SPIKE1 Activates ROP GTPase to Modulate Petal Growth and Shape

Abstract

Plant organ growth and final shape rely on cell proliferation and, particularly, on cell expansion that largely determines the visible growth of plant organs. Arabidopsis (Arabidopsis thaliana) petals serve as an excellent model for dissecting the coordinated regulation of patterns of cell expansion and organ growth, but the molecular signaling mechanisms underlying this regulation remain largely unknown. Here, we demonstrate that during the late petal development stages, SPIKE1 (SPK1), encoding a guanine nucleotide exchange factor, activates Rho of Plants (ROP) GTPase proteins (ROP2, ROP4, and ROP6) to affect anisotropic expansion of epidermal cells in both petal blades and claws, thereby affecting anisotropic growth of the petal and the final characteristic organ shape. The petals of SPK1 knockdown mutants were significantly longer but narrower than those of the wild type, associated with increased anisotropic expansion of epidermal cells at late development stages. In addition, ROP2, ROP4, and ROP6 are activated by SPK1 to promote the isotropic organization of cortical microtubule arrays and thus inhibit anisotropic growth in the petal. Both knockdown of SPK1 and multiple rop mutants caused highly ordered cortical microtubule arrays that were transversely oriented relative to the axis of cell elongation after development stage 11. Taken together, our results suggest a SPK1-ROP-dependent signaling module that influences anisotropic growth in the petal and defines the final organ shape.

ROP GTPases Structure-Function and Signaling Pathways

“…The interdigitation of the cells is compromised in the spk1 mutant and in the rop2 rop6 rop4RNAi mutant, and cells display transverse MT arrays. Furthermore, the levels of active ROP2 and ROP6 are lower in spk1 than in wild-type plants (Ren et al, 2016). Taken together, these results indicated that SPK1 is responsible for activating ROP2, ROP6, and likely ROP4, which together regulate the growth of petal epidermal cells.…”

GTP ase ROP 2 binds and promotes activation of target of rapamycin, TOR , in response to auxin

“…To show directly that ROP2 induces TOR signaling activation in response to auxin treatment, we depleted ROPs 2, 4, and 6 ( Fig 3A and B; Ren et al, 2016). First, we found that knockout of only ROP2 substantially reduces the level of ROPs immunoprecipitated by TOR to levels similar to that observed in rop2 rop6 and rop2 rop6 ROP4 RNAi plants, strongly suggesting that ROP2 plays a pivotal role in TOR association ( Fig 3C).…”

Spatio-temporal orientation of microtubules controls conical cell shape in Arabidopsis thaliana petals

“…We therefore investigated the role of SPK1 during conical cell development by analyzing the spk1-4 mutant, which was predicted to have aberrant messenger RNA splicing [50]. Our results showed that the spk1-4 mutant also displayed swollen apexes of conical cells (Fig 3), as observed in the rop multiple mutants.…”