Title

Interaction of the vacuolar H+-pyrophosphatase AVP1 with the secondary transporters AtNHX1 and SOS1 in Arabidopsis thaliana: Implications in salt tolerance and mineral nutrition

Date of Completion

January 2006

Keywords

Biology, Plant Physiology

Degree

Ph.D.

Abstract

Feeding humanity is a challenge for this new millennium. The difficulties are further exacerbated by the loss of agricultural land to salinization and desertification. ^ Transgenic plants overexpressing AVP1 (AVP1 OX), encoding a type-I H+-pumping pyrophosphatase, are more salt- and drought- tolerant than their control counterparts. They accumulate more Na+ in their leaves and have a higher PPi-dependent Cat2+ uptake than controls. Transgenic plants overexpressing AtNHX1, encoding a tonoplast Na+(K+)/H + antiporter, are also salt tolerant. They accumulate more Na + and have an enhanced tonoplast Na+/H+ transport activity. In order to establish if an increased proton electrochemical gradient coupled with a greater Na+(K+)/H + exchange activity in the tonoplast would further improve salt tolerance, I generated AVP1OX/AtNHX1OX double transgenic lines. AVP1/AtNHX1 overexpressing plants are more salt tolerant than the individual transgenic lines. These results are consistent with an additive effect of AVP1 and AtNHX1. Double transgenics also display novel phenotypes, with boosted root hair development and enhanced hypocotyl elongation, suggesting an improvement in K+ homeostasis. ^ The Arabidopsis loss-of-function mutants sos1-1, sos2-2, and sos3-1 are sensitive to low concentrations of Na+ and grow poorly under low potassium conditions. I generated Arabidopsis plants that simultaneously have each one of these loss-of-function mutations and also overexpress AVP1. AVP1OX/ sos1-1, AVP1OX/sos2-2 and AVP1OX/sos3-1 lines are as salt sensitive as the original sos mutants. These results are consistent with an epistatic effect of sos mutations over AVP1 overexpression regarding salt-tolerance. Interestingly, all plants homozygous for the sos1-1 allele, encoding a plasma membrane Na +(K+)/H+ antiporter, showed reduction in shoot biomass and inhibition of root hair growth when grown under either low potassium or low iron concentrations. AVP1OX/ sos1-1 plants also showed a reduction in PM H+-ATPase activity with a concomitant reduction in their rhizosphere's acidification capacity. The enhanced sensitivity of AVP1OX plants to exogenous auxin was suppressed in AVP1OX/sos1-1 plants. Additionally, all plants homozygous for the sos1-1 allele showed root swelling in the presence of Fusicoccin; this response was delayed under low potassium conditions. These phenotypes suggest that sos1-1 has an influence on nutrient uptake, very likely, by controlling the PM H+-ATPase under conditions where the induction of this pump is required.^