The Procambium Specification Gene Oshox1 Promotes Polar Auxin Transport Capacity and Reduces Its Sensitivity toward Inhibition

Abstract

The auxin-inducible homeobox gene Oshox1 of rice (Oryza sativa) is a positive regulator of procambial cell fate commitment, and its overexpression reduces the sensitivity of polar auxin transport (PAT) to the PAT inhibitor 1-N-naphthylphthalamic acid (NPA). Here, we show that wild-type rice leaves formed under conditions of PAT inhibition display vein hypertrophy, reduced distance between longitudinal veins, and increased distance between transverse veins, providing experimental evidence for a role of PAT in vascular patterning in a monocot species. Furthermore, we show that Oshox1 overexpression confers insensitivity to these PAT inhibitor-induced vascular-patterning defects. Finally, we show that in the absence of any overt phenotypical change, Oshox1 overexpression specifically reduces the affinity of the NPA-binding protein toward NPA and enhances PAT and its sensitivity toward auxin. These results are consistent with the hypothesis that Oshox1 promotes fate commitment of procambial cells by increasing their auxin conductivity properties and stabilizing this state against modulations of PAT by an endogenous NPA-like molecule.During development of multicellular organisms, most cells enter specific differentiation programs that overtly change their structure and specialize them toward a distinct function. Cell differentiation is preceded by the process of cell fate commitment, in which the developmental potential of the cell becomes restricted to a certain fate or subset of fates, but the explicit demonstration and realization of that developmental pathway is not yet apparent (Maclean and Hall, 1987). Cell fate commitment has been divided into the processes of specification and determination (Slack, 1991). However, the definitions of these processes are, by nature, operational, and knowledge of the molecular nature underlying the effective functional properties of specified or determined cell states in plants is still mostly lacking.The vascular tissues of plants represent an attractive system for the study of cell fate commitment. In fact, vascular tissues consist of several distinct cell types arranged in a network of continuous strands to form a system of exquisitely complex topography (Steeves and Sussex, 1989). Yet, all types of highly specialized vascular cells derive from an anatomically homogeneous population of meristematic cells, the procambium, or provascular tissue. We have previously developed assays with which different stages of procambial cell fate commitment in rice (Oryza sativa) could be distinguished (Scarpella et al., 2000). However, despite the detailed studies available on different aspects of vascular development (for review, see Sachs, 1981Sachs, , 2000Fukuda, 1996Fukuda, , 1997Nelson and Dengler, 1997; Berleth et al., 2000; Aloni, 2001; Dengler, 2001; Dengler and Kang, 2001;Kuriyama and Fukuda, 2001;Ye, 2002), the molecular mechanisms underlying procambial cell fate commitment remain elusive (e.g. Savidge, 2001). Nevertheless, it is possible that the plant hormone auxin may be involv...

TheRADICLELESS1gene is required for vascular pattern formation in rice

“…Alternatively, primary defects in auxin perception or response could give rise to the vascular defects of ral1. Treatments of wild-type rice leaves with increasing concentrations of polar auxin transport inhibitors increase the distance between longitudinal veins and decrease that between commissural veins (Scarpella et al, 2002). Therefore, ectopic accumulation of auxin near source regions in the wild-type rice leaf results in vascular pattern alterations opposite to those induced by the ral1 mutation.…”

Grain Number,Plant Height,and Heading Date7Is a Central Regulator of Growth, Development, and Stress Response

“…The expression of an auxin-inducible gene Oshox1, which regulates the sensitivity of polar auxin transport (Scarpella et al, 2002), increased in OXGhd7 HJ19 plants (Fig. 7C).…”

A Homeodomain Leucine Zipper Gene from Craterostigma plantagineum Regulates Abscisic Acid Responsive Gene Expression and Physiological Responses

“…This is particularly important given that it is likely that ectopic expression of CpHB-7 could titrate out complexes of endogenous Arabidopsis HDZips that are involved in ABA responses. HDZip functional analysis must therefore be interpreted with caution, nevertheless the Pro 35S :HDZip strategy has allowed insight into the function of ATHB6 (Himmelbach et al, 2002), ATHB-8 (Baima et al, 2001), ATHB13 (Hanson et al, 2001) and Oshox1 (Scarpella et al, 2002).…”