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Auxin regulation of primary root growth in Arabidopsisand rice was compared by analyzing root growth in response to changes in auxin levels. A bell-shaped root-growth curve was identified in both Arabidopsisand rice in response to change in auxin levels. In Arabidopsis, cell division was the main regulator of root growth in response to auxin; in rice, auxin promoted root growth by regulating cell division and cell length. The expression levels of PLETHORA (PLT) genes in response to change in auxin level followed a bell-shaped curve and closely correlated with cell division in Arabidopsisbut not in rice, implying that PLTgene expression plays key role to control root growth in Arabidopsis. The level of auxin in Arabidopsiswas optimal for primary root elongation, while in rice it was higher than optimal. These differences may explain the species-dependent development of root systems.Graphical AbstractThe role and developmental mechanisms of the primary root in Arabidopsisand rice are similar but different.

We previously reported that exogenous application of auxin to Arabidopsis seedlings resulted in downregulation of indole-3-acetic acid (IAA) biosynthesis genes in a feedback manner. In this study, we investigated the involvement of the SCFTIR1/AFB-mediated signaling pathway in feedback regulation of the indole-3-pyruvic acid-mediated auxin biosynthesis pathway in Arabidopsis. Application of PEO-IAA, an inhibitor of the IAA signal transduction pathway, to wild-type seedlings resulted in increased endogenous IAA levels in roots. Endogenous IAA levels in the auxin-signaling mutants axr2-1, axr3-3, and tir1-1afb1-1afb2-1afb3-1also increased. Furthermore, YUCCA(YUC) gene expression was repressed in response to auxin treatment, and expression of YUC7and YUC8increased in response to PEO-IAA treatment. YUCgenes were also induced in auxin-signaling mutants but repressed in TIR1-overexpression lines. These observations suggest that the endogenous IAA levels are regulated by auxin biosynthesis in a feedback manner, and the Aux/IAA and SCFTIR1/AFB-mediated auxin-signaling pathway regulates the expression of YUCgenes.

His-Asp phosphorelays are evolutionary-conserved powerful biological tactics for intracellular signal transduction. Such a phosphorelay is generally made up of “sensor histidine (His)-kinases”, “response regulators”, and “histidine-containing (HPt) phosphotransmitters”. Results from recent intensive studies suggested that, in the higher plant Arabidopsis thaliana, His-Asp phosphorelays may be widely used for propagating environmental stimuli, such as phytohormones (e.g., ethylene and cytokinin). In this study, we characterized, in vitro, the putative cytokinin-responsive CKI1 His-kinase, in terms of His-Asp phosphorelays. It was demonstrated for the first time that the receiver domain in this sensor exhibits a strong phosphohistidine phosphatase activity toward some ArabidopsisHPt phosphotransmitters (AHP1 and AHP2), suggesting the functional importance of the receiver domain for a presumed interaction of the sensor His-kinase with other His-Asp phosphorelay components.

When brassinosteroid (BR)-deficient mutant (det2) or wild-type (WT) seedlings were treated with brassinolide (BL), the most active BR, for 3 h, the abundance of PIN4and PIN7transcripts decreased, and there were fewer PIN4and PIN7transcripts in det2than in the WT. This suggests that BL selectively regulates the PINgene in a complex manner.