Don’t be MIZguided, know where to grow!

Plant organs shift their directional growth in response to environmental stimuli through tropisms. Arabidopsis roots exhibit positive hydrotropism (towards water) and negative phototropism (away from light). In a recent study, Pang and colleagues demonstrated that root phototropism is regulated by the activity of two proteins in the elongation zone that also play essential roles in hydrotropism.

Credit: Candy1812, stock.adobe.com

Plants have evolved tropisms that allow for differential growth in response to environmental gradients, including light, gravity, water, touch, temperature, nutrients, and pathogens¹, which enables them to grow towards resources (e.g., root positive hydrotropism) and avoid unnecessary stressors. Root hydrotropism is dependent on the genes MIZU-KUSSEI1 (MIZ1) and MIZ2, whose names are derived from the Japanese words for “water” (mizu) and “tropism” (kussei)². MIZ1 encodes an endoplasmic reticulum membrane-associated protein with a largely unknown molecular function, while MIZ2 encodes a guanine nucleotide exchange factor for ADP ribosylation factor (GNOM)^2,3,4. In addition to lacking hydrotropic responses, roots of both miz1 and miz2 mutants also show substantially reduced negative phototropism—hinting at some underlying interaction between these tropic responses^2,3.

A recent study by Pang et al. provides further evidence that root hydrotropism and phototropism are regulated to at least some degree by shared molecular pathways⁵. Building on previous findings that MIZ1 expression in the cortex of the transition and elongation zones is necessary for hydrotropism⁶, Pang et al. examined the hydro- and phototropic responses of tissue-specific complemented, transgenic miz1 and miz2 mutant roots. They showed that functional MIZ1-GFP expressed under a cortex-specific promoter is able to fully rescue the phototropic response of miz1 plants, with roots demonstrating wild-type-like curvature in response to blue light exposure⁵. In contrast, expression of MIZ1-GFP in the root cap, meristem, epidermis, or endodermis is not sufficient to rescue phototropic responses in this mutant. In parallel, Pang and colleagues also demonstrated complete recovery of both hydrotropic and phototropic responses when MIZ2/GNOM is expressed in the epidermis, cortex, or stele of miz2 roots, but not when it is expressed in the root cap or endodermis.

Pang and colleagues concluded that MIZ1 functions in the cortex of the elongation zone of roots, where it is necessary for a full phototropic response, while MIZ2/GNOM functions in the epidermis, cortex or stele to promote phototropism. These findings suggest that both phototropism and hydrotropism are at least partially mediated through homologous pathways. While it is logical that both MIZ1 and MIZ2/GNOM function in the same cell type (cortex), as it was previously demonstrated that MIZ1 activity depends on MIZ2/GNOM, MIZ2/GNOM’s tissue-specific ability to promote tropic responses in the epidermis or stele remains an intriguing mystery. Furthermore, additional work is needed to understand how MIZ1 and MIZ2/GNOM act on downstream factors to induce phototropism. For instance, future studies should focus on investigating if MIZ1 or MIZ2/GNOM act directly on key molecular components involved in phototropic signaling, such as the phototropin and phytochrome photoreceptors or the kinase PKS1.