Functional Analyses Of SLAH3 And NAXT1 Genes In Nitrogen Regulation In Arabidopsis

SLAH3 is one of the members of SLAC/SLAH family in Arabidopsis.SLAH3,a plasma membrane-localized protein,functions as a nitrate outward channel.SLAH3 was reported earlier to play central role in the process of nitrate-dependent alleviation of ammonium toxicity.NAXT1,a nitrate efflux transporter,is one of the seven members in the family of NAXT.To determine whether NAXT1 is also involved in the process of nitrate-dependent alleviation of ammonium toxicity,we analyzed naxt1 mutants under the conditions of low nitrate,high ammonium and low p H.The results suggested that naxt1 mutant did not exhibit similar phenotype to slah3 mutant under low nitrate,high ammonium and low p H conditions.We also examined naxt1 slah3 double mutant under the under low nitrate,high ammonium and low p H conditions.The naxt1 slah3 double mutant showed identical phenotype to slah3 under various conditions tested.The results suggested NAXT1 may not contribute to the process of nitrate-dependent alleviation of ammonium toxicity.Since NAXT1 has a close homolog in Arabidopsis genome.It is also possible that functional redundancy cause no obvious phenotype observed in naxt1 single mutant.In order to determine whether SLAH3 and NAXT1 participate in the nitrate sensing,we employed the method of Split Root System to analyze slah3 and naxt1 mutants.The results showed that the numbers of lateral roots grown on the high nitrate medium and numbers of the lateral roots grown on the low nitrate medium were almost equal in slah3.The numbers of lateral roots grown on the medium with high nitrate was significantly more than the numbers of the lateral roots grown on the medium with low nitrate in naxt1-1 mutant or in Col-0 wildtype.The results suggested that SLAH3 may play a role in nitrate sensing,but NAXT1 is likely unrelated to this process.In this study,we also observed that slah3 mutants show early flowering phenotype compared with Col-0 under long-day condition.The number of rosette leaves was reduced in slah3 mutants.By using q-PCR,the transcription levels of flowering-regulating genes FT and FLC were examined.The results slowed that the transcripts of FT were increased and the expression level of FLC was decreased in slah3 mutant,consistent with the early flowing phenotype of slah3.