Summary: Cytokinin is known to have negative effects on de novo auxin‐induced root formation. However, the regulatory mechanisms of root initiation by both cytokinin and auxin are poorly understood. In this study, we characterized a rice mutant, termed crown rootless5 (crl5), which produced fewer crown roots and displayed impaired initiation of crown root primordia. The expression of CRL5, which encodes a member of the large AP2/ERF transcription factor family protein, was observed in the stem region where crown root initiation occurs. Exogenous auxin treatment induced CRL5 expression without de novo protein biosynthesis, which also required the degradation of AUX/IAA proteins. A putative auxin response element in the CRL5 promoter region specifically interacted with a rice ARF, demonstrating that CRL5 may be a direct target of an ARF, similar to CRL1/ADVENTITIOUS ROOTLESS1 (ARL1) that also regulates crown root initiation. A crl1 crl5 double mutant displayed an additive phenotype, indicating that these two genes function in different genetic pathways for crown root initiation. In addition, ProACT:CRL5/WT showed a cytokinin‐resistant phenotype for crown root initiation, and also up‐regulated the expression of two negative regulators of cytokinin signaling, OsRR1 and OsRR2, which were downregulated in crl5. Transgenic plants that over‐expressed OsRR1 under the control of the CRL5 promoter in a crl5 mutant background produced a higher number of crown roots than the crl5 plant. Taken together, these results indicate that auxin‐induced CRL5 promotes crown root initiation through repression of cytokinin signaling by positively regulating type‐A RR, OsRR1.

Rice production faces the challenge to be enhanced by 50% by year 2030 to meet the growth of the population in rice-eating countries. Whereas yield of cereal crops tend to reach plateaus and a yield is likely to be deeply affected by climate instability and resource scarcity in the coming decades, building rice cultivars harboring root systems that can maintain performance by capturing water and nutrient resources unevenly distributed is a major breeding target. Taking advantage of gathering a community of rice root biologists in a Global Rice Science Partnership workshop held in Montpellier, France, we present here the recent progresses accomplished in this area and focal points where an international network of laboratories should direct their efforts.

The lignocellulosic biorefinery industry can be an important contributor to achieving global carbon net zero goals. However, low valorization of the waste lignin severely limits the sustainability of biorefineries. Using a hydrothermal reaction, we have converted sulfuric acid lignin (SAL) into a water-soluble hydrothermal SAL (HSAL). Here, we show the improvement of HSAL on plant nutrient bioavailability and growth through its metal chelating capacity. We characterize HSAL’s high ratio of phenolic hydroxyl groups to methoxy groups and its capacity to chelate metal ions. Application of HSAL significantly promotes root length and plant growth of both monocot and dicot plant species due to improving nutrient bioavailability. The HSAL-mediated increase in iron bioavailability is comparable to the well-known metal chelator ethylenediaminetetraacetic acid. Therefore, HSAL promises to be a sustainable nutrient chelator to provide an attractive avenue for sustainable utilization of the waste lignin from the biorefinery industry.

Internal oxygen diffusion from shoot to root tips is enhanced by the formation of aerenchyma (gas space) in waterlogged soils. Lysigenous aerenchyma is created by programmed cell death and subsequent lysis of the root cortical cells. Rice (Oryza sativa) forms aerenchyma constitutively under aerobic conditions and increases its formation under oxygen-deficient conditions. Recently, we have demonstrated that constitutive aerenchyma formation is regulated by auxin signaling mediated by Auxin/indole-3-acetic acid protein (AUX/IAA; IAA). While ethylene is involved in inducible aerenchyma formation, the relationship of auxin and ethylene during aerenchyma formation remains unclear. Here, we examined the effects of oxygen deficiency and ethylene on aerenchyma formation in the roots of a rice mutant (iaa13) in which auxin signaling is suppressed by a mutation in the degradation domain of IAA13 protein. The results showed that AUX/IAA-mediated auxin signaling contributes to ethylene-dependent inducible aerenchyma formation in rice roots. An auxin transport inhibitor abolished aerenchyma formation under oxygen-deficient conditions and reduced the expression of genes encoding ethylene biosynthesis enzymes, further supporting the idea that auxin is involved in ethylene-dependent inducible aerenchyma formation. Based on these studies, we propose a mechanism that underlies the relationship between auxin and ethylene during inducible aerenchyma formation in rice roots.

Significance: In recent years, phenotypic plasticity has received attention for improving plant adaptability to variable environments. For more than half a century, it has been known that rice and cereal plants develop different types of lateral roots (LRs), unlike the dicot model plant Arabidopsis . Despite the importance of plastic LR development under variable water conditions, the molecular mechanisms regulating LR types are unknown. Here, we report the regulatory mechanism of LR primordium size in rice, an important determinant of LR type. We identified two WUSCHEL -related homeobox (WOX) transcription factors that opposingly regulate LR primordium size. Our findings form the basis for improving root phenotypic plasticity for sustainable crop production under variable environments.