Heterotrimeric G Protein In Arabidopsis Root Development Of The Auxin-mediated Mechanism Of Action

The signaling mediated by heterotrimeric G protein in higher plants is, to large extent, through its interaction with phytohormones, such as auxin, gibberellic acid, abscisic acid, ethylene and brassinosteroids, etc. In present study, the possible cross-talk of heterotrimeric G protein and auxin in the regulation of lateral root development was explored using null mutants of the heterotrimeric G protein subunits, i.e. Gαsubunit mutant gpa1-3, Gβsubunit mutant agb1-2, the double mutantsαβ36, as compared with their wildtype, Columbia (Col). The major results are as followings:1. Lateral root emerged from the second day after germination. The numbers of lateral root primodium are more in gpa1-3 than those in the wildtype, Col, within two days of germination, and then the increase of lateral root primodium became slowly, and the development of lateral roots was mainly in the III-V stage. Whereas the lateral root primodium emerged much rapidly in agb1-2 andαβ36, and the total numbers of lateral root were much more, as compared with those in gpa1-3 and Col. These results implied that heterotrimeric G protein might directly or indirectly involved in the regulation of development of lateral root primodium in Arabidopsis.2. No obvious differences were observed in the effects of either IAA treatment or TIBA treatment, an inhibitor of IAA transport, on primary root growth among different genotypes investigated, suggesting that heterotrimeric G protein had no relation to primary root growth.3. Exogenous auxin treatment had stimulated effects on the development of lateral roots and the numbers of lateral roots increased with increase in IAA or NAA concentration. A significant inhibition of lateral root development was observed after TIBA treatment. Furthermore, the differences of the numbers of lateral roots among different genotypes were diminished after TIBA treatment.4. Endogenous IAA contents in roots were quite different among different genotypes and gpa1-3 mutant had the highest IAA content and agb1-2 mutant the lowest. Removing the tip of shoot on 4 day after germination, IAA content of roots increased observably in agb1-2 andαβ36 mutants, while no obvious increase in IAA content was observed with the same treatment in gpa1-3 mutant.5. RT-PCR results showed that the expression of TIR1 was the lowest in agb1-2 roots, whereas the expression of CDC2a in agb1-2 roots was the highest among four genotypes investigated.According to the above results and other reports, it was suggested that heterotrimeric G protein was involved in the auxin-meidated lateral root initiation and development of Arabidopsis, but the different subunits of G protein had different roles. GPA1 exerted its effect on auxin-mediated lateral root initiation and development largely through increasing the expression of TIR1, a gene encoding IAA receptor, and thus the downstresm of signaling pathway. AGB1, as a negative regulator, affected lateral root initiation and development and this effect was related to polar transport of IAA, but not to the entering pathway of IAA to cells.