XVI International Botanical Congess

Expression patterns of presumed orthologs, maize zmrs1, and rice osh15, suggest a role in marking shoot internodes for these homeobox genes (Jackson et al. 1994, Sato et al. 1998). In a rice mutant of osh15 cortical sclerenchyma and vasculature is affected, and internodes shortened due to reduced cell numbers and height, effects suggested to follow from functions related to either intercalary meristem activity or switching on differentiation into sclerenchymatous cells in the hypodermis (Sato et al. 1999). Considering histological aspects of shoot development in rice, maize, and monocots (Kaufman 1959, Sharman 1942, Tomlinson 1995) I suggest an alternative explanation that accounts for the range of effects observed in the mutant. This hypothesis predicts additional effects that might be observed in the rice mutant and explains certain developmental differences in rice and maize.