Potassium Deficiency Inhibits Lateral Root Development In Tobacco Seedlings By Changing Auxin Distribution

Potassium(K)is a fundamental macronutrient required for plant growth and development.The K concentration in soil solution is generally within the range of 0.025?5 mM,while in the rhizosphere,its concentration is usually less than 0.3 mM(Schroeder et al.1994).Even in fertilized fields,K availability can be affected by environmental conditions such as drought and soil density(Kuchenbuch et al.1986;Liebersbach et al.2004).Therefore,plants may frequently experience K deficiency(LK)during their life cycle.Plants exhibit various mechanisms to adapt to different nutrient supply conditions.Among these mechanisms,the plasticity of root development is vital.Plant root systems continuously branch and form lateral roots(LRs).These LRs develop from founder cells in the pericycle,the outermost layer of the vascular cylinder(stele)of the root.Auxin plays a dominant role in LR initiation and the later stages of LR development.Tobacco is an economically important crop worldwide.Little is known of the mechanism by which LR growth responds to LK or the extent to which this response is linked to auxin.In this study,in order to explore the mechanism of auxin involved the K deficiency on root development of tobacco seedlings,we analyzed root morphology,auxin distribution in plants and PIN genes expressions in two K treatments(2 and 0.01 mM).We also examined the effects of the auxin polar transport inhibitor N-1-naphthylphthalamic acid(INPA)and exogenously applied auxin(NAA)on LR formation and elongation in tobacco seedlings.The results are listed as follows:1.Compared with normal nutrient(2 mM),K deficiency decreased shoot and root dry weight by more than 50%in tobacco seedlings.Further results showed that repressed root growth by K deficiency mainly resulted from decreased formation and elongation in first-order LRs rather than in second-order LRs comparison with normal nutrient treatment.Time-course experiments showed that first-order LR formation and elongation significantly increased slower in response to K deficiency since 7 d and ultimately resulted in?25%less than that in normal nutrition.2.To assess whether auxin participated formation and elongation of first-order LR modulated by K deficiency,endogenous IAA concentration was analyzed in the leaves and roots of tobacco seedlings.IAA concentration was decreased by 34%,32%and 43%,respectively,in first and second leaves and roots in response to K deficiency relative to control treatment.Expression of DR5::GUS under K-deficiency was less in the newly-grown leaf,junction and LR tip(RT),in comparison with normal nutrition.These results suggested that K deficiency attenuated auxin level in the leaves and roots relative to normal nutrition.To examine the effect of K deficiency on auxin polar transport from the aerial part to the root,we measured auxin transport using radiolabelled IAA.Potassium deficiency reduced[3H]IAA polar transport to 50%from the junction to the LR subjected to K-deficient supply compared with control treatment.Compared with control treatment,qRT-PCR showed that the expression levels of PIN1,PIN1b,PIN4 and PIN9 in the root were reduced by K deficiency.Based on the above data,PIN1,PIN1b,PIN4 and PIN9 might play the pivotal role in down-regulated auxin transport from the shoot to root by K deficiency.3.We examined first-order LR formation and elongation responses to the application of(i)the auxin polar transport inhibitor NPA and(ii)exogenous NAA to determine whether decreased auxin level(regulated by K deficiency)was responsible for belowground changes in tobacco root morphology.Application of 60 nM NPA under normal nutrition decreased first-order LR formation and elongation and the expression level of DR5::GUS in the junction and root tip to the similar level as that under K-deficient treatment.Conversely,Application of 20 nM NAA counteracted the effects of K deficiency on first-order LR formation and elongation and the expression level of DR5::GUS in the junction and root tip of tobacco seedlings.In conclusion,K deficiency inhibited LR formation and elongation in tobacco plants via mediating auxin distribution.