Replication of cowpea mosaic virus in protoplasts isolated from immune lines of cowpeas.

In previous work, some lines from a collection of cowpeas (Vigna unguiculata, Walp.) were found to be immune to a cowpea mosaic virus strain (CPMV-SB) under specified conditions. At 24 hr after inoculation, protoplasts from a susceptible and most immune lines were sources of similar amounts of virus, but very little virus was recovered from protoplasts of line Arlington. Now we have compared the responses of Arlington, several other immune lines, a local lesion host (a selection out of the cowpea cross Chinese red x Iron), and a systemic host (Blackeye 5) after inoculation under a variety of conditions and by means of three types of virus assay. Replication of CPMV-SB apparently did occur in Arlington protoplasts, though to a low level only. For lines other than Arlington, in vitro inoculation of protoplasts initiated similar "growth curves" except that the onset of virus increase was delayed in some. Contrasting results were obtained with protoplasts isolated from leaves that had been heavily inoculated with CPMV-SB. The hypersensitivity of Chinese red x Iron seedlings then seemed to influence virus replication in the isolated protoplasts; little or no virus could be recovered from the protoplasts of the mechanically inoculated primary leaves of immune lines. Seedlings of the immune lines also were immune or highly resistant to graft inoculation. The results have implications regarding general mechanisms of immunity to CPMV-SB and indicate that immunity in this system is not an exaggerated form of hypersensitivity.