Directed overexpression of PHYA locally suppresses stem elongation and leaf senescence responses to far‐red radiation

Abstract

Ectopic overexpression of an oat PHYA cDNA in tobacco under the cauliflower mosaic virus 35S promoter results in plants with reduced morphological responses to far-red radiation (FR). We have tested the hypothesis that it is possible to molecularly 'mask' stems and leaves to FR-induced elongation and senescence responses by targeting the overexpression ol'PHYA with appropriate promoters. Oat PHYA was expressed in tobacco {Nicotiana tahacum L. cv Xanthi) under the 35S and two Arabidopsis promoters: UBQl and CAB. The internodes of wild type, UBQ.PHYA, and CAB.PHYA plants, which exhibited little or no oat PHYA overexpression, responded to locali/.ed FR treatments with a marked increase in elongation. In contrast, 35S:PHYA plants, which overexpressed PHYA to high levels in all parts of the shoot, did not respond to FR treatments directed to their stems. Leaf senescence responses to FR were remarkably localized, and sensitivity to FR was also inversely correlated with the local PHYA expression level. Thus, chlorophyll content, specific leaf weight, and nitrate reductase activity in leaf spots treated with FR were highly reduced in wild type and UBQ:PHYA plants, but not in the CAB.PHYA and 35S:PHYA counterparts. Our results suggest that it may be feasible to obtain transgenie crop plants in which certain organs or tissues are made 'blind' to phytochrome-perceived signals of canopy density, but whose general photoniorpbogenic competence is not greatly disturbed.

Phytochrome-interacting transcription factors PIF4 and PIF5 induce leaf senescence in Arabidopsis

“…Red light has long been considered to be a key negative signal for plant senescence, because pulses of red light substantially delay dark-induced senescence (DIS) in a few plant species, supposedly via phytochromes 24 and a low ratio of red/far-red light causes senescence in tobacco leaves 25 . Red light activates and far-red light inactivates phytochromes; darkness also causes slow inactivation of phytochromes 23 .…”

Expression of Enzymes Involved in Chlorophyll Catabolism in Arabidopsis Is Light Controlled

“…The most visible effect of senescence is Chl degradation. For instance, illumination of tobacco (Nicotiana tabacum) leaves with far red causes a drop in Chl content, which does not occur in Nicotiana plants overexpressing oat (Avena sativa) phyA (Rousseaux et al, 1997). We have observed that the levels of mRNA of two enzymes involved in Chl catabolism in Arabidopsis, products of the AtCLH1 and AtCLH2 genes, dramatically increase (by almost two and one order of magnitude, respectively) upon illumination with white light.…”

Nitric Oxide Signaling During Senescence and Programmed Cell Death in Leaves

“…It was shown that leaves of soybean (Guiamet et al, 1989) and sunflower (Rousseaux et al, 1996) senesced more quickly when the red:far-red ratio of the light they received was decreased, and that far-red light induces chlorophyll loss in tobacco leaves (Rousseaux et al, 1997). It also has been shown that tobacco and overexpressing oat phytochrome A display both delayed leaf senescence (Cherry et al, 1991) and an inhibited response to the senescence promoting effects of far-red light (Rousseaux et al, 1997).…”

“…It also has been shown that tobacco and overexpressing oat phytochrome A display both delayed leaf senescence (Cherry et al, 1991) and an inhibited response to the senescence promoting effects of far-red light (Rousseaux et al, 1997). Weaver and Amasino (2001) examined a hy2/hy3 phytochrome double mutant line, in which there is no phytochrome B and low levels of all other phytochromes (Parks and Quail, 1991;Somers et al, 1991), and observed it to behave much like wild-type controls, after both whole-plant and individual leaf dark treatments.…”