Effect of 4-CPA, SA and ABA on the expression of HbPIP2;1, HbTIP1;1 and HbPIP1;1 in the latex and inner bark of virgin trees As shown in Fig. 2, all hormones tested induced significant up-regulation of the HbPIP2;1 gene at the transcription level in both the inner bark tissues and the laticifers, but the response was more marked in the latter. The level of HbPIP2;1 transcripts in both tissues were found to increase significantly at 4 h, then progressively until 16 h after the auxin treatment ( Fig. 2A and D). HbPIP2;1 up-regulation by SA and ABA was at a maximum, and was highly significant in the inner soft bark tissues, reaching a plateau 8 h after the treatments ( Fig. 2B and C). In the laticifers, the expression of HbPIP2;1 was the lowest and was progressively up-regulated by SA treatment ( Fig. 2E), while its expression was transiently but significantly up-regulated 8 h after ABA treatment ( Fig. 2F)