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

Ectopic overexpression of an oat PHYA cDNA in tobacco under the cauliflower mosaic virus 35S promoter results in plants with reduced morphological responses to farred 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 of PHYA with appropriate promoters. Oat PHYA was expressed in tobacco (Nicotiana tabacum L. cv Xanthi) under the 35S and two Arabidopsis promoters: UBQ1 and CAB. The internodes of wild type, UBQ:PHYA, and CAB:PHYA plants, which exhibited little or no oat PHYA overexpression, responded to localized 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 transgenic crop plants in which certain organs or tissues are made 'blind' to phytochrome-perceived signals of canopy density, but whose general photomorphogenic competence is not greatly disturbed.