Pollen tube NAD(P)H oxidases act as a speed control to dampen growth rate oscillations during polarized cell growth

Abstract

Reactive oxygen species (ROS) produced by NAD(P)H oxidases play a central role in plant stress responses and development. To better understand the function of NAD(P)H oxidases in plant development, we characterized the Arabidopsis thaliana NAD(P)H oxidases RBOHH and RBOHJ. Both proteins were specifically expressed in pollen and dynamically targeted to distinct and overlapping plasma membrane domains at the pollen tube tip. Functional loss of RBOHH and RBOHJ in homozygous double mutants resulted in reduced fertility. Analyses of pollen tube growth revealed remarkable differences in growth dynamics between Col-0 and rbohh-1 rbohj-2 pollen tubes. Growth rate oscillations of rbohh-1 rbohj-2 pollen tubes showed strong fluctuations in amplitude and frequency, ultimately leading to pollen tube collapse. Prior to disintegration, rbohh-1 rbohj-2 pollen tubes exhibit high-frequency growth oscillations, with significantly elevated growth rates, suggesting that an increase in the rate of cell-wall exocytosis precedes pollen tube collapse. Time-lapse imaging of exocytic dynamics revealed that NAD(P)H oxidases slow down pollen tube growth to coordinate the rate of cell expansion with the rate of exocytosis, thereby dampening the amplitude of intrinsic growth oscillations. Using the Ca(2+) reporter Yellow Cameleon3.6, we demonstrate that high-amplitude growth rate oscillations in rbohh-1 rbohj-2 pollen tubes are correlated with growth-dependent Ca(2+) bursts. Electrophysiological experiments involving double mutant pollen tubes and pharmacological treatments also showed that ROS influence K(+) homeostasis. Our results indicate that, by limiting pollen tube growth, ROS produced by NAD(P)H oxidases modulate the amplitude and frequency of pollen tube growth rate oscillations.