Studies Of NPY Genes On Gravitropism In Arabidopsis

Gravity is an important environment signal for plants, effecting its growth and development. Plants can sense the direction of gravity and orient their growth to ensure that roots are anchored in soil and that shoots grow upward. Gravitropism have been studied extensively using Arabidopsis genetics, but the exact mechanisms are not fully understood. Gravitropism, phototropism and auxin-regulated organogenesis are important for morphogenesis. It was reported that auxin-regulated organogenesis and phototropism used analogous mechanisms for which at least one NPH3-like gene, one AGC kinase, and one ARF were required. NPH3-like gene is a big gene family, which NPY genes are included. It was reported that root gravitropic responses required both auxin response factors (ARF7and ARF19) and AGC kinase (PID, WAG1, WAG2and D6PKs). This paper researched whether NPY genes were required for gravitropic responses. The main results were as follows:(1) The expression patterns of the NPY genes were shown by GUS staining. All of the five NPY genes were highly expressed in primary root tips, indicating that they may play a key role in Arabidopsis root gravitropism.(2) The single and double npy mutants did not display obvious gravitropism defects; all of the npy triple mutants except npyl npy2npy4had weak gravitropism defects; all of the npy quadruple mutants except npyl npy2npy3npy4had stronger gravitropism defects than the npy triple mutants. The npyl npy2npy3npy4npy5quintuple mutants showed the strongest gravitropic phenotypes, second is npy2npy3npy4npy5quadruple mutants. Systematic phenotypic analysis of npy mutant demonstrated that the five NPY genes all contributed to gravitropism redundancily.(3) Auxin transport inhibitor NPA inhibits gravitropism and promotes the growth of root hair. Our data showed that NPA slightly decreased the agravitropism in npy2npy3npy4npy5quadruple mutants and npy2npy3npy4npy5quadruple mutants were more resistant to NPA in length and dencity of root hair than wild type, indicating NPY genes deseased the inhibitor function of NPA to gravitropism and increased the promote function of NPA to root hair, which uncovered that NPY genes are related to auxin transportion.(4) Auxin is accumulated in quiescent center and columella cells in wild type. The expression pattern of auxin reporter DR5::GFP in npyl npy2npy3npy4npy5quintuple mutants was shown by confocal. DR5::GFP not only detected in quiescent center and columella cells, but also in the lateral root cap in npy quintuple mutants, indicating auxin transportion was affected in npy quintuple mutants.(5) PIN2gene encoded a polar-auxin-transport efflux. Our data indicated NPYs and PIN2had synergistic interactions since npy2npy3npy4npy5pin2quintuple mutants showed stronger phenotype than either pin2single mutant or npy2npy3npy4npy5quadruple mutant in both agravitropism and the phenotype of root tip bending out of the plates with exogenous IAA and NAA.(6) PIN2located in upper side of epidermis of root tip in wild type. Our data showed that there was no PIN2polarity in epidermis of root tip in npyl npy2npy3npy4npy5quintuple mutants, probably causing the restrain of the polar auxin transportion from root tip to elongation zone through epidermis.In summary, NPY genes played an essential role in root gravitropic responses in Arabidopsis. The research of gravitropism is very important to clarify the function of gravity to biological evolution and help developing cultivation techniques in different gravity environments. This paper uncovered that plants used an AGC kinase-NPH3-like protein-ARF module analogous to those in auxin-regulated organogenesis and phototropism to regulate plant root gravitropic responses, which providing foundation for further understanding the exact mechanisms of gravitropism.