The Role of the Distal Elongation Zone in the Response of Maize Roots to Auxin and Gravity

Abstract

between the meristem and the point at which rapid elongation is initiated as a special zone in which growth is nearly isodiametric. They referred to this region as the postmitotic "isodiametric" growth zone, The cells that show rapid graviinduced enhancement of elongation rate (Ishikawa et 1991) include cells in the postmitotic isodiametric growth zone (compare Baluska et al., 1990, andIshikawa et al., 1991).There is evidence that cells between the meristem and the CEZ also play a special role in the ability of roots to adapt to inhibitory levels of auxin. Burstrom (1957) reported that the elongation of roots of wheat seedlings was initially inhibited by IAA but SOOn recovered even in the presente of the hormone. The resumption of elongation resulted from initiation of rapid elongation in cells near the meristem including the apical region of the elongation zone. Cells that were in the main 'One at the time Of auxin treatment remained inhibited. Similar data were obtained by Hejnowicz (1961) using wheat roots and bY Goodwin (1972) using roots Of PhkUm.In this study, we examined the auxin response of cells throughout the apical region of the root. Because IAA redistribution is thought to mediate gravitropic responses, we compared the elongation rate distribution pattern in auxintreated roots with that of gravistimulated roots. In view of the suggestion that adaptation to localized changes in auxin leve1 contributes to the kinetics of gravitropism (Evans, 1991), we also examined the gravitropic response of roots exposed to IAA before stimulation. We discuss the surprising finding that roots whose elongation rate is reduced to zero by auxin treatment still exhibit a rapid and strong gravitropic response upon gravistimulation. Preliminary results of these studies were reported by Evans et al. (1990) and Ishikawa and Evans (1992).We used a video digitizer system to (a) measure changes i n the pattern of longitudinal surface extension in primary roots of maize (Zea mays 1.) upon appkcation and withdrawal of auxin and (b) compare these patterns during gravitropism in control roots and roots pretreated with auxin. Special attention was paid to the distal elongation zone (DEZ), arbitrarily defined as the region between the meristem and the point within the elongation zone at which the rate of elongation reaches 0.3 of the peak rate. For roots i n aqueous solution, the basal limit of the DEZ is about 2.5 mm behind the tip of the root cap. Auxin suppressed elongation throughout the elongation zone, but, after 1 to 3 h, elongation resumed, primarily as a result of induction of rapid elongation i n the DEZ.Withdrawal of auxin during the period of strong inhibition resulted i n exceptionally rapid elongation attributable to the initiation of rapid elongation in the DEZ plus recovery i n the main elongation In a study of surface extension patterns in maize roots (Ishikawa et al., 1991), we reported that the pattern of differential elongation responsible for gravitropic curvature is complex. During rapid downward curvature, ...

Specialized Zones of Development in Roots: View from the Cellular Level

“…The length of this region thus represents about 5.5% of the entire growth zone. Subsequent studies confirmed the uniqueness of the cells located in the PIG zone, not only from a morphometrical point of view, but also on account of their specific cytological and physiological properties (Barlow et al, 1991;BaluSka et al, l992,1994BaluSka et al, l992, ,1996aBaluSka et al, l992, , 1996cIshikawa and Evans, 1992, 1993, 1995.…”

Impacts of Aluminum on the Cytoskeleton of the Maize Root Apex. Short-Term Effects on the Distal Part of the Transition Zone1

“…Most of the observations were of cells of the epidermis and of the two to three outermost cortex cell files (outer cortex) in the DTZ and in the middle part of the TZ, where we viewed major changes to the cytoskeleton (for additional details on zone specifications, see Sivaguru and Horst, 1998). However, the root-zone specifications of the nutrient solutiongrown seedlings may vary from those of the plants grown in humid air (Ishikawa and Evans, 1993). Hence, we cannot exclude overlapping from the DTZ to the proximal part of the meristematic zone.…”

NO signaling is a key component of the root growth response to nitrate inZea maysL.

“…42 Actually, since 1993 it has been evidenced that cells belonging to this zone are strongly auxin-responsive and accomplish dramatic rearrangements of the cytoskeleton, being subjected to a series of fundamental changes in their cytoarchitecture. 50 A recent study conducted on maize demonstrated that the transition zone plays central roles in both sensing and adapting to root hypoxia. 51 The authors also observed that the oxygen deprivation of roots induces local NO emission in the TZ, that is essential for the successful acclimation of the entire maize root to oxygen deprivation.…”