| The objective of this work was to examine the role of auxin (IAA) and gibberellins (GA) in the control of root growth. Genotypes of pea which differ in stem height were analyzed. Analysis involved measurement of root growth, endogenous auxin (IAA) levels, and response to hormone treatment. There was no difference between the root growth and IAA levels of wild-type and le dwarf plants, which have a moderate reduction in gibberellin (GA) biosynthesis. There was also no difference between the root growth and IAA levels of slender (la cry-s) and brassinosteroid deficient mutants (lka, lkb) as compared to wild-type. Root length and IAA levels of severe dwarf, GA-deficient nana (na) plants were decreased, as compared to wild-type and le dwarf plants. Root length and IAA levels of moderately severe dwarf lines ls and lh also had a reduction in root length and IAA level, though not as severe as roots of nana plants. There is thus a good correlation between the extent of root elongation and IAA level in the root tips.; Three day-old seedlings of all lines showed no response to applied GA. However, 5 day-old nana seedlings did show increased root elongation rates in response to applied GA, while 5 day old tall roots did not respond. Despite the varying endogenous IAA levels (and the corresponding root elongation) in the genotypes, all lines responded similarly to applied IAA; root elongation was rapidly inhibited following treatment. GA applications to shoots of nana plants caused a significant increase in the endogenous IAA level in the roots, which corresponded with increased root elongation. Similar responses to hormone treatment were observed when excised root tips of the various genotypes were grown in sterile culture. These results suggest that IAA can either inhibit or promote growth, depending on its location in the root. IAA from exogenous treatments may act at a region of the root which inhibits growth, while endogenous IAA acts to promote growth. |
