INHIBITION OF PHENYLALANINE AMMONIA-LYASE ACTIVITY CAUSES THE DEPRESSION OF LIGNIN ACCUMULATION OF SECONDARY WALL THICKENING IN ISOLATED ZINNIA MESOPHYLL-CELLS

Isolated mesophyll cells of Zinnia elegans L. cv. Canary Bird differen tiate into tracheary elements in differentiation (D) medium. These ele ments develop lignified secondary wall thickenings. The influence of 2 -aminoindan-2-phosphonic acid (AIP), an inhibitor of phenylalanine amm onia-lyase (PAL), on lignification of Zinnia tracheary elements was ex amined. The mesophyll cells were cultured in D and AIP media. The latt er medium, in which 100 mu M AIP was added to the D medium, inhibited PAL activity, though the differentiation proceeded. Morphological diff erences of secondary wall thickenings cultured in these two types of m edia were investigated under an UV microscope and a transmission elect ron microscope. The secondary wall thickenings at 96 h in the D medium showed strong UV absorption. The fibrillar structure of the thickenin gs observed clearly at 72 h was covered with electron opaque materials by 96 h. The secondary wall thickenings at 96 h in the AIP medium sho wed weak UV absorption. The thickenings at 96 h had a cracked appearan ce. Furthermore, the thickenings showed a little irregular or wavy arr angement of cellulose microfibrils and had many pores and spaces betwe en microfibrils. From these results, the role of lignin accumulation i n the formation of secondary wall thickenings was discussed.