Abstract:

In this study，the roles of cotyledons and apical bud in adventitious root formation on hypocotyls of cucumber seedlings were analyzed. The results suggested that cotyledon is required for adventitious root formation. In the seedlings with two cotyledons，the adventitious root could form normally. During adventitious root formation，the content of endogenous IAA increased firstly and then  decreased，while the content of GA₃ showed an opposite changing pattern. Removing one of the cotyledons resulted in significantly lower levels of IAA and GA₃，reduced expression of IAA- and GA-responsive genes（eg. CsARL1，CsGH3.5，CsGASA4，etc.），and therefore led to shorter root zone and less root numbers on cucumber hypocotyls. Complete removal of two cotyledons diminished the accumulation of endogenous IAA and GA₃ in hypocotyls and reasonably diminished adventitious root formation entirely. Removal of apical bud had no significant effect on adventitious root formation. The inhibitory effects on adventitious root formation caused by removal of one cotyledon could be partially reversed by application of exogenous IAA. On the other hand，foliar spraying with IAA-transport inhibitor caused an inhibitory effect on adventitious root formation. Different from IAA，application GA₃ to the hypocotyls inhibited the adventitious root formation，while foliar spraying of GA biosynthesis inhibitor increased root formation. GA exerted an inhibition on the IAA-responsive genes during adventitious root formation. These results suggested that the cotyledon-generated IAA was essential for adventitious root formation in cucumber hypocotyls. Furthermore，application of IAA and GA₃ individually or simultaneously could not induce adventitious root formation in the hypocotyls without cotyledons attached，suggesting that IAA was not the only necessary factor. Removal of cotyledons led to a substantial decrease in the contents of soluble proteins and soluble sugars in the basal hypocotyls，and this might be another reason for the diminished adventitious root formation. Taken together，these results suggested that cotyledons，as the major resource of auxin，proteins and sugars，were necessary for adventitious root formation in cucumber hypocotyls.

Key words: cucumber, cotyledon, apical bud, hypocotyl, adventitious root, IAA