EXPRESSION OF A NOVEL-TYPE SMALL PROLINE-RICH PROTEIN GENE OF ALFALFAIS INDUCED BY 2,4-DICHLOROPHENOXIACETIC ACID IN DEDIFFERENTIATED CALLUS CELLS

Differential screening of a cDNA library of 2,4-dichlorophenoxiacetic acid (2,4-D)-treated alfalfa (Medicago sativa) callus tissues resulted in the isolation of a 571 bp cDNA clone (MsPRP5) encoding for a proli ne-rich protein (84 amino acids) with a specific repeat unit of TPVLPP RK/(R)GRPPPVPP. In addition, a characteristic amino acid block (PPVYK) previously found in other proline-rich proteins also occurs in the C- terminal region of MsPRP5. At the N-terminal, a signal peptide similar to leader sequences of extracellular proteins can be predicted. Accor ding to the northern analysis, the corresponding gene is not expressed or is weakly expressed in differentiated vegetative organs and somati c embryos. However the accumulation of MsPRP5 mRNA is auxin concentrat ion-dependent in dedifferentiated callus tissue. An increase in the am ount of steady-state mRNA was detected already 20 min after auxin shoc k (100 mu M 2,4-D). Maximum expression was observed at 24-48 h in the presence of 2,4-D. Elevated expression was also found in cells recover ing after heat shock and wounding stress. In synchronized alfalfa cell s, the transcript level of MsPRP5 gene fluctuated during cell cycle pr ogression with peaks in G1/S phase cells. Considering the structural f eatures and expression properties of MsPRP5, this clone may represents a new type of proline-rich protein gene which responds to hormonal sh ock and some other stresses as well.