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 Environment1 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 (rop11CA) 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 rop11CA suppressed ammonium toxicity, similar to auxin resistant mutants. In this Addendum article we discuss these findings and their implications.