Regulation of proline biosynthesis in kiwifruit buds with and without hydrogen cyanamide treatment

Proline accumulates in kiwifruit (Actinidia deliciosa [A. Chev.] C. F. Liang et A. R. Ferguson var. deliciosa cv. Hayward) buds in spring during budbreak and flower differentiation. However, proline accumulation occurred earlier, and to a level approximately 6.5 times greater, in buds treated with the dormancy breaking chemical hydrogen cyanamide (HC). Peak proline accumulation coincided with peak pyrroline-5-carboxylate reductase (P5CR, EC 1.5.1.2) activities in both HC-treated and control (untreated) vines. Peak levels of P5CR activity were the same in treated and control vines, suggesting that the activity of P5CR does not regulate proline biosynthesis. Ornithine-δ-aminotransferase (OAT, EC 2.6.1.13) activity declined over the sampling period and, consequently, was not correlated with proline accumulation in buds from either treated or control vines. This suggests that ornithine does not make a significant contribution to the proline pool in this system. Pyrroline-5-carboxylate synthetase (P5CS, EC number not assigned) and P5CR have been cloned from a kiwifruit bud cDNA library. Northern hybridisation analyses indicate that both P5CS and P5CR are transcriptionally regulated. The PSCS northern hybridisation data, together with the P5CR and OAT enzyme data, suggest that glutamate provides the carbon skeletons for proline synthesis and that P5CS transcription has a significant role in the regulation of proline biosynthesis in kiwifruit buds. This role for P5CS is consistent with those reported in other plants and demonstrates a conservation of the pathway, and its overall regulation in quite diverse plants. The only difference in the regulation of proline biosynthesis between kiwifruit buds from control and HC-treated vines would appear to be temporal.