LEC1/AFL-B3 transcription factors are involved in the regulation of several aspects of seed maturation, including the
synthesis of storage compounds (seed storage proteins, lipids) as well as the acquisition of dormancy and desiccation tolerance.
LEC1 has been shown to be involved in the regulation of seed lipid synthesis. Transcripts of several genes encoding lipid biosynthetic
enzymes were found to be enriched in LEC1 over-expressing seedlings. These include genes encoding three components of the Acetyl CoA
carboxylase complex (ACCase), and genes encoding an enoyl-ACP reductase MOSAIC DEATH 1 (MOD1/EAR), FATTY ACID ELONGASE1 (FAE1 or KCS1),
a stearoyl-ACP desaturase FATTY ACID BIOSYNTHESIS 2 (FAB2/SSI2) and others (Mu et al. 2008). LEC1 also up-regulates the transcription
of several oleosin genes the products of which are important for oil body formation (Mu et al. 2008). Furthermore, the transcript
abundance of WRINKLED1 (WRI1) was found to be elevated when LEC1 was constitutively expressed (Mu et al. 2008). As
regulator of lipid biosynthetic genes (Baud et al. 2007), WRI1 is known as a direct target of LEC2. Using an inducible LEC2
expression system, WRI1 expression was found to be enhanced after four hours of induction (Baud et al. 2007).
The HAP3 subunit LEC1 is able to activate the cruciferin (CRC) seed storage protein promoter when co-expressed with the HAP2
subunit NF-YC2 and the bZIP transcription factor bZIP67 (Yamamoto et al. 2009). The same or a similar complex with LEC1-LIKE (L1L)
substituting LEC1 has been shown to activate the promoter of a sucrose synthase gene thereby reducing the amount of soluble sugars and
increasing substrates for starch synthesis (Yamamoto et al. 2009). LEC1 increases the expression of its homologue L1L (Mu et al. 2008).
LEC1 regulation of maturation genes can also be mediated by FUS3 and ABI3 (Kagaya et al. 2005b). The direct interaction of LEC2, FUS3
and ABI3 via the RY element (CATGCA) has been proven by several independent approaches (Braybrook et al. 2006, Ezcurra et al. 2000,
Kroj et al. 2003, Mönke et al. 2004, Reidt et al. 2000, Roschzttardtz et al. 2009). Via binding to ABRE elements (ACGT),
two bZIP factors, AtbZIP10/25, participate in ABI3-mediated regulation of SSP genes. Physical interactions have been reported between
ABI3 and bZIP proteins (Lara et al. 2003) and are discussed for other B3 domain proteins LEC2 and FUS3 (Vicente-Carbajosa and Carbonero 2005).
Activation of SSP genes by LEC1 (indirectly), FUS3 and ABI3 (directly) depend on ABA (Kagaya et al. 2005a). ABI3 mediates ABA
responsive gene expression by interaction with the bZIP protein ABI5 (Nakamura et al. 2001). Binding to RY and ABRE elements promotes
transcription of EARYL METHIONINE-LABELLED6 (AtEM6) which encodes a late embryogenesis abundant protein (LEA) and the
RESPONSIVE TO DROUGHTNESS29 (RD29) gene, the gene product of which is involved in acquisition of desiccation tolerance
(Bies-Ethéve et al. 1999, Carles et al. 2002, Nakamura et al. 2001, Nakashima et al. 2006). FUS3 and ABI3 have
been shown to bind the RY element in the promoter of a seed-specifically expressed succinate dehydrogenase gene (SDH2-3, Roschzttardtz et al. 2009).
bZIP heterodimers consisting of either bZIP10 or bZIP25 and bZIP53 interact with ABRE elements in the same promoter (Roschzttardtz et al. 2009),
providing further proof for bZIP-B3 factor cooperation in the maturation gene control.
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