered that seed longevity is severely
reduced in these two legume species. Also, in these mutants, the accumulation of raffinose
oligosaccharide family (RFO) sugars that takes place at the final steps of seed maturation is
impaired. Mature Mtabi5 and Psabi5 mutant seeds contain respectively 2-fold less stachyose
and verbascose than in wild type seeds. MtABI5 also plays a role in the accumulation of a
restricted subset of Late Embryogenesis Abundant (LEA) proteins. A proteomic analysis on
the heat stable protein fraction revealed strongly reduced levels of EM1 and several D-34
LEA polypeptides. Transcriptome analysis on developing Mtabi5 mutants during seed
development confirmed that the biochemical phenotypes of RFO accumulation and LEAs
were also regulated at the transcriptional level. Surprisingly, this analysis also revealed a
strong deregulation of transcripts encoding photosynthesis-associated nuclear genes. This
result was confirmed by a transcriptome analysis of hairy roots ectopically over-expressing
MtABI5. Further biochemical analysis demonstrated elevated chlorophyll and carotenoid
levels in mature seeds of the Psabi5 and Mtabi5 mutants compared to wild type seeds,
suggesting that in these legumes, ABI5 is involved in the dedifferentiation of chloroplasts.
These results demonstrate that in legumes, ABI5 links RFO, chloroplasts and longevity. The
causal relationship between these factors will be discussed.
Loading...