A cDNA subtraction library enriched for mRNAs encoding ESTs that increased in abundance during SDs (SD: short-days) was constructed by suppression subtractive hybridization (SSH) from leaf tissues of soybean. The proteins predicted to be encoded by the mRNAs were inferred to be involved in diverse functions such as transcription, signal transduction, programmed cell death, protein, nucleic acid and carbohydrate macro-molecule degradation, cell-wall modiWcation, primary metabolism, secondary metabolism and stress response. Of these, regulation and stress response associated clones occupied higher portions. As expected (Lin et al. 1999; Asamizu et al. 2000) few of these mRNAs were present in the existing soybean EST libraries (Shoemaker et al. 2002).
GmRAV had a prominent role in negatively controlling plant growth
The results herein suggested that GmRAV (GmRAV: Soybean RAV-like DNA-binding protein) was involved in the negative regulation of photosynthesis and growth that led to senescence (Gan and Amasino 1997). The dwarf GmRAV transgenic plants had reduced chlorophyll content in their leaves and low rates of photosynthesis, and their growth and development of roots, stems, leaves and flowers were inhibited compared to the wild-type plants. Days to flowering and to maturity were signiWcantly increased.
GmRAV induced in SDs inhibited development of vegetative organs
The mRNA abundance of GmRAV in soybean leaves, roots and stems in SDs was higher than that in LDs (LD: Long-day). Equally, soybeans grown in SDs exhibited smaller and yellower leaves, shorter stem than soybean in LDs. SDs repress the growth of soybean vegetative organs, reduce growth rate and accelerate leaf senescence with less chlorophyll content and photosynthesis rates. Moreover, SDs promote the transition to flowering, the development of reproductive organs and seed maturation. During constitutive expression in transgenic tobacco plants GmRAV was a growth repressor of leaves, roots and stems. The enhanced expression of GmRAV in SDs induced plants compared to that in LDs inhibited the growth of soybean leaves, roots and stems, reduced chlorophyll content in leaves and lowered rates of photosynthesis. Though flowering time and maturity in soybean in SDs were earlier, transgenic tobacco plants expressing GmRAV inhibited tobacco flowering. Thus, increased GmRAV transcript abundance may have reduced photosynthesis, caused nutrient deWciency and increased the juvenile phase by interfering with the induction of floral initiation. Both developmental cues and environmental signals control the timing of flowering. Many plants pass through a juvenile phase, in which flowering cannot occur, to ensure that sufficient reserves accumulate to sustain floral development and seed set (Simpson and Dean 2002). The juvenility trait is important to high seed yield from soybean production in tropical regions like South America and South Asia.
GmRAV transcripts in soybean and tobacco were associated with the senescence of leaves treated with darkness or ABA
GmRAV transcript abundance in soybean leaves was enhanced during dark and ABA (ABA: Abscisic acid) induced senescence which caused chlorophyll contents decrease. GmRAV was also involved in the positive promotion of dark and ABA induced senescence based on the analysis of transgenic tobacco plants overexpressing GmRAV. Seedlings of transgenic tobaccos by darkness and ABA treatments exhibited the symptom of visible yellowing and chlorophyll degradation.
In conclusion, the GmRAV gene appears to encode a transcript and protein implicated in the control of growth and development of soybean. The gene may provide avenues to manipulate meristem development, including nodulation, plant height, root length, photosynthetic rate, flowering time, the length of the juvenility period and the maturity date. Each of these developmental events has profound implications on the production of soybeans around the world (Fehr 1987). Alleles of GmRAV will be exciting new tools for soybean breeding and selection.
