Adventitious Root Growth and Cell-Cycle Induction in Deepwater Rice1

Abstract

Deepwater rice (Oryza sativa) is adapted to survive conditions of severe flooding over extended periods of time. During such periods adventitious roots develop to provide water, nutrients, and anchorage. In the present study the growth of adventitious roots was induced by treatment with ethylene but not auxin, cytokinin, or gibberellin. Root elongation was enhanced between 8 and 10 h after submergence. The population of cells in the S phase and expression of the S-phase-specific histone H3 gene increased within 4 to 6 h. Within 6 to 8 h the G2-phase population increased. Cell-cycle activation was accompanied by sequential induction of a cdc2-activating kinase homolog, R2, of two cdc2 genes, cdc2Os-1 and cdc2Os-2, and of three cyclin genes, cycA1;3, cycB2;1, and cycB2;2, but only induction of the R2 gene expression preceded the induction of the S phase, possibly contributing to cell-cycle regulation in the G1 phase. Both cdc2 genes were expressed at slightly higher levels during DNA replication. Transcripts of the A-type cyclin accumulated during the S and G2 phases, and transcripts of the B-type cyclins accumulated during the G2 phase. Cyclin expression was induced at all nodes with a similar time course, suggesting that ethylene acts systemically and that root primordia respond to ethylene at an early developmental stage.

Acclimation to soil flooding — sensing and signal-transduction

“…It is conceivable that these treatments may have evoked some extra ethylene production. Earlier, Suge (1985) found in the same species a synergistic effect of gibberellin, when applied together with ethylene, but this could not be reproduced in a later study (Lorbiecke and Sauter 1999). It cannot be ruled out that other hormones do play a role in other species, particularly when these do not constitutively produce adventitious root primordia, such as rice does, but there is as yet no information available.…”

“…Whether this also applies to monocotyledonous species is not clear, since experimental evidence is scarce. Lorbiecke and Sauter (1999) did not find an effect of applied auxin on adventitious root formation in rice (except via ethylene, see below), whereas Zhou et al (2003) claimed that endogenous auxin is critical for adventitious root development in the same species.…”

“…Application of cytokinin and gibberellin to deepwater rice did induce some roots, but not nearly as many as did ethylene (Lorbiecke and Sauter, 1999). It is conceivable that these treatments may have evoked some extra ethylene production.…”

“…In deepwater rice, Lorbiecke and Sauter (1999) distinguish four stages of adventitious root development: (i) an initiation phase, followed by (ii) development into a root primordium, and then subsequent (iii) arrest of growth until an appropriate stimulus causes (iv) further development and emergence of the root from the stem epidermis. The first three stages seem part of the constitutive development course of the rice stem nodes.…”

Strigolactones fine-tune the root system

“…Adventitious roots forming from Arabidopsis hypocotyls initiate from a cell layer proximate to the central cylinder and in this regard show similarity to the lateral roots that emerge from pericycle founder cells (Casimiro et al 2001). In many important crop species (including rice, maize and wheat), adventitious roots are formed via vascular cambial cell divisions whereby phloem initials start to divide to produce a primordium (Naija et al 2009) callus in which tracheids differentiate, elongate and eventually form the center of a complete root primordium (Bollmark et al 1988;Lorbiecke and Sauter 1999;Rasmussen and Hunt 2010).…”