Abstract
During seed germination, emerging roots display positive gravitropism and penetrate into the soil for nutrition and anchorage. Tomato (Solanum lycopersicum) seeds germinated in the presence of 1-methylcyclopropene (1-MCP), an inhibitor of ethylene action, failed to insert roots into Soilrite and grew in the air, forming loops. Time-lapse video imaging showed that 1-MCP-grown root tips retained positive gravitropism and made contact with the surface of Soilrite but failed to penetrate into the Soilrite. Time-course studies revealed that the effect of 1-MCP was most prominent when seed imbibition and germination were carried out in the continual presence of 1-MCP. Conversely, 1-MCP was ineffective when applied postgermination after penetration of roots in the Soilrite. Furthermore, treatment with 1-MCP caused a reduction in DR5::β-glucuronidase auxin-reporter activity and modified the expression of SlIAA3 and SlIAA9 transcripts, indicating interference with auxin signaling. The reduced ethylene perception mutant, Never-ripe, displayed decreased ability for root penetration, and the enhanced polar auxin transport mutant, polycotyledon, showed a nearly normal root penetration in the presence of 1-MCP, which could be reversed by application of auxin transport inhibitors. Our results indicate that during tomato seed germination, a coaction between ethylene and auxin is required for root penetration into the soil.
Highlights
During seed germination, emerging roots display positive gravitropism and penetrate into the soil for nutrition and anchorage
Using the Arabidopsis MID1-COMPLEMENTING ACTIVITY1 (MCA1) transcript for sequence alignment, we identified the putative tomato MCA1 transcript (EST SGN-E 1245095 in Unigene SGN-U565830) with high homology to AtMCA1. quantitative reverse transcription (qRT)-PCR showed that the expression of the MCA1-like tomato gene was elevated in 1-MCP-treated root tips (Fig. 4A), whereas in hypocotyls, its expression was reduced by the 1-MCP treatment (Fig. 4B)
We examined in a time course whether the removal of 1-MCP would restore the level of DR5 expression in tomato root tips, along with the resumption of root penetration into the Soilrite
Summary
During seed germination, emerging roots display positive gravitropism and penetrate into the soil for nutrition and anchorage. The roots of plants perform multifaceted roles, of which two functions are foremost: to firmly anchor the plant into the soil, and to supply water and nutrients to the aerial parts of the plant (Hodge et al, 2009) To efficiently perform these functions, roots are endowed with a variety of sensory mechanisms for detecting gravity, water, nutrients, Consistent with the classic Cholodny-Went hypothesis, a large number of reports have indicated that the differential growth of roots after gravistimulation is initiated by asymmetric redistribution of the plant hormone auxin (Rashotte et al, 2001; Ottenschlager et al, 2003; Muday and Rahman, 2008; Vanneste and Friml, 2009). The molecular details regarding the modes of interaction between the auxin and ethylene pathways are still unclear, numerous studies point out that there is extensive cross talk between the hormones in the regulation of root growth
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