Probing the hormonal activity of fractionated molecular humic components in tomato auxin mutants

Abstract

We progressively reduced the complexity of humic matter by a mild sequential removal of unbound or free components, weakly, and strongly bound molecules. The auxin-like response of residues from each step was tested using tomato (cv. Micro-Tom) seedlings expressing DR5 auxin synthetic promoter fused to the β-glucuronidase (GUS) reporter gene and the low auxin-sensitivity diageotropica (dgt) mutant. Both exogenous auxin and humic matter promoted lateral root emergence in the control, but failed to induce lateral roots in the dgt mutant. When strongly bound components were removed from humic matter by breaking the ester and ether bonds, the humic residues lost their ability to induce the DR5::GUS activity and lateral root emergence. However, these capacities were retained in the free or weakly bound molecules. These findings confirm that auxin-like activity in humic matter is associated with complex hydrophobic structures, whose simplification by hydrolysis may release auxin-like molecules.

Agricultural uses of plant biostimulants

“…These results were interpreted as suggesting that hydrophobic interactions of humic acids in the rhizosphere may release auxin-like compounds that promote root growth. Support for this suggestion was reported in two independent studies using the auxin-synthesis promoter DR5 fused to a GUS reporter gene in transgenic tomato (Canellas et al 2011;Dobbss et al 2010).…”

“…Promotion of root system development is the most commonly reported initial effect of humic acids on plant growth. For example, enhancement of lateral roots or general increased seedling root growth has been reported with tomato (Adani et al 1998;Canellas et al 2011), Arabidopsis Canellas et al 2010), wheat (Tahir et al 2011;Peng et al 2001), maize (Canellas et al 2002 andCanellas et al 2009;Eyheraguibel et al 2008;Jindo et al 2012), pepper (Cimrin et al 2010), and Lantana camara (Costa et al 2008). Shoot growth promotion by humic acids has also been reported with cucumber (Mora et al 2010), tomato (Adani et al 1998;Lulakis and Petsas 1995), wheat (Tahir et al 2011), maize (Eyheraguibel et al 2008), and pepper (Cimrin et al 2010).…”

Humic substances stimulate maize nitrogen assimilation and amino acid metabolism at physiological and molecular level

“…However, because of the complexity of HS' nature, the relationship between their molecular structure and biological activity is not yet completely clear. In the last decades, several studies correlated biological activity to humic chemical features [31][32][33][34][35][36][37][38][39][40][41]. The positive effects of different HS in relation to their chemical structure were shown by Muscolo et al [35] and Nardi et al [36] on Pinus nigra callus metabolism and Zea mays seedling growth.…”

Physiological responses to humic substances as plant growth promoter

“…Figure 4 Visualization of β-glucoronidase (GUS) activity in the root of DR5:GUS transgenic tomato. Seedlings were grown for 4 days in a 2-mM CaCl 2 medium and then incubated for 4 days in water (A), 10 −6 M indole acetic acid (B) and 2.5 mM C of humic substances isolated from cattle manure vermicompost after free lipid removal (C) (Adapted from Canellas et al [22]). …”

“…These results were further corroborated by employing tomato (cv. Micro-Tom) seedlings expressing DR5:GUS ( Figure 4) and low auxin-sensitive diageotropica (dgt) mutant [22].…”