Os05g0145000

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Annotated Information

Function

Figure 1. Functional complementation of ptb1. (a) Plant phenotype of ptb1. (b) Plant phenotype of the PTB1-transformed ptb1 plants (ptb1–PTB1).(c) Panicle phenotype of ptb1. (d) Panicle phenotype of the ptb1–PTB1 transgenic plants. (from reference).

The domestication-related POLLEN TUBE BLOCKD 1(PTB1), a RING-type E3 ubiquitin ligase, positively regulates the rice panicle seed setting rate by promoting pollen tube growth. The PTB1 positively regulates rice PSSR through controlling pollen tube growth. Shuangcheng et al(2013) support the hypothesis that PTB1 is an important maternal sporophytic factor of pollen tube growth and a key modulator of the rice panicle seed setting rate.
There are many genes that related to the grain yield such as GS3/5, GW2/5/8, GL3.1 and TGW6, but as we know now, no genes regulates PSSR(which is one of the most important yield determinants) have been functionally characterized. Rice PSSR is impressionable to outside environmental conditions, which often cause large rice yield losss and threatens the safety of rice production. Nowadays, compared with the parents, many hybrid rice varieties exhibit sharp decreases in PSSR, the finding of the PBT1 gene has implications for the improvement of rice yield.
PTB1 has pleiotropic effects on PT growth control. First, it has a decisive role in entering and early growth of the STT.Second, PTB1 functions during the later navigation of PTs in the STT because a lower level of PTB1 expression leads to different numbers of PTs growing in this process.Third, PTB1 promotes growth from the chalazal end of the sporophytic STT to the micropyle because a moderate expression of PTB1 leads to the growth of few PTs in this female location.Finally, PTB1 functions as a promoting agent based on the observation that a higher expression of this gene encourages more PTs to complete their full journey.

Expression

Figure 2. Expression pattern(from reference).

The natural variation in expression of PTB1 which is affected by the promoter haplotype and the environmental temperature, correlates with the rice panicle seed setting rate. The GUS gene test in rice showed that PTB1 was differentially expressed in various rice tissues: the expression of PTB1 was relatively weak in the root, shoot and leaf, and the expression was strong in the stigma of the pistil and the spikelet hulls of the pre-emergence panicle. But , in rice pollen grains,the expression was undetectable. The results of Q-PCR was further confirmed this temporal and spatial expression patterns by showing that PBT1 was expressed prior in the reproductive organs such as the pistil, young panicle,pre-emergence panicle and post-emergence panicle. This pattern corresponded well to the role PBT1 played in the pollen tube growth.
The transient expression of PBT1-YFP fusion protein in rice protoplast cells showed that the PBT1 localized preferentially to the cytoplasm and although the PBT1 protein was predicted to contain 4 transmembrane segments, it was weakly expressed on the membrane.

Evolution

Figure. 3. (from reference).

Figure. 4. (from reference).

Figure. 5. (from reference).

• PBT1 may be a domestication-related gene: Shuangcheng et al(2013) found that in 107 rice accessions(42 indica cultivars, 44 japonica cultivars and 21 O.rufipogon accessions), there were 23variation site within the promoter region of PBT1. The sequence variation analysis showed that completely identified 16 haplotypes in O. rufipogon, but only 3 haplotypes were present in O. sativa. The result suggested that the PBT1 locus was selected by ancient humans during the domestication of rice.

The haplotype of promoter influence the lever of PBT1 expression and the final PSSRs: the two allelic variation in PBT1 promoter(CATT and TCGT) were determined by 4 SNPs(-1,156, -1,069 -931, -621). The two haplotypes were significantly related to PBT1 expression(Fig. 3).

• PBT1 causes PSSR decrease un der abnormal temperature: we know that rice PSSR can be affected by environmental temperature during the flowering stage, and it always results into a great loss in rice yields. Under the abnormal temperature, the PBT1 expression lever was significantly down-regulated alike with the decrease of PSSR, and the change of PSSR and PBT1 expression were significantly correlated(Fig. 4).
• PTB1 has a role in promoting the amount and depth of growth of PTs in the rice reproductive tract, indicating that the level of PTB1 expression positively regulates PT growth in

a dose-dependent manner. However, considering its role as an E3 ubiquitin ligase, the PTB1 protein itself might not act as a direct PT promoter reagent. Some researchers propose that PTB1 recruits its substrate (an unknown inhibitor of PT growth) for target degradation, thereby promoting PT growth by controlling the amount and depth of growth of PTs and, subsequently, positively affecting rice PSSR and ultimately improving grain yields(Fig. 5).

Labs working on this gene

Rice Research Institute, Sichuan Agricultural University; State Key Laboratory of Hybrid Rice, Sichuan Agricultural University; Key Laboratory of Crop Genetic Resources and Improvement, Sichuan Agricultural University, Ministry of Education