Co-regulation of root hair tip growth by ROP GTPases and nitrogen source modulated pH fluctuations

Abstract

Growth of plant cells involves tight regulation of the cytoskeleton and vesicle trafficking by processes including the action of the ROP small G proteins together with pH-modulated cell wall modifications. Yet, little is known on how these systems are coordinated. In a paper recently published in Plant Cell and Environment we show that ROPs/RACs function synergistically with NH4NO3-modulated pH fluctuations to regulate root hair growth. Root hairs expand exclusively at their apical end in a strictly polarized manner by a process known as tip growth. The highly polarized secretion at the apex is maintained by a complex network of factors including the spatial organization of the actin cytoskeleton, tip-focused ion gradients and by small G proteins. Expression of constitutively active ROP mutants disrupts polar growth, inducing the formation of swollen root hairs. Root hairs are also known to elongate in an oscillating manner, which is correlated with oscillatory H(+) fluxes at the tip. Our analysis shows that root hair elongation in wild type plants and swelling in transgenic plants expressing a constitutively active ROP11 (rop11(CA)) is sensitive to the presence of NH4(+) at concentrations higher than 1 mM and on NO3(-). The NH4(+) and NO3(-) ions did not affect the localization of ROP in the membrane but modulated pH fluctuations at the root hair tip. Actin organization and reactive oxygen species distribution were abnormal in rop11CA root hairs but were similar to wild type root hairs when seedlings were grown on medium lacking NH4(+) and / or NO3(-). These observations suggest that the nitrogen source-modulated pH fluctuations may function synergistically with ROP regulated signaling during root hair tip growth. Interestingly, under certain growth conditions, expression of rop11 (CA) suppressed ammonium toxicity, similar to auxin resistant mutants. In this Addendum article we discuss these findings and their implications.

Figure 1

A model for regulation of root hair tip growth by ROP GTPases and pH oscillations dependent on nitrogen supply. GTP bound ROPs activate downstream effectors which directly affect actin organization, vesicular trafficking and localized ROS production as well as indirectly affecting the localization of membrane proteins involved in ion/proton fluxes. High concentrations of nitrogen ions in the growth medium increase pH oscillations at the apex of growing root hairs. In turn downstream ROP effectors sense the changes in pH and adjust their function accordingly. pH oscillations affect tip growth independent of ROPs via changes of wall pH and possibly through additional unknown factors. Dashed lines indicate that these effects were not confirmed experimentally.

Figure 2

Changes in apoplastic and cytoplasmic pH fluctuations, following application of NH₄NO₃, NH₄Cl or KNO₃. (A) Apolplastic pH (pH_ex) following treatments with either NH₄NO₃, NH₄Cl or KNO₃. Note the increase pH fluctuations induced by either NH₄NO₃ and NH₄Cl but not by KNO₃. (B) Cytoplasmic pH (pH_cyt) following treatments as above. Note the changes in pH fluctuations induced by NH₄NO₃ and the subtler effects of NH₄Cl.