The tapping panel dryness (TPD) syndrome of rubber is characterized by the reduction or ultimately total cessation of latex flow upon tapping, due to physiological disorders in the bark tissue. The protein pattern in the cytoplasm from healthy and TPD tree latex cells was compared by electrophoresis. Two polypeptides (P15 and P22) of 15 and 22 kDa, respectively, were found to accumulate in the cytosol of the TPD-affected trees, whereas a 29 kDa polypeptide (P29) appeared de novo. P15 and P22 were identified as REF (Hev b1) and SRPP (Hev b3), respectively, two proteins proposed to be involved in rubber biosynthesis. P29 appeared to be a new member of the patatinlike protein family. Specific molecular probes were designed for a detailed characterization of REF and SRPP gene expression and RFLP mapping. This allowed the demonstration that REF and SRPP display very similar expression profiles. They are highly over-expressed by the tappinginduced metabolic activation, although not by wounding per se, or ethylene or ABA. In addition to this similarity in gene expression, they were found to share one common locus in the genome. No significant difference in REF and SRPP gene expression was observed between healthy and TPD trees, indicating that their TPD-related accumulation in the cytosol was not transcriptionally regulated. Western blot analysis demonstrated that osmotic lysis of the sedimentable organelles (lutoids) in vitro caused the release of REF and SRPP from the rubber particle membrane into the cytosol. A mechanism of cellular delocalization as a consequence of the lutoids instability is proposed to explain REF and SRPP accumulation in the cytosol of TPD trees.
First attempts to discriminate between tapping panel dryness (TPD) and bark necrosis (BN), two Hevea sp. bark diseases leading to the cessation of latex production, showed differences in latex biochemical characteristics (1). Further, contrary to TPD, BN is characterized by inner phloem necrosis starting at the rootstock/scion junction (RS/S) and spreading upward to the tapping cut. Recent etiological (3) and epidemiological studies did not provide evidence of a causative pathogen for BN, but showed that BN is favored by a combination of various stresses (2). Searching for molecular markers of BN using sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses highlighted differential expression of some proteins in the latex and bark, especially a 67-kDa protein, which accumulates in the inner phloem of the BN trees. This protein was identified by peptide microsequencing as a linamarase (cyanogenic β-glucosidase). This led to the suspicion of the involvement of cyanogenesis in the spread of the syndrome inside the inner bark. The cDNAs of enzymes involved in cyanide (CN) metabolism (linamarase, hydroxynitrile lyase, and cyanoalanine synthase) were cloned from our Hevea sp. phloem specific cDNA library. In addition, the most BN-susceptible rubber clones were shown to exhibit higher cyanide potentions in the leaves and bark, together with low cyanoalanine synthase (CAS) gene expression and activity. Furthermore, linamarine (the cyanogene glucoside substrate of linamarase) was shown to accumulate in the phloem at the base of the trunk, especially above the rootstock/scion junction. The results of biochemical and gene expression studies associated with recent ecophysiological advances (2) strongly suggest a possible cell decompartmentalization near the RS/S junction, resulting in a local release of toxic concentration of highly diffusive CN. This, combined with a lethal imbalance between cyanogenic and CN-detoxifying activities (CAS) in the phloem of BN trees, could lead to poisoning of neighboring cells and to the spread of tissue necrosis toward the tapping cut. In conclusion, after providing evidence of exogenous factors favoring BN (2), this report highlights endogenous disorders that may be at the origin of this physiological disease leading to BN. References: (1) D. Nandris et al. Eur. J. For. Pathol. 21:325, 1991. (2) D. Nandris et al. Plant Dis. 88:1047, 2004.
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