| Summary: | Barley developmental mutants <i>tweaky spike</i> (<i>tw</i>) with disturbed auxin pathways possess a unique feature of an increased level of mouldy germinating grains (MGG), which serves as a convenient model to investigate the effects of plant immunity-related substances. The effects of the auxin 2,4-dichlorophenoxyacetic acid (2,4-D), auxin inhibitors, salicylic acid (SA), and <i>trans</i>-cinnamic acid (TCA) were studied using the <i>tw-WT</i> system in surface-sterilized and unsterilized germinating grains under high rates of natural infection. Significant differences among the allelic <i>tw</i> mutants were revealed at the natural MGG level and in response to 2,4-D, SA, and TCA. The most effective means against MGG were sterilization and TCA. 2,4-D inhibited root growth in <i>tw</i> and <i>tw</i><sub>2</sub> mutants, occurring only in unsterilized and not sterilized germinating grains, while the opposite was observed for TCA and SA. The <i>tw</i> mutations influenced variations in the seed-borne fungal spectra, decreasing the frequency of <i>Bipolaris sorokiniana</i> and increasing <i>Fusarium</i> spp. Hypochlorite-based surface sterilization methods should be used with caution in studies where the action of exogenous 2,4-D will be analysed in germinating grains. Auxin pathway disturbances specific for pleiotropic <i>tw</i> mutants are generally restricted to organogenesis but not to germination events.
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