Molecular Mechanism Underlying Rice Gravitropism Mediated By The Factors Controlling The Lateral Auxin Transport

To meet the challenge of increasing damands in grain yield and cope with the globally extreme climate changes,it is an effective strategy to breed high-yield and stress-resistant rice varieties.Tiller angle is an essential determinator for rice architecture,and an optimum tiller angle ensures the high yield per unit area of crop.Thus,exploring the regulatory mechanism of rice tiller angle will provide great theoretical guidance for breeding practice.From prostrate growth and fewer grains per panicle in wild rice,to erect growth and more grains per panicle in cultivated rice,the domestication greatly promoted the increase of rice yield.The prostrate growth of wild rice is controlled by a key gene PROSTRATE GROWTH 1(PROG1)encoding a C2H2 zinc-finger transcription factor,and overexpression of this gene in cultivated rice causes prostrate growth phenotype.However,the corresponding regulatory mechanism has not been fully elucidated.LAZY1(LA1)is the first identified gene that controls rice tiller angle.LA1 could influence the polar auxin transport,and further influence the lateral transport of auxin to regulate shoot gravitropism and tiller angle in rice.In this research,we conducted in-depth study for genetic regulatory relationship between these two critical genes in rice tiller angle regulation.Our study found that just like LA1,PROG1 negatively regulated auxin asymmetric distribution upon gravity stimulation to mediate shoot gravitropism in rice.Double mutant genetic analysis confirmed that the shoot gravitropism and tiller angle regulatory pathways of PROG1 and LA1 overlapped,and PROG1 may function at upstream of LA1 in the overlapping pathway.Overexpression of LA1 could partically rescue the increased tiller angle phenotype in prog1-D,which indicated that PROG1 located at upstream of LA1 in shoot gravitropism and tiller angle regualtion in rice.Further,we demonstrated that PROG1 could repress the expression of LA1 to regulate shoot gravitropism and tiller angle in rice.Additionally,we found that the inhibition of PROG1 to LA1 transcription was light-dependent in coleoptiles.SUPPRESSORS OF LAZY1 1(SOL1)encodes DWARF3(D3),a key factor in strigolactones signaling pathway.The sol1 mutation could repress the increased tiller angle phenotype in prog1-D by recovering the expression of LA1.Collectly,the discovery of PROG1-LA1 module provides a new idea for genetic regulation and also enriches the genetic regulatory mechanism of rice tiller angle.Transporter proteins located at membrane play essential roles in auxin polar transport,in which PIN-FORMED(PIN)family is especially important as auxin efflux transporters.To investigate the relationship between the LA1-core regulatory pathway and PIN family genes in controlling shoot gravitropism of rice,we generated transgenic plants of PIN protein family in this research.Among these trangenic lines,the OsPIN10 a overexpressed trangenic plants showed defective shoot gravitropism and increased tiller angle.Further research showed that OsPIN10 a negatively regulated lateral auxin transport in shoot gravitropism.We also found that OsPIN10 a regulated rice tiller angle by influencing the expression levels of WUSCHEL RELATED HOMEOBOX6(WOX6)and WOX11,which acted downstream of LA1 in the LA1-core regulatory pathway.Interestingly,in addition to tiller angle,OsPIN10 a overexpressed trangenic plants also showed shallower root morphology due to defective root gravitropism.Further observation of auxin reporter line DR5:GFP-N7 in OsPIN10 a overexpressed transgenic plants showed that overexpressed OsPIN10 a significantly influenced the distribution and intensity of GFP signals in roots,which suggested that overexpressed OsPIN10 a influenced auxin content and distribution in roots.Starch staining experiments showed that overexpression of OsPIN10 a also reduced the starch content in root tips.Thus,these results suggested that overexpression of OsPIN10 a influenced auxin distribution in roots and starch content of root tips,and resulting in defective root gravitropism.In addition,we found that defective root gravitropism of OsPIN10 a overexpressed transgenic lines led to special root architecture,which showed more distributiom in shallow layer of soil and extended in all directions.Special root architecture of OsPIN10 a overexpressed transgenic lines improves phosphate uptake in rice,providing potential targets for root morphology and nutrient uptake.In conclusion,our study enriched the genetic regulatory network of tiller angle and gravitropism in rice by elaborating genetic relationship between PROG1 and LA1,two crucial genes in rice tiller angle regulation.We also found OsPIN10 a negatively regulated shoot and root gravitropism in rice,and further regulated tiller angle and root architecture of rice,respectively.Changes of root architecture even improved Pi utilization efficiency,and provided new gene resource for improving Pi utilization efficiency of rice.