The Decrease in Auxin Polar Transport Down the Lupin Hypocotyl Could Produce the Indole-3-Acetic Acid Distribution Responsible for the Elongation Growth Pattern

Abstract

The variation of indole-3-acetic acid (IAA) transport along Lupinus albus L. hypocotyls was studied using decapitated seedlings and excised sections. To confirm that the mobile species was IAA and not IAA metabolites, dual isotope-labeled IAAs, [5-3H]IAA + [1-'4CJIAA, were used. After apical application to decapitated seedlings, the longitudinal distribution of both isotopes at different transport periods showed that the velocity of IAA transport was higher in the apical, elongating region than in the basal, nongrowing region. This variation in velocity was not a traumatic consequence of decapitation because after application of IAA to the basal region of decapitated seedlings, both the velocity and intensity of IAA transport were lower than in the apical treatment. The variation in IAA transport down the hypocotyl was confirmed when it was measured in excised sections located at different positions along the hypocotyl. The velocity and, to a greater extent, the intensity of IAA transport decreased from the apical to the basal sections. Consequently, if the amount of IAA reaching the apical zones of lupin hypocotyl were higher than the IAA transport capacity in the basal zones, accumulation of mobile IAA might be expected in zones located above the basal region. In fact, an IAA accumulation occurred in the elongating region during the first 4-h period of transport after apical treatment with IAA. It is proposed that the fall in IAA transport along the hypocotyl might be responsible for the IAA distribution and, consequently, for the growth distribution reported in this organ. An indirect proof of this was obtained from experiments that showed that the excision of the slowly transporting basal zones strongly reduced the growth in the remaining part of the organ, whereas excision of the root caused no significant modification in growth during a 20-h period.The frequently described correlations between the longitudinal distribution of IAA and growth in stem axes suggest that in some tissues, cell elongation is regulated mainly by the endogenous levels of auxin (15,18,22,26,28,29). In plant systems, such as coleoptiles or hypocotyls, the apex is the main source of auxin. Therefore, it can be suspected that basipetal polar auxin transport might be involved in supplying auxin for growth in these systems. Auxin transport has been intensively investigated for decades, and several hypotheses have been formulated to account for the characteristics of the process (see reviews by Goldsmith [2], Kaldewey [7], Rubery [17]). However, the actual mechanism responsible for IAA distribution, which presumably causes the growth distribution, remains uncertain.Contrary to those reported for sunflower hypocotyls (1), some data suggest that mobile auxin is involved in the regulation of elongation growth in etiolated lupin hypocotyls. Thus, [5-3H]IAA-fed decapitated seedlings showed a wave of unaltered LAA in the elongation zone that correlated with the distribution of growth (12), as well as with the distribution of endogenous ...

The contrasting role of auxin in submergence‐induced petiole elongation in two species from frequently flooded wetlands

“…If auxin is involved in this growth pattern, two explana tions seem possible: ( 1) a classical longitudinal gradient in auxin content exists (high in tip, low at basal parts, Sanchez-Bravo et al 1993) with an opposing gradient in responsiveness (low at tip, high at basal part, as men tioned in Hoson et al 1992) resulting in a similar growth rate along the petiole, or (2 ) both auxin content and tis sue responsiveness are evenly distributed over the peti ole. Upon submergence the growth pattern did not change for Rumex palustris, unlike in Ranunculus scel eratus, where segmental growth declined along the api cal-basal gradient.…”

An anionic class III peroxidase from zucchini may regulate hypocotyl elongation through its auxin oxidase activity

“…The phytohormone auxin is involved in diverse developmental processes, many of which depend on regulated auxin gradients (Went and Thimann 1937). For example, a gradient in auxin concentration from tip to base is believed to be responsible for diVerential elongation rates in diVerent regions of shoots (Sánchez-Bravo et al 1992). Strict control of the endogenous IAA concentration is therefore of great importance.…”

Superroot, a recessive mutation in Arabidopsis, confers auxin overproduction.

“…Suttle (1991) has demonstrated for sunflower hypocotyls that this decrease in auxin polar transport is due to a loss of function of the IAA efflux carrier with increased age. As discussed by Kaldeway (1984) and Sánchez-Bravo et al (1992), the existence of a decrease in basipetal IAA transport might produce the so-called barrier effect. This would imply that if the amount of IAA to be transported from the apical part of the hypocotyl to its base were higher than the capacity to transport IAA, IAA would accumulate in the lower regions of the hypocotyl, where it would exert its effects.…”

“…Sánchez-Bravo et al (1992) showed that the auxin polar transport in basal zones of the lupin hypocotyl is slower and that these zones receive lower amounts of transported IAA than the apical, elongating regions. Suttle (1991) has demonstrated for sunflower hypocotyls that this decrease in auxin polar transport is due to a loss of function of the IAA efflux carrier with increased age.…”