Abstract
SummaryThe rice root system is important for growth. The crosstalk between auxin and cytokinin mediates root initiation and elongation. However, it remains unclear how the transcriptional network upstream of the auxin and cytokinin signalling pathways determines root development. Here, we observed that the knockdown of OsNAC2, which encodes a NAC transcription factor, increased the primary root length and the number of crown roots. OsNAC2 predominantly expressed in primary root tips, crown roots and lateral root primordia, implying it influences root development. Molecular analyses revealed that the expressions of auxin‐ and cytokinin‐responsive genes were affected in OsNAC2‐overexpressing (OsNAC2‐OX;ON7 and ON11), RNA interference (OsNAC2‐RNAi;RNAi25 and RNAi31) and CRISPR/Cas9 plants. Additionally, OsNAC2 can directly bind to the promoters of IAA inactivation‐related genes (GH3.6 and GH3.8), an IAA signalling‐related gene (OsARF25), and a cytokinin oxidase gene (OsCKX4). Furthermore, genetic analysis of ON11/osgh3.6 and RNAi31/osckx4 homozygote confirmed that OsCKX4 and OsGH3.6 functioned downstream of OsNAC2. The mRNA levels of CROWN ROOTLESS (CRL) genes and cyclin‐dependent protein kinase (CDK) genes increased in OsNAC2‐RNAi and OsNAC2‐cas9 lines while reduced in OsNAC2‐OX lines. Thus, we describe that OsNAC2 functions as an upstream integrator of auxin and cytokinin signals that affect CRL and CDK production to regulate cell division during root development. This novel auxin‐OsNAC2‐cytokinin model should provide a new insight into the understanding of NAC TFs and crosstalk of auxin and cytokinin pathway, and can be potentially applied in agriculture to enhance rice yields by genetic approaches.
Highlights
Rice (Oryza sativa L.) is one of the most important staple foods worldwide
To characterize the OsNAC2 function in root development, the spatial and temporal expression profiling of OsNAC2 were investigated in transgenic plants expressing b-glucuronidase (GUS) driven by the native promoter of OsNAC2 promoter (1,947 bp)
GUS staining was detected in the apex and base of lateral roots, the lateral root primordia and crown roots (Figure 1b, a’ to d’)
Summary
The root system is important for the absorption of nutrients and water, anchoring of the plant and hormone biosynthesis, all of which are critical for crop growth and yield (Coudert et al, 2010). Root morphological abnormalities were observed in a number of rice mutants defective in auxin biosynthesis or signalling. The ectopic expression of OsGH3.2 in rice leads to an IAA-deficient morphology, including the production of relatively few crown roots and root hairs (Du et al, 2012). The ectopic expression of OsAUX1, which is related to polar auxin transport, enhances lateral root initiation (Yu et al, 2015). Rice osiaa mutants are insensitive to auxin and gravitropic stimuli, with fewer crown roots than in normal rice plants (Nakamura et al, 2006).
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