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Seed storage reserves accumulated during embryogenesis in higher plants are crucial
for plant propagation, providing carbon and energy during germination prior to seedling
establishment. In mature Arabidopsis seeds, storage lipids and proteins are the major
storage compounds, each accounting for 30% - 45% of the seed dry weight [1]. The past decade has witnessed a substantial progress in identification and characterization
of genes involved in the de novo fatty acid (FA) biosynthesis and triacylglycerol (TAG) assembly pathways [[1,4] and references therein]. This advancement is particularly evident in the model plant
Arabidopsis, largely owing to the sequencing and release of its relatively compact
genome in the year 2000 [5]. Moreover, characterization of transcription factors (TFs) has led to the identification
of several master regulator genes that play critical regulatory roles in this biological
process, including ABI3 (ABSCISIC ACID INSENSITIVE 3), LEC1 (LEAFY COTYLEDON 1), LEC2 and FUS3 (FUSCA 3) [6-17]. These TFs interact with each other and form complex regulatory networks [18-23], regulating multiple aspects of seed development including storage reserve accumulation
through interaction with cognate cis-acting DNA elements in the promoter regions of target genes. ABI3, FUS3 and LEC2
contain a plant-specific 'B3' DNA-binding domain which targets RY-repeat regulatory
elements, whereas LEC1 and L1L (LEC1-LIKE) contain a NF-YB domain binding to the CCAAT
boxes in the promoter region [24,25]. Additional TFs such as WRINKLED 1 (WRI1), a member of plant-specific APETALA 2 (AP2)
- ethylene response element binding factor (EREB) family, is also known to control
transcription of many FA biosynthetic genes [26], and recent studies show it acts via binding to the AW-box motif present in the promoter
region of 19 FA biosynthetic genes [27]. Moreover, ABI4 (an AP2 family protein) and various basic-leucine zipper (bZIP) TFs
including ABI5 or EEL (ENHANCED EM [EMBRYO MORPHORGENESIS] LEVEL) are known regulators
of the expression of SEED STORAGE PROTEIN (SSP) genes, which act in the same signalling network but downstream of ABI3 [28,29]. Distinct regulatory mechanisms are present in controlling the accumulation processes
of oils and proteins, perhaps with cross-talk to coordinate the synthesis of seed
storage compounds. This coordination could help to explain the well-documented negative
correlation (correlation coefficient ranging from -0.60 to -0.90) between oil and
protein content in seeds of many oleaginous species [[3] and references therein]. Moreover, several TFs, such as LEC1, ABI3 and FUS3, have
been demonstrated to regulate many genes in the synthesis of both oils and storage
proteins in developing seeds [30-32].
In contrast to the great advancement in characterizing individual genes involved in
the accumulation of seed storage reserves, the relationship of their expression and
regulation is not well understood. A more holistic view of this biological process
at the systems level would prove beneficial in developing strategies to further enhance
seed yield and oil content, as well as in the modification of oil composition.
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