Disease relevance of NPH4

• The msg1 leaves were also resistant to 2,4-D-induced chlorosis [1].

High impact information on NPH4

• By contrast, gravitropism and auxin-induced gene expression in the root, which were promoted by functionally redundant NPH4/ARF7 and ARF19 proteins, were inhibited by shy2-2, but not by bdl protein [2].
• Global gene expression analysis revealed that auxin-induced gene expression is severely impaired in the arf7 single and arf7 arf19 double mutants [3].
• An Arabidopsis mutant (nonphototropic hypocotyl4-1 [nph4-1]) that is a null for ARF7 showed strongly reduced expression of integrated auxin-responsive reporter genes and natural genes that were monitored in Arabidopsis leaf mesophyll protoplasts [4].
• Expression of the reporter and natural genes was restored in an auxin-dependent manner when protoplasts were transfected with a 35S:ARF7 effector gene, encoding a full-length ARF7 protein [4].
• Transfection of effector genes encoding other ARF activators restored auxin-responsive gene expression to varying degrees, but less than that observed with the ARF7 effector gene [4].

Biological context of NPH4

• Both arf19 and arf7 mutants isolated from our forward genetic screens are auxin resistant and the arf19arf7 double mutant had stronger auxin resistance than the single mutants and displayed phenotypes not seen in the single mutants [5].
• Subsequently, we generated double mutants. arf7 arf19 has a strong auxin-related phenotype not observed in the arf7 and arf19 single mutants, including severely impaired lateral root formation and abnormal gravitropism in both hypocotyl and root [3].
• Moreover, the auxin signaling events mediating phototropism are genetically correlated with the abundance of the NPH4 protein [6].
• AUXIN RESPONSE FACTOR7 (ARF7) is one of five ARF transcriptional activators in Arabidopsis thaliana that is proposed to regulate auxin-responsive expression of genes containing TGTCTC auxin response elements in their promoters [4].
• NPH4/ARF7 and ARF19 function as transcriptional activators, suggesting that auxin may control senescence in part by activating gene expression [7].

Associations of NPH4 with chemical compounds

• These characteristics of msg2 are similar to those of another auxin-insensitive mutant, non-phototropic hypocotyl4 (nph4), which is a loss-of-function mutant of AUXIN RESPONSE FACTOR7 (ARF7) (Harper et al., 2000) [8].
• Here we show that ARF19 and ARF7 not only participate in auxin signaling, but also play a critical role in ethylene responses in Arabidopsis (Arabidopsis thaliana) roots, indicating that the ARFs serve as a cross talk point between the two hormones [5].
• Inhibition of HDAC activity by trichostatin A also results in LR formation in the slr-1 mutant, but not in the slr-1 arf7 arf19 triple mutant, suggesting that normal HDAC activity is required for the mIAA14-mediated inactivation of ARF7/19 functions in LR initiation [9].
• They did not undergo hypocotyl growth curvature at any of the IAA concentrations tested. msg1 is recessive and is located on chromosome 5. msg 1 hypocotyl growth is resistant to 2,4-dichlorophenoxyacetic acid (2,4-D), but the roots are as sensitive to 2,4-D as the wild type [1].
• These LBD/ASL proteins are localized in the nucleus, and dominant repression of LBD16/ASL18 activity inhibits lateral root formation and auxin-mediated gene expression, strongly suggesting that these LBD/ASLs function downstream of ARF7- and ARF19-dependent auxin signaling in lateral root formation [10].

Other interactions of NPH4

• The NPH4 locus encodes the auxin response factor ARF7, a conditional regulator of differential growth in aerial Arabidopsis tissue [11].
• PICKLE is required for SOLITARY-ROOT/IAA14-mediated repression of ARF7 and ARF19 activity during Arabidopsis lateral root initiation [9].
• Thus, NPH2 and NPH3 appear to act as signal carriers in a phototropism-specific pathway, whereas NPH4 is required for both phototropism and gravitropism and thus may function directly in the differential growth response [12].
• Both MP and NPH4 interact strongly and selectively with themselves and with each other, and are expressed in vastly overlapping domains [13].
• This indicated that the NPH2, NPH3, and NPH4 proteins likely function downstream of NPH1 photoactivation [12].

References