A PSK receptor was first identified in carrot (Daucus carota) as a leucine-rich repeat receptor kinase (LRR-RK) (Matsubayashi et al., 2002) . Sequence information from the carrot protein (DcPSKR1) was used to identify an ortholog in Arabidopsis, AtPSKR1 (At2g02220). described a mutant with a Ds insertion in AtPSKR1 (pskr1-1), and found that callus derived from the mutant was less sensitive to the growth-promoting effects of PSK in culture. However, they observed little difference in overall plant growth between wild-type, pskr1-1 and AtPSKR1ox, an over-expression line. The most prominent characteristic of pskr1-1 was that vegetative tissues in mature plants lost their ability to form callus. Unlike wild-type plants, leaf discs from the fully expanded leaves of pskr1-1 plants were less capable of producing callus, while unexpanded leaves retained callus-forming capacity. AtPSKR1 over-expressing plants (AtPSKR1ox) showed delayed senescence, and, as a result, leaves continued to expand, resulting in larger leaves than the wild-type.