Keara A. Franklin

Phytochrome functions in Arabidopsis development

Light signals are fundamental to the growth and development of plants. Red and far-red light are sensed using the phytochrome family of plant photoreceptors. Individual phytochromes display both unique and overlapping roles throughout the life cycle of plants, regulating a range of developmental processes from seed germination to the timing of reproductive development. The evolution of multiple phytochrome photoreceptors has enhanced plant sensitivity to fluctuating light environments, diversifying phytochrome function, and facilitating conditional cross-talk with other signalling systems. The isolation of null mutants, deficient in all individual phytochromes, has greatly advanced understanding of phytochrome functions in the model species, Arabidopsis thaliana. The creation of mutants null for multiple phytochrome combinations has enabled the dissection of redundant interactions between family members, revealing novel regulatory roles for this important photoreceptor family. In this review, current knowledge of phytochrome functions in the light-regulated development of Arabidopsis is summarised.

High Temperature-Mediated Adaptations in Plant Architecture Require the bHLH Transcription Factor PIF4

Exposure of Arabidopsis plants to high temperature (28 degrees C) results in a dramatic change in plant development. Responses to high temperature include rapid extension of plant axes, leaf hyponasty, and early flowering. These phenotypes parallel plant responses to the threat of vegetational shade and have been shown to involve the hormone auxin. In this work, we demonstrate that high temperature-induced architectural adaptations are mediated through the bHLH transcriptional regulator PHYTOCHROME INTERACTING FACTOR 4 (PIF4). Roles for PIF4 have previously been established in both light and gibberellin (GA) signaling, through interactions with phytochromes and DELLA proteins, respectively. Mutants deficient in PIF4 do not display elongation responses or leaf hyponasty upon transfer to high temperature. High temperature-mediated induction of the auxin-responsive gene IAA29 is also abolished in these plants. An early flowering response to high temperature is maintained in pif4 mutants, suggesting that architectural and flowering responses operate via separate signaling pathways. The role of PIF4 in temperature signaling does not, however, appear to operate through interaction with either phytochrome or DELLA proteins, suggesting the existence of a novel regulatory mechanism. We conclude that PIF4 is an important component of plant high temperature signaling and integrates multiple environmental cues during plant development.

Mutant Analyses Define Multiple Roles for Phytochrome C in Arabidopsis Photomorphogenesis

The analysis of Arabidopsis mutants deficient in the A, B, D, and E phytochromes has revealed that each of these phytochrome isoforms has both distinct and overlapping roles throughout plant photomorphogenesis. Although overexpression studies of phytochrome C (phyC) have suggested photomorphogenic roles for this receptor, conclusive evidence of function has been lacking as a result of the absence of mutants in the PHYC locus. Here, we describe the isolation of a T-DNA insertion mutant of phyC (phyC-1), the subsequent creation of mutant lines deficient in multiple phytochrome combinations, and the physiological characterization of these lines. In addition to operating as a weak red light sensor, phyC may perform a significant role in the modulation of other photoreceptors. phyA and phyC appear to act redundantly to modulate the phyB-mediated inhibition of hypocotyl elongation in red light and to function together to regulate rosette leaf morphology. In addition, phyC performs a significant role in the modulation of blue light sensing. Several of these phenotypes are supported by the parallel analysis of a quadruple mutant deficient in phytochromes A, B, D, and E, which thus contains only active phyC. Together, these data suggest that phyC has multiple functions throughout plant development that may include working as a coactivator with other phytochromes and the cryptochrome blue light receptors.

phytochrome B and PIF4 Regulate Stomatal Development in Response to Light Quantity

Stomata are pores on the surfaces of leaves that regulate gas exchange between the plant interior and the atmosphere [1]. Plants adapt to changing environmental conditions in the short term by adjusting the aperture of the stomatal pores, whereas longer-term changes are accomplished by altering the proportion of stomata that develop on the leaf surface [2, 3]. Although recent work has identified genes involved in the control of stomatal development [4], we know very little about how stomatal development is modulated by environmental signals, such as light. Here, we show that mature leaves of Arabidopsis grown at higher photon irradiances show significant increases in stomatal index (S.I.) [5] compared to those grown at lower photon irradiances. Light quantity-mediated changes in S.I. occur in red light, suggesting that phytochrome photoreceptors [6] are involved. By using a genetic approach, we demonstrate that this response is dominated by phytochrome B and also identify a role for the transcription factor, PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) [7]. In sum, we identify a photoreceptor and downstream signaling protein involved in light-mediated control of stomatal development, thereby establishing a tractable system for investigating how an environmental signal modulates stomatal development.

Plant chromatin feels the heat.

Temperature is a key environmental signal regulating plant development, but the mechanisms by which plants sense small changes in ambient temperature have remained elusive. Kumar and Wigge (2010) now reveal that eviction of the histone variant H2A.Z from nucleosomes performs a central role in plant thermosensory perception.

Improvement of Horticultural and Ornamental Crops Through Transgenic Manipulation of the Phytochrome Family of Plant Photoreceptors

Abstract Consumer demands drive continuous developments in the production of horticultural and ornamental crops. In addition to improvements in quality and nutritional content, crop producers must supply an increased variety of products to a year-round market. The availability of many horticultural and ornamental crop products is dependent on the timing of reproductive development. The time at which many plants initiate sexual or vegetative reproduction is governed by a number of interacting environmental factors such as daylength, light quality and temperature. Artificial manipulation of the growing environment is therefore frequently used to ensure production meets retailers marketing programs, a strategy which can often result in high energy costs. An alternative approach involves the manipulation of genes encoding proteins responsible for perceiving and transducing environmental stimuli, in particular, the genes encoding the phytochrome family of plant photoreceptors. Alterations in the expression of genes encoding phytochromes can modulate not only the timing of reproductive development, but also plant architecture. Such approaches can therefore be used to modulate a variety of phenotypic traits such as height, lateral branching and harvest yield, while enabling growers to tailor crop reproduction to their marketing needs. In this review, we will discuss examples of crop improvement using transgenic manipulation of phytochrome expression, along with benefits and disadvantages of such approaches.