Promotion of root development by slightly alkaline pH involves an auxin mediated adaption mechanism

The present study performed experiments to identify the superior pH for Arabidopsis root growth base on its development (radical root length and lateral root density); and then approached the mechanism by which optimized pH primes root development. The results showed that neutral to slightly alkaline pH (7.0−8.0) in medium was the optimum range in which the plant had longer primary roots and denser lateral roots. Auxin reporter DII:VENUS transgenic line cultured in standard (5.8) and slightly alkaline (7.5) pH indicated that in pH 7.5 conditions, there was less fluorescence in the root cap compared to in acidic conditions. Furthermore, the DR5:Luciferase transgenic plant showed pH 7.5 accelerated the auxin oscillation frequency, was 17.83% shorter than that in pH 5.8. Later, a series of mutant germplasms showed slightly alkaline conditions promoting root development were independent from PLT transcript factors, but mainly mediated by auxin transportation. Transcriptomic dynamic analysis showed pH 7.5 conditions could trigger the changes in the pathways of plant hormones signal transduction and ABC transporter. The mutants of the ABC transporter coding genes were thus tested and the results showed that mutant abcb20 could block the slightly alkaline (7.5) pH promoting root development completely, as well as several other mutants blocking it partially.