Ethylene, a gaseous plant hormone, plays a crucial role in regulating various aspects of plant growth and development. This study delves into the intricate mechanisms by which ethylene modulates root architecture, specifically focusing on its influence on lateral root formation and the polar transport of auxin, a key growth hormone also known as indole-3-acetic acid (IAA).
Researchers employed a combination of genetic and molecular techniques to dissect these processes in Arabidopsis thaliana. They discovered that mutants deficient in auxin influx (aux1, lax3) and efflux (pin3, pin7) proteins exhibited reduced sensitivity to the inhibitory effects of ethylene on lateral root development. These mutants also showed a diminished response to ethylene-induced stimulation of auxin transport, when treated with 1-aminocyclopropane-1-carboxylic acid (ACC), an ethylene precursor. Conversely, pin2 and abcb19 mutants displayed typical wild-type responses to ACC.
The study further revealed that both ACC and IAA treatments led to an increase in the levels of transcripts encoding auxin transport proteins. This upregulation was found to be dependent on ethylene signaling components ETR1 and EIN2, as well as the auxin receptor TIR1. Intriguingly, the effects of ACC on transcript levels and lateral root development persisted even in the tir1 mutant, suggesting the involvement of independent signaling pathways.
Further investigation into auxin dynamics showed that ACC enhanced auxin-induced gene expression in the root apex region. However, it simultaneously reduced gene expression in areas where lateral roots typically emerge. Moreover, ACC treatment resulted in a decrease in free IAA levels in whole roots, indicating a shift in auxin distribution. Conversely, aminoethoxyvinylglycine (AVG), an inhibitor of ethylene synthesis, exhibited opposite effects on auxin-dependent gene expression, reinforcing the role of ethylene in modulating auxin activity. These findings strongly suggest that ethylene exerts its influence on root development by altering auxin distribution patterns within the root system, impacting Iaa Transport.
To visualize the changes in auxin transporter localization, the researchers used PIN3- and PIN7-GFP fusion proteins. They observed that ACC treatment led to an increase in PIN3-GFP and PIN7-GFP fluorescence, while AVG treatment resulted in a decrease. This observation is consistent with the proposed role of PIN3 and PIN7 in mediating ethylene-enhanced auxin transport. Notably, ACC treatment abolished the localized depletion of PIN3- and PIN7-GFP fluorescence that is normally observed below the sites of lateral root primordia formation.
These comprehensive results converge to suggest a compelling mechanism: ethylene treatment elevates PIN3 and PIN7 expression levels. This, in turn, leads to enhanced auxin transport, preventing the necessary localized accumulation of auxin required for initiating and driving lateral root formation. By manipulating iaa transport, ethylene effectively fine-tunes root architecture in response to environmental and developmental cues.
