Dynamics of Aerenchyma distribution in the cortex of sulfate-deprived adventitious roots of maize

BACKGROUND AND AIMS: Aerenchyma formation in maize adventitious roots is induced in nutrient solution by the deprivation of sulfate (S) under well-oxygenated conditions. The aim of this research was to examine the extent of aerenchyma formation in the cortex of sulfate-deprived adventitious roots along the root axis, in correlation with the presence of reactive oxygen species (ROS), calcium levels and pH of cortex cells and root lignification. METHODS: The morphometry of the second whorl of adventitious (W2) roots, subject to S-deprivation conditions throughout development, was recorded in terms of root length and lateral root length and distribution. W2 roots divided into sectors according to the mean length of lateral roots, and cross-sections of each were examined for aerenchyma. In-situ detection of alterations in ROS presence, calcium levels and pH were performed by means of fluorescence microscopy using H(2)DCF-DA, fluo-3AM and BCECF, respectively. Lignification was detected using the Wiesner test. KEY RESULTS: S-deprivation reduced shoot growth and enhanced root proliferation. Aerenchyma was found in the cortex of 77 % of the root length, particularly in the region of emerging or developing lateral roots. The basal and apical sectors had no aerenchyma and no aerenchyma connection was found with the shoot. S-deprivation resulted in alterations of ROS, calcium levels and pH in aerenchymatous sectors compared with the basal non-aerenchymatous region. Lignified epidermal layers were located at the basal and the proximal sectors. S-deprivation resulted in shorter lateral roots in the upper sectors and in a limited extension of the lignified layers towards the next lateral root carrying sector. CONCLUSIONS: Lateral root proliferation is accompanied by spatially localized induced cell death in the cortex of developing young maize adventitious roots during S-deprivation.     

Comparative spatiotemporal analysis of root aerenchyma formation processes in maize due to sulphate, nitrate or phosphate deprivation

Nitrate (N), phosphate (P) or sulphate (S) deprivation causes aerenchyma formation in maize (Zea mays L.) nodal roots. The exact mechanisms that trigger the formation of aerenchyma under these circumstances are unclear. We have compared aerenchyma distribution across the nodal roots of first whorl (just emerging in 10-day-old seedlings), which were subject to S, N or P deprivation over a period of 10?days in connection with oxygen consumption, ATP concentration, cellulase and polygalacturonase activity in the whole root. The effect of deprivation on aerenchyma formation was examined using light and electron microscopy, along with in situ detection of calcium and of reactive oxygen species (ROS) by fluorescence microscopy. Aerenchyma was not found in the root base regardless of the deprivation. Programmed cell death (PCD) was observed near the root tip, either within the first two days (-N) or a few days later (-S, -P) of the treatment. Roots at day?6 under all three nutrient-deprived conditions showed signs of PCD 1?cm behind the cap, whereas only N-deprived root cells 0.5?cm behind the cap showed severe ultrastructural alterations, due to advanced PCD. The lower ATP concentration and the higher oxygen consumptions observed at day?2 in N-, P- and S-deprived roots compared to the control indicated that PCD may be triggered by perturbations in energy status of the root. The peaks of cellulase activity located between days?3 (-N) and 6 (-P), along with the respective alterations in polygalacturonase activity, indicated a coordination which preceded aerenchyma formation. ROS and calcium seemed to contribute to PCD initiation, with ROS possessing dual roles as signals and eliminators. All the examined parameters presented both common features and characteristic variations among the deprivations.     

Aerenchyma formation in roots of maize during sulphate starvation

Young maize ( Zea mays L., Poaceae) plants were grown in a complete, well-oxygenated nutrient solution and then deprived of their external source of sulphate. This treatment induced the formation of aerenchyma in roots. In addition to the effect of sulphate starvation on root anatomy, the presence and location of superoxide anions and hydrogen peroxide, and changes in calcium and pH were examined. By day 6 of sulphate deprivation, aerenchyma started to form in the roots of plants and the first aerenchymatous spaces were apparent in the middle of the cortex. S-starvation also induced thickening of the cell walls of the endodermis. Active oxygen species appeared in groups of intact mid-cortex cells. Formation of superoxide anion and hydrogen peroxide was found in degenerating cells of the mid-cortex. Very few nuclei in the cortex of S-starved roots fluoresced, being shrunken and near to the cell wall. By day 12 of S-deprivation, a fully developed aerenchyma was apparent and there were only a few 'chains' of cells bridging hypodermis to endodermis and stele of roots. Cell walls of endodermis of S-starved roots increased 68% in thickness. Intensive fluorescence in the cell walls of the endodermal, hypodermal and to a lesser extent of epidermal cells was observed due to the formation of active oxygen species, while there was no fluorescence in the cortical cells. There was a higher Ca concentration in the cells walls of the endodermis and epidermis, compared to the rest of the S-starved root tissues. A higher pH was observed, mainly in the cell walls of the hypodermis and to a lesser extent in the cell walls of the endodermis. Superoxide anion and hydrogen peroxide was found in degenerating cells of the root cortex. There was no fluorescence of nuclei in the cortex of S-starved roots.