Effect of different oxygen and Na+ levels on root radial anatomy in two Italian rice genotypes

The very peculiar radial anatomy of rice roots reflects the plant’s adaptation to semiaquatic growing conditions. The formation of aerenchyma, providing aeration of the root tissues by gas exchanges with the shoot, is constitutive in rice, although the extent and the precocity of its development may be affected by oxygen availability in the growth medium. The function of aerenchyma is implemented by strong barriers in the root peripheral layers which prevent radial oxygen loss. These barriers are the lignified sclerenchyma cell layer and, externally to this, the exodermis, which is reinforced, following two developmental stages, by the formation of Casparian bands (CBs) in radial and transverse cell walls and, later on, by the deposition of suberin lamellae to the inner surface of the anticlinal and tangential cell walls. These peripheral apoplastic barriers, as well as CBs and suberin lamellae deposited in the endodermis, confer selectivity in solute uptake, by impeding the apoplastic flow of water and solutes to the stele bypassing the trans-membrane transport. Since the apoplastic flow has been recognized as the main pathway by which Na+ enters rice shoot, the aptitude to modulate root porosity and development of apoplastic barriers in response to oxygen availability and to salt stress are expected to be crucial in conferring salt stress tolerance. Aim of the present work is to analyse, in rice, the effect of the presence of oxygen and/or NaCl on the formation of aerenchyma and on the development of both CBs and suberin lamellae in exodermis and endodermis. With this purpose, plants of Baldo and Vialone Nano, two Italian cultivars, were grown in hydroponic culture under a long-term (30 days) mildsalinity treatment (25 mM NaCl). These experiments were conducted in both aerobic and hypoxic conditions, miming the stagnant growing medium through the addition of a final concentration of 0.1% agar to the nutrient solution. A histological analysis was performed on adventitious roots by cutting cross sections at different distances from the tip. The development of aerenchyma along the root was detected by light microscopy observation under bright field; staining root sections with fluorochromes and dyes for suberin and lignin allowed to detect the deposition of CBs and suberin lamellae as well as the lignification of the sclerenchyma layer. The results highlighted an effect of the presence of oxygen and/or NaCl in the growth medium on the histological development of rice root apoplastic barriers in terms of both precocity and intensity.