Cell expansion and microtubule behavior in ray floret petals of Gerbera hybrida: Responses to light and gibberellic acid

Abstract

It is still unclear how light and gibberellins are integrated to regulate petal size. Here, we report that light improves both the length and the width of the ray floret petals in G. hybrid, but GA₃ promotes only the petal length. It is also revealed that the control of the petal size by light and GA₃ depends on modulating the cell size, which is governed by the behavior of cortical microtubule.

Light and gibberellins are important regulators of plant organ growth. However, little is known about their roles in petal size determination. Here, we report how light and gibberellic acid (GA₃) signals are integrated to regulate the ray floret (Rf) size in Gerbera hybrida. The inflorescences of G. hybrida at stages 1.5 were cultivated in vitro for 9 d followed by the determination of the Rf petal size. Results demonstrated that the light signal significantly enhanced both the length and the width of Rf petals, but GA₃ promoted only the petal length. Moreover, GA₃ displayed a synergistic positive effect on the length but an antagonistic effect on the width with the light signal. Measurements of the petal cells revealed that the cell size, not the cell number, exhibited a dominant contribution to the petal size in response to light and GA₃ signals. Furthermore, light and GA₃ signals not only induced an obvious reorientation of cortical microtubules (MTs) into transverse arrays but also promoted the recovery of the MT lengths in petal cells following oryzalin (an MT depolymerizing agent) treatment. Importantly, disruption of the MT lengths and arrays by oryzalin could inhibit the cell expansion and the petal enlargement induced by light or/and GA₃ signals. Taken together, it is concluded that the control of the petal size by light and GA₃ signals mainly depends on modulating the cell size and, moreover, the organization of the cortical MTs plays a crucial role in the control of the cell size and hence the Rf petal growth.