PS 65-5 - Rapid changes in root gene expression in response to nitrogen availability: Linking molecular biology, plant physiology, and soil biogeochemical processes

Thursday, August 9, 2012
Exhibit Hall, Oregon Convention Center
Timothy M. Bowles1, Philipp A. Raab2 and L.E. Jackson2, (1)Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH, (2)Land, Air and Water Resources, University of California, Davis, Davis, CA
Background/Question/Methods

Plant root nitrogen (N) uptake and assimilation systems are highly responsive to external N availability.  Expression patterns of genes involved in these systems may serve as a sensitive “plant’s eye view” of soil N availability, even in situations when N cycling is so rapid that plant/microbial N uptake and N mineralization are tightly coupled and inorganic N does not accumulate.  This would allow better understanding of N mineralization-immobilization dynamics, and thus factors that lead to pulses of N excess and periods of N deficiency in agroecosystems.  We explored the relationship between soil biogeochemical processes and dynamic plant nutrient uptake by using novel applications of molecular biology techniques coupled with conventional metrics of soil N availability at an organic farm in the Sacramento Valley, California.  Following N treatments (6.5 and 65 µg-NH4+-N g-1 soil) designed to simulate a nutrient patch, we measured changes in expression of tomato (Solanum lycopersicum L.) root genes involved in N uptake and assimilation as well labile soil N pools, bioassays for microbial N transformations, and root/shoot N concentration.  Since organic agriculture relies on microbial transformations of soil organic matter to render N available for strong plant demand, it is an excellent system to test these interactions.

Results/Conclusions

Tomato root genes responded rapidly to N additions and to the subsequent changes in soil inorganic N concentrations.  The high N treatment significantly increased soil ammonium (NH4+) and nitrate (NO3-) pools after 48 hours and significantly increased expression of an NH4+ transporter, LeAMT2, and glutamine synthetases, LeGS and LeGTS1.  These genes also trended toward higher expression levels under the low N treatment, in spite of the lack of a detectable increase in soil inorganic N for this treatment.  Root N concentration was significantly higher in both the low and high N treatments relative to the control after 120 hours.  Rapid depletion of soil NH4+ after 48 hours indicates high N demand and likely high nitrification rates, since soil NO3- levels remained elevated 120 hours following N treatments.  No differences in gene expression were observed 120 hours after the treatments.  Soil microbial biomass carbon did not differ among the treatments, suggesting that N was not limiting to microbial abundance.

The high sensitivity of these N uptake and assimilation genes to soil N cycling makes them strong candidates for diagnostic indicators of plant N availability, which could facilitate adaptive nutrient management on organic farms.