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

As a general group, long-lived perennial plants probably present the most c hallenging obstacles to the researcher, breeder or propagator utilizing mic roculture as a tool. These challenges appear during all stages of the micro culture process, but are probably most resplendent during the stabilization phase. What may be particularly frustrating is that much of this 'recalcit rance' is genetically driven and is thus difficult to control by environmen tal and nutritional manipulations in mitroculture. Perennials have complex seasonal cycles and life cycles, which complicate control of their growth i n 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 domi nated by strong episodic or determinant shoot growth are some of the most r ecalcitrant species because stabilized shoot cultures cannot be readily gen erated. In some cases, episodic growth may be tied closely to phase state a nd can thus be controlled by manipulating phase; nevertheless, adequate con trols have not been identified for many problematic plants. Another trait c ontributing to recalcitrance of perennials is the relatively slow growth ra te in microculture. Slow growth complicates such procedures as selection of transformed tissues. The high phenolic content of many perennial tissues c an interfere with the efficacy of transgenic traits such as beta-glucuronid ase. Developmentally determined growth characteristics such as plagiotropis m may persist through all stages of microculture and complicate the recover y of commercially useful micropropagules. Although some technical approache s can occasionally circumvent immediate microculture limitations. general s olutions await the development of a deeper understanding of physiological b ases of such genetically predetermined phenomena.