Abstract
Rice MADS29 has recently been reported to cause programmed cell death of maternal tissues, the nucellus, and the nucellar projection during early stages of seed development. However, analyses involving OsMADS29 protein expression domains and characterization of OsMADS29 gain-of-function and knockdown phenotypes revealed novel aspects of its function in maintaining hormone homeostasis, which may have a role in the development of embryo and plastid differentiation and starch filling in endosperm cells. The MADS29 transcripts accumulated to high levels soon after fertilization; however, protein accumulation was found to be delayed by at least 4 days. Immunolocalization studies revealed that the protein accumulated initially in the dorsal-vascular trace and the outer layers of endosperm, and subsequently in the embryo and aleurone and subaleurone layers of the endosperm. Ectopic expression of MADS29 resulted in a severely dwarfed phenotype, exhibiting elevated levels of cytokinin, thereby suggesting that cytokinin biosynthesis pathway could be one of the major targets of OsMADS29. Overexpression of OsMADS29 in heterologous BY2 cells was found to mimic the effects of exogenous application of cytokinins that causes differentiation of proplastids to starch-containing amyloplasts and activation of genes involved in the starch biosynthesis pathway. Suppression of MADS29 expression by RNAi severely affected seed set. The surviving seeds were smaller in size, with developmental abnormalities in the embryo and reduced size of endosperm cells, which also contained loosely packed starch granules. Microarray analysis of overexpression and knockdown lines exhibited altered expression of genes involved in plastid biogenesis, starch biosynthesis, cytokinin signalling and biosynthesis.
Highlights
Hormones play a crucial role in regulating development of seeds, starting from early embryo and endosperm patterning till its maturation (Sun et al, 2010)
Analyses involving OsMADS29 protein expression domains and characterization of OsMADS29 gain-of-function and knockdown phenotypes revealed novel aspects of its function in maintaining hormone homeostasis, which may have a role in the development of embryo and plastid differentiation and starch filling in endosperm cells
The process of plastid differentiation is accompanied by induction of several nucleus-encoded plastid genes related to starch biosynthesis, such as ADP-glucose pyrophosphorylase (AgpS) and GRANULE BOUND STARCH SYNTHASE (GBSS) (Miyazawa et al, 1999)
Summary
Hormones play a crucial role in regulating development of seeds, starting from early embryo and endosperm patterning till its maturation (Sun et al, 2010). Cytokinins are required during early endosperm development (5–10 days after pollination (DAP) in maize, 6–15 DAP in rice) where their presence coincides with active cell division, which corresponds to the grain filling stage (Lur and Setter, 1993; Yang et al, 2000, 2002). The molecular aspects of differentiation of starch-storing amyloplasts from the undifferentiated proplastids are largely unknown, cytokinins have been shown to be involved in the process of grain filling in cereals (Yang et al, 2000). Upon addition of cytokinin (6-benzylaminopurine) to the medium, proplastids in BY-2 cells differentiate into amyloplasts and develop starch granules (Miyazawa et al, 2002). Cytokinins have been shown to regulate plastid biogenesis and function in Lupinus luteus, maize, and tobacco (Kusnetsov et al, 1998; Polanska et al, 2007; Brovko et al, 2010)
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