TEMPORAL AND PHOTOREGULATED EXPRESSION OF 5 TOMATO PHYTOCHROME GENES

Quantitative measurements of the absolute amounts of mRNAs transcribed from each of five phytochrome genes (PHYA, PHYB1, PHYB2, PHYE, PHYF) throughout the life cycle of a tomato plant and in response to changes in ambient light conditions are reported here. From their lowest leve l in unimbibed seed, all five transcripts increase by from 10- to 1000 -fold during the first 24 h following the onset of imbibition, both in continuous darkness and in a greenhouse. In a greenhouse and on a who le-plant basis, all continue to increase throughout day 6, after which all but PHYE decline over the next week to a plateau at about one-hal f of the maximal value. PHYE mRNA differs in that it continues to incr ease in abundance during the first 2-3 weeks and thereafter remains at that maximal level. In adult plants, on a whole-plant basis and in de creasing order of abundance, PHYA, PHYB1, PHYE, PHYB2, and PHYF transc ripts were present at approximately 120, 40, 40, 15, and 8 mu mol mu g (-1) of poly(A)(+)-enriched RNA, respectively. The data are consistent with the demonstrated roles of phytochromes A and B1 during seedling development and lead to the suggestion that phytochrome E might have a more important role in mature plants. Somewhat unexpectedly, PHYA and PHYB2 expression patterns are very similar. In seedlings, PHYA and PH YB2 exhibit the greatest increase in expression following a light-to-d ark transition, as well as the greatest decrease following a dark-to-l ight transition. PHYA and PHYB2 are also similar in that both exhibit comparable variation on a natural diurnal cycle, while PHYB1 also exhi bits variation but with a markedly different phase. The diurnal variat ion in expression of PHYA, PHYB1 and PHYB2 is consistent with the poss ibility that one or more of the phytochromes they encode is important with respect not only to photoperiodic behavior but also to the regula tion of other events whose photosensitivity varies during a diurnal cy cle.