A Temperature-Sensitive Chlorophyll b -Deficient Mutant of Sweetclover ( Melilotus alba )

Abstract

The ch4 mutant of sweetclover (Melilotus alba) has previously been demonstrated to be partially deficient in chlorophyll and to have a higher ratio of chlorophyll a to b than normal plants. We were able to substantiate these findings when plants were grown at 23°C and lower (permissive temperatures). However, when grown at 26°C (nonpermissive temperature) the plants produced small yellow leaves which exhibited one-twentieth the chlorophyll content of normal plants. Affected leaves did not increase their chlorophyll content when plants were incubated at permissive temperatures, but leaves which developed at the lower temperature contained increased amounts of chlorophyll. Similarly, only new leaves, not previously grown leaves, exhibited the yellow phenotype when the mutant plant was shifted from the permissive temperature to the nonpermissive temperature. Ribulose 1,5-bisphosphate carboxylase activity was decreased by half, relative to normal plants, in the mutant plants grown at the nonpermissive temperature, indicating that general protein synthesis was not greatly impaired and that the effect of the mutation was perhaps specific for chlorophyll content. HPLC analysis indicated that carotenoid content was not diminished to the same extent as chlorophyll and we have determined that the thylakoid protein kinase is not altered, as is the case for other chlorophyll b-deficient mutants. Experiments suggest that changes in photoperiod may be able to modulate the effect of temperature. One type of mutation in higher plants and algae that has been widely studied results in reduced expression of Chl b. This pigment comprises approximately one-fourth of the Chl in most higher plants, functions in a light-harvesting capacity (i.e. having no known photochemical role), and is bound in pigment-protein complexes associated with both PSI and PSII. Mutants defective in the expression of Chl b can generally be placed in one of two phenotypes: those which have no detectable Chl b and which 'Supported having ratios of Chl a/b >5 versus values of approximately 3 for the normal plants. Such mutants have been described in barley (Hordeum vulgare) (14), pea (Pisum sativum) (15, 27), maize (Zea mays) (16, 25), wheat (Triticum aestivum) (10, 12), sweetclover (Melilotus alba) (13, 21, 22, 26, 30), Chlamydomonas reinhardii (24), and other species. Such mutants are usually selected on the basis of decreased pigment content and the major physiological effect is that the fluence rate of light needed to saturate photosynthetic electron transport is increased (15, 16). It is currently unclear how many genotypes can give rise to the Chl b-deficient phenotype. Chl blacking mutants of sweet-clover were found by complementation analysis to be allelic whereas a Chl b-deficient mutant was able to genetically complement these mutants, suggesting its lesion was in a different gene (13, 26). It has also been demonstrated that thylakoid membranes of some mutants lacking Chl b still contain significant amounts of the Light-Harvesting Complex apoproteins which bind Chl b (e.g. barley chlorina-f2 and Chlamydomonas pg 113) (4, 24) whereas other such mutants (e.g. sweetclover chS) appear to contain very little of these polypeptides (22). Although Chl b mutants generally appear to have an increased thylakoid protein kinase specific activity (21, 24),some Chl b-deficient and-lacking mutants have a thylakoid protein kinase activity with altered affinity for ATP (21). The effect of light, temperature, and other abiotic environmental factors on the plasticity of the photosynthetic apparatus in normal plants has been studied in a number of laboratories (for a review, see Ref. 3). Mutants lacking Chl b appear to be relatively insensitive to environmental conditions. However, evidence in the literature suggests that mutants only deficient in Chl b can have their pigment contents strongly influenced by growth conditions and that their expression of Chl b may be more plastic than that in the corresponding normal plants. In the Chl b-deficient Oy-yg mutant of maize the amount of Chl b expressed varies markedly in an inverse relationship to the light intensity during growth (16, 25). The relative proportion of pigment in the Chl b-containing Light-Harvesting Complex of the CD3 mutant of wheat has been reported to increase as the leaves aged (10). Such Chl b-deficient mutants have not been sufficiently studied to determine if conditionality or phenotypic plasticity is common to this class. While in the process ofgrowing the ch4 Chl b-deficient mutant of sweetclover for a previously reported study (22), it was observed that plants grown with artificial lighting and at ambient temperatures exhibited rather different properties over the course of the year. To determine whether this mutant might be environmentally responsive, we 329 www.plantphysiol.org on August 26, 2020-Published by Downloaded from