TITLE: Hairy root generation in common bean (Phaseolus vulgaris L.) and selection of Agrobacterium rhizogenes clones
AUTHORS: Ronal Pacheco, Georgina Estrada-Navarrete, Noreide Nava, Jorge Solis-Miranda, Carmen Quinto

[DESCRIPTION]
The common bean (Phaseolus vulgaris L.) is one of the legumes used to study the molecular mechanisms that regulate mycorrhizal and rhizobial symbioses. To study these mechanisms, the generation of transgenic hairy roots is a very advantageous method for applying genetic approaches. However, the generation of hairy roots is a difficult task that requires a lot of skill and experience. Here we show a new version of an optimized protocol for the generation of hairy roots in common bean. This version includes photos and videos showing the procedure for preparing the Agrobacterium rhizogenes culture and the infection procedure, which were not included in the original version. Additionally, we included a step for growing hairy roots under hydroponic conditions.


[STEPS]
SECTION: Seeds disinfection (when necessary)
1. Immerse the common bean seeds in 96 % volume ethanol for 5 min  and wash them three times with sterile water.

SECTION: Seeds disinfection (when necessary)
2. Immerse the seeds in 2 % volume  sodium hypochlorite for 5 min and wash them three times with sterile water.
 



SECTION: Seeds disinfection (when necessary)
3. Keep the seeds at 4 °C in a disinfected container, e.g., sterile  or reused Petri dishes previously disinfected with 96 % volume ethanol.




SECTION: Seeds germination (1st day)
4. Placed disinfected seeds, using sterile forceps, in a metal tray on a wet paper towel, previously sterilized in an autoclave. Leave 2 cm between the seeds.

 
 
  

SECTION: Seeds germination (1st day)
5.  Cover the metal tray with aluminum foil and incubate it in a growth chamber at 28 °C  for 2760 min to 2880 min in the dark. 

 

SECTION: Preparation of the inoculum of A. rhizogenes K599 (2nd day)
6. Spread 150 µL-200 µL of the inoculum in Petri dishes containing solid LB medium with the appropriate selection antibiotic. 
 



SECTION: Preparation of the inoculum of A. rhizogenes K599 (2nd day)
7. Incubate the Petri dishes inoculated in the previous step, for approximately 1800 min at 30 °C. 
 

SECTION: Preparation of the inoculum of A. rhizogenes K599 (2nd day)
7.1. Scratch off this thin layer of dried culture with a sterilized yellow tip or something similar. Transfer this culture to an   or   


 
Video 1
SECTION: Preparation of the inoculum of A. rhizogenes K599 (2nd day)
7.2. Add LB liquid medium to the Eppendorf tube and centrifugate  8000 rpm, 1 min. Finally, homogenize the content using a micropipette. The content must be viscous, but liquid enough to be pipetted (Fig. 3).

 
SECTION: Preparation of the inoculum of A. rhizogenes K599 (2nd day)
7.3. Prepare the inoculum in Eppendorf tubes (preferably 0.6 ml) by mixing an equal volume 50 % (v/v)of the liquid content previously obtained, and 80 % volume glycerol. Mix tubes by inversion and immediately place them in liquid nitrogen; finally, store the inoculum at -80 °C.
 




SECTION: Seedling infection by A. rhizogenes K599 (3rd day)
9. Carefully puncture in the infection zone of the hypocotyl several times using a sterile needle tip (0.4 mm). Apply the inoculum on the wounded zone, directly from the plates, using an autoclaved micropipette tip (Video 3).

 
Video 3
SECTION: Seedling infection by A. rhizogenes K599 (3rd day)
8. Expose the infection zone (Video 2, Fig 4).

 
Video 2.
 

SECTION: Generation of hairy roots
10. Place the infected seedlings on the top of plastic tubes, i.e.,   containing B & D medium (Table 1). Place plastic tubes inside glass tubes containing autoclaved deionized water and cover the glass tubes with plastic caps to prevent water evaporation (Video 3, Fig. 5). 
 

 


SECTION: Generation of hairy roots
11. Place the glass tubes on racks and incubate in a growth chamber at around 25 °C, 960 min light/480 min 8 h dark for 3-5 days post-infection (dpi). 
 

SECTION: Generation of hairy roots
13. Once the hairy roots have emerged, remove the primary root by cutting the stem 2 cm below the hairy root callus.
SECTION: Generation of hairy roots
12. After removing the caps, the plants should be incubated at about 28 °C  to promote growthuntil hairy roots emerge (10-13 dpi)

SECTION: Generation of hairy roots
14. Transfer the seedlings to autoclaved glass tubes containing B & D medium and seal the tube hole with parafilm or adhesive plastic. Maintain plants under this condition for around 4320 min  at 28 °C to promote hairy root growth. 

 



SECTION: Removal of untransformed roots (selection of suitable plants)
15. Observe the fully developed hairy roots (15 to 16 dpi) using an epifluorescence microscope to remove non-fluorescent roots. 
 

SECTION: Uses of hairy roots
16. Depending on the intended use of the roots, you can proceed as follows.
SECTION: Selection of A. rhizogenes clones
17. 
Extraction of total RNA from hairy roots using an appropriate protocol and cDNA synthesis. For RNA extraction from common bean root tissue, we recommend the following protocol.
dx.doi.org/10.17504/protocols.io.8epv5jq24l1b/v1

 

SECTION: Selection of A. rhizogenes clones
18. Quantify transcript levels of the gene of interest by qPCR