Soilless plant growth media influence the efficacy of phytohormones and phytohormone inhibitors.

Norman B Best,Bruce R Cooper,Gnanasiri S Premachandra,Joshua S Budka,Brandon J Bishop,Burkhard Schulz,Cliff T Johnston,Elliot Brown,Thomas Hartwig,Devi P V Potluri,Malcolm Bennett

doi:10.1371/journal.pone.0107689

Abstract

Plant growth regulators, such as hormones and their respective biosynthesis inhibitors, are effective tools to elucidate the physiological function of phytohormones in plants. A problem of chemical treatments, however, is the potential for interaction of the active compound with the growth media substrate. We studied the interaction and efficacy of propiconazole, a potent and specific inhibitor of brassinosteroid biosynthesis, with common soilless greenhouse growth media for rice, sorghum, and maize. Many of the tested growth media interacted with propiconazole reducing its efficacy up to a hundred fold. To determine the molecular interaction of inhibitors with media substrates, Fourier Transform Infrared Spectroscopy and sorption isotherm analysis was applied. While mica clay substrates absorbed up to 1.3 mg of propiconazole per g substrate, calcined clays bound up to 12 mg of propiconazole per g substrate. The efficacy of the gibberellic acid biosynthesis inhibitor, uniconazole, and the most active brassinosteroid, brassinolide, was impacted similarly by the respective substrates. Conversely, gibberellic acid showed no distinct growth response in different media. Our results suggest that the reduction in efficacy of propiconazole, uniconazole, and brassinolide in bioassays when grown in calcined clay is caused by hydrophobic interactions between the plant growth regulators and the growth media. This was further confirmed by experiments using methanol-water solvent mixes with higher hydrophobicity values, which reduce the interaction of propiconazole and calcined clay.

Highlights

Biochemical treatments, in addition to mutant studies, are very effective approaches to study the function of endogenous signal substances such as phytohormones
It has a high permeability and helps prevent soil compaction but suffers from low water retention [48]. Vermiculite is another thermal expanded soilless medium, which is produced by exfoliation of the silicate mineral mica (Fig. 2 E–F). It is structured in two tetrahedral sheets for every one octahedral sheet (2:1) giving it a very high cation exchange capacity (CEC, 100– 150 meq/100 g) and moderate water retention capacity [47]
The choice of media substrate is rarely considered an important aspect of the experimental design and is primarily based on optimizing the growth conditions for the plant

Summary

Introduction

Biochemical treatments, in addition to mutant studies, are very effective approaches to study the function of endogenous signal substances such as phytohormones. GA is a tetracyclic dihydroxy lactonic acid first identified from culture filtrates of the fungus Gibberella fujikuroi, whereas BRs were first identified in pollen of Brassica napus and are polyhydroxylated steroidal hormones (Fig. 1) [3,4]. Both GAs and BRs are major plant growth regulators (PGRs) and show substantial overlap in the developmental processes that they affect. GAs and BRs can have opposite functions as exhibited by their effects on sex determination in maize [8,9]

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Discussion

Conclusion