THE EXPRESSION AND ACCUMULATION OF SOYBEAN VEGETATIVE-CELL THIOL PROTEASE IS TEMPORALLY AND DEVELOPMENTALLY-REGULATED

Plants possess a salvage pathway in which proteins accumulated during prior developmental stages are catabolized and recycled as building bl ocks for new rounds of synthesis. Soybean leaves and pods accumulate v egetative storage proteins prior to and during reproductive stage that function as transient nitrogen resources. These proteins are mobilize d to provide carbon and nitrogen for the synthesis of seed storage pro teins during seed maturation. We have isolated two thiol protease cDNA s that correspond to the two isoforms that are expressed in, soybean l eaves and pods. Immunoblot analysis with an antipeptide antibody elici ted against the 15 C-terminal amino acids of the leaf protease cross-r eact with an identical Mr polypeptide in leaves, pods, roots, flowers and cotyledons after 9 days of seedling growth. The distribution of th e protease in various soybean organs indicates the thiol protease is a vegetative cell protease. The expression of the vegetative protease m RNA in soybean leaves occurs during leaf expansion but only after the leaves are sufficiently mature to undergo the sink to source transitio n. In contrast thiol protease gene expression in pods occurs throughou t seed maturation. The leaves of podded and depodded soybean plants bo th express the thiol protease mRNAs, however depodding soybean plants partially represses thiol protease gene expression. The expression of the leaf protease is subject to temporal control with higher levels of expression occuring in the late afternoon in plants exposed to the am bient day/night cycle. The accumulation of the proteins encoded by the vegetative thiol protease mRNAs are likely to be responsible for amin o acid salvage from vegetative storage protein.