Recalcitrance of woody and herbaceous perennial plants: Dealing with genetic predeterminism

As a general group, long-lived perennial plants probably present the most challenging obstacles to the researcher, breeder or propagator utilizing microculture as a tool. These challenges appear during all stages of the microculture process, but are probably most resplendent during the stabilization phase. What may be particularly frustrating is that much of this 'recalcitrance' is genetically driven and is thus difficult to control by environmental and nutritional manipulations in mitroculture. Perennials have complex seasonal cycles and life cycles, which complicate control of their growth in microculture. As shoot cultures have provided useful tools for overcoming these limitations, the inability to establish stabilized shoot cultures is a major form of recalcitrance. Plants having seasonal growth dynamics dominated by strong episodic or determinant shoot growth are some of the most recalcitrant species because stabilized shoot cultures cannot be readily generated. In some cases, episodic growth may be tied closely to phase state and can thus be controlled by manipulating phase; nevertheless, adequate controls have not been identified for many problematic plants. Another trait contributing to recalcitrance of perennials is the relatively slow growth rate in microculture. Slow growth complicates such procedures as selection of transformed tissues. The high phenolic content of many perennial tissues can interfere with the efficacy of transgenic traits such as beta-glucuronidase. Developmentally determined growth characteristics such as plagiotropism may persist through all stages of microculture and complicate the recovery of commercially useful micropropagules. Although some technical approaches can occasionally circumvent immediate microculture limitations. general solutions await the development of a deeper understanding of physiological bases of such genetically predetermined phenomena.