An efficient hairy root system for validation of CRISPR/Cas system activities in cotton.

The hairy root induction system has been widely applied in studying gene expression and function in plant species due to its rapidity and efficiency. The hairy root system is an efficient tool for evaluating the activities of CRISPR/Cas systems. Cotton hairy roots were primarily induced through cotton tissue culture under aseptic conditions and by injecting cotton stem tips under non-aseptic conditions. However, both methods are lab-intensive and time-consuming. In this study, an efficient cotton hairy root induction procedure was established via infecting cotton hypocotyls with Agrobacterium rhizogenes under non-sterile conditions. Cotton seedlings with expanded cotyledons were decapitated with a slanted cut, and the residual hypocotyl (maintained 1 cm apical portion) was inoculated with A. rhizogenes. Over 90% of the infected explants from all three tested varieties could produce hairy roots after 8 days of inoculation. The effictiveness of the method was tested by overexpressing two reporter genes (eGFP and GUS). The transformation efficiency of the GUS and eGFP were ranged from50-68.18% and 40.9-68.18%. In addition, the editing efficiency of target sites in different CRISPR/Cas systems were also tested in hairy root. This method provided technical support for screening suitable target sites for cotton gene editing.