In the global agricultural panorama, the fluctuation of soil pH emerges as a formidable conundrum. Approximately 40% of the world’s arable land is beleaguered by overly acidic or alkaline soils. This not only curtails the growth conditions for crops but also precipitates a direct decline in crop yields. As sessile organisms, plants are compelled to initiate an intricate array of physiological and metabolic regulatory mechanisms to acclimate to these pH vicissitudes. Notably, the rhizospheric pH is directly intertwined with the plant’s nutrient absorption efficiency and root growth, thereby exerting a profound impact on overall plant growth. During this regulatory process, Root growth inhibition (RGI) stands as a pivotal response of plants to extreme pH conditions, epitomizing their growth strategies in adverse environments.
Auxin and jasmonates play an indispensable role in modulating RGI. Auxin, a crucial plant growth regulator, intervenes in cell expansion and differentiation, thus directly partaking in the regulation of root growth. Jasmonates, on the other hand, are engaged in signal transduction in response to environmental stresses, assisting plants in confronting external challenges. Although extant research has intimated the significant role of these two hormones in plant growth regulation, the precise mechanisms underlying their response and adaptation to soil pH changes, as well as their interaction dynamics, remain shrouded in mystery.
Recently, Ajay K. Pandey’s research team from the National Agri-Food Biotechnology Institute (NABI) in India published a research paper titled “Jasmonates Regulate Auxin-Mediated Root Growth Inhibition in Response to Rhizospheric pH in Arabidopsis thaliana” in Plant, Cell & Environment. This study endeavors to disclose the molecular mechanisms through which plants perceive and respond to alterations in rhizospheric pH, with a particular focus on how auxin and jasmonates interact under diverse pH conditions to modulate root growth.
The research team initially scrutinized the impact of different rhizospheric pH values on the primary root length (PRL) of Arabidopsis thaliana. The findings revealed that, in contrast to neutral pH (5.8), both extremely acidic (pH 3.8) and alkaline (pH 7.8) conditions markedly inhibited root growth, and this inhibition was reversible. When plants were transferred from extreme pH conditions to a neutral pH environment, root growth could be restored.
Subsequently, the team employed auxin synthesis and signal transduction inhibitors, such as NAA (an auxin analogue), NPA (an auxin transport inhibitor), and TIBA, to explore the role of auxin in rhizospheric pH-mediated root growth inhibition. Concurrently, they utilized jasmonate biosynthesis and signal transduction inhibitors, like MeJA (methyl jasmonate), to investigate the crucial role of jasmonates in this process.
By leveraging auxin-responsive reporter genes, such as DR5rev::GFP and DII:VENUS, the research team observed that auxin accumulation in the root tip was intimately correlated with the rhizospheric pH value. Under acidic conditions, auxin accumulation diminished, while it increased under alkaline conditions. Additionally, through the Jas9:VENUS reporter gene, the researchers monitored the dynamics and distribution of jasmonates under different rhizospheric pH conditions, revealing a negative correlation between jasmonate accumulation and rhizospheric pH.
Further gene expression analysis and confocal microscopy observations divulged that jasmonates regulate auxin accumulation through the GH3 gene family, thereby governing root growth. In acidic conditions, the elevation of jasmonates prompts the upregulation of GH3 gene expression, facilitating auxin degradation, reducing auxin accumulation, and ultimately inhibiting root growth. Conversely, in alkaline conditions, the low jasmonate content fails to suppress auxin accumulation, leading to an augmented auxin accumulation and consequent root growth inhibition.
Utilizing jasmonate signaling mutants, namely jar1-11 and coi1-16, the research team corroborated the indispensability of jasmonate signaling in the rhizospheric pH response. Specifically, the jar1-11 mutant exhibited insensitivity to root growth inhibition induced by acidic conditions, while the coi1-16 mutant was insensitive to that caused by alkaline conditions.
Ultimately, the study proposes a working model delineating how rhizospheric pH orchestrates root growth by modulating the interaction between jasmonates and auxin. In acidic conditions, high jasmonate levels promote auxin degradation by upregulating the expression of GH3 genes, resulting in diminished auxin accumulation and subsequent root growth inhibition. In alkaline conditions, the low jasmonate content fails to restrain auxin accumulation, leading to an increased auxin accumulation and consequent root growth inhibition.
This research unravels the mechanism by which rhizospheric pH governs root growth in Arabidopsis thaliana through the interaction of auxin and jasmonates. It proffers strategies for agricultural improvement to enhance crop adaptation to extreme soil pH conditions. It not only augments our comprehension of how plants adapt to environmental changes but also holds the promise of cultivating stress-resistant crops, thereby safeguarding global food security.