Increasing UV-B Induces Biphasic Leaf Cell Expansion in Phaseolus vulgaris, Suggesting Multiple Mechanisms for Controlling Plant Growth

Abstract

Leaf expansion, comprising cell division and cell enlargement, is controlled by light quality and quantity. The role of UV-B irradiance on leaf cell enlargement has not been determined. We studied the effect of a wide range of UV-B irradiances on the cell-enlargement-driven expansion of Phaseolus vulgaris L., cv. Contender (bush bean) leaf discs. Our growth method allowed separation of the cell enlargement phase of leaf expansion from the cell division phase. In two series of experiments with different types of UV-B screening filters, the effect of increasing levels of UV-B on the area of excised P. vulgaris leaf discs was investigated. One set of experiments utilized polyester (UV-B-absorbing) and cellulose acetate (UV-B-transmitting) filters. The other set utilized UV-B-absorbing and UV-B-transmitting acrylic filters. Regardless of which type of filter was used for screening, high (above summer solstice) levels of supplemental UV-B inhibited cell enlargement in a linear, dose-dependent manner, resulting in smaller leaf discs than treatment with UV-B-absorbing filters. Conversely, low levels of supplemental UV-B enhanced cell enlargement in a linear, dose-dependent manner, resulting in larger leaf discs than did treatment with UV-B-absorbing filters. The results suggest a biphasic response to UV-B, and that there is an optimum UV-B level that results in maximum leaf expansion by cell enlargement.

Ultraviolet Radiation as a Limiting Factor in Leaf Expansion and Development

“…It was notable that the field data also showed clear effects of longer wavelength UV‐A wavelengths of cell‐wall peroxidases, since only the zero‐UV field environment showed no upregulation of peroxidase activity. Reductions in LER under increased UV‐B (21,58) and UV‐A (4) have been reported, but our data suggests both are related to induction of cell‐wall peroxidase. The role of cell‐wall peroxidases in regulating cell expansion is well documented (59,60), and similar observations of growth inhibition following induction of cell‐wall peroxidase have been noted in response to herbivory (61) and in the induction of systemic resistance (62), and it is likely that induction of peroxidase may regulate growth following pathogen attack (63).…”

“…UV‐B has also been shown to reduce leaf cell size in several species [ e.g. (13,18–20)], although increases in cell size have been observed at lower doses (21). Cell expansion is a product of turgor and cell‐wall extensibility, or the ability of the cell wall to expand or irreversibly extend during the growth cycle (22), with the rate of cell enlargement (GR) described by the equation GR = Ext( T ‐ Y ), where Ext is cell‐wall extensibility, T is cell turgor, and Y is the cell wall yield threshold (23).…”

Species‐specific effect of UV‐B radiation on the temporal pattern of leaf growth

“…Although the difference in leaf length:width ratio in B. pendula caused by UV‐B radiation was in the opposite direction to that reported by Hectors et al (2010), it nevertheless may reflect that cell expansion was affected by UV‐B radiation and that this effect was compensated for before the completion of leaf growth as it coincides with the transient effects on leaf expansion rate that we report. Compensatory responses during different phases of leaf growth have been reported elsewhere (Barkan et al 2006, Wargent et al 2009a). In B. pubescens , the absence of a change in leaf length:width ratio as a result of UV‐B radiation and lack of difference in growth rate during expansion, may suggest that a difference in cell number rather than cell expansion could be responsible for the decreased final size in this species.…”

UVR8 in Arabidopsis thaliana regulates multiple aspects of cellular differentiation during leaf development in response to ultraviolet B radiation

“…Exposure to UV‐B leads to a dose‐dependent reduction of leaf size in a diverse range of plant species (Liu et al ., 1995; Nogues et al ., 1998; Searles et al ., 2001; Barkan et al ., 2006), most likely by inhibition of epidermal cell expansion and cell division (Gonzalez et al ., 1998; Nogues et al ., 1998). Here we show that UV‐B significantly reduced the number of epidermal cells per leaf in both uvr8 2 and wild type, which indicates that most of the UV‐mediated leaf area reduction in wild‐type Arabidopsis is explained by changes in cell division, and that this aspect of UV‐B response is independent of UVR8.…”