Cytokinin treatment modifies litchi fruit pericarp anatomy leading to reduced susceptibility to post-harvest pericarp browning.

Litchi (Litchi chinensis Sonn.) is a subtropical fruit known for its attractive red pericarp color, semi-translucent white aril and unique flavor and aroma. Rapid post-harvest pericarp browning strictly limits litchi fruit marketing. In the current research, we hypothesized that modification of litchi fruit pericarp anatomy by hormone application may reduce fruit susceptibility to post-harvest pericarp browning. In this context, we hypothesized that cytokinin treatment, known to induce cell division, may yield fruit with thicker pericarp and reduced susceptibility for fruit surface micro-crack formation, water loss and post-harvest pericarp browning. Exogenous cytokinin treatment was applied at different stages along the course of litchi fruit development and the effect on fruit pericarp anatomy, fruit maturation and postharvest pericarp browning was investigated. Interestingly, cytokinin treatment, applied 4 weeks after full female bloom (WFB), during the phase of pericarp cell division, led to mature fruit with thicker pericarp, reduced rate of post-harvest water loss and reduced susceptibility to post-harvest pericarp browning, as compared to non-treated control fruit. Histological sections ascribe the difference in pericarp anatomy to increased cell proliferation in the parenchymatic tissue and the highly-lignified brachysclereid cell layer. In contrast, exogenous cytokinin treatment applied 7 WFB, following the phase of pericarp cell division, significantly increased epidermal-cell proliferation but had no significant effect on overall fruit pericarp thickness and only minor affect on post-harvest water loss or pericarp browning. Interestingly, the late cytokinin treatment also significantly postponed fruit maturation-associated anthocyanin accumulation and chlorophyll degradation, as previously reported, but had no effect on other parameters of fruit maturation, like total soluble sugars and total titratable acids typically modified during aril maturation. In conclusion, exogenous cytokinin treatment at different stages in fruit development differentially modifies litchi fruit pericarp anatomy by induction of cell-type specific cell proliferation. Early cytokinin treatment during the phase of pericarp cell division may prolong litchi fruit storage by reducing fruit susceptibility to post-harvest water loss and pericarp browning.

Gene expression profiling in juvenile and mature cuttings of Eucalyptus grandis reveals the importance of microtubule remodeling during adventitious root formation.

BACKGROUND: The ability to form adventitious roots (AR) is an economically important trait that is lost during the juvenile-to-mature phase change in woody plants. Auxin treatment, which generally promotes rooting in juvenile cuttings, is often ineffective when applied to mature cuttings. The molecular basis for this phenomenon in Eucalyptus grandis was addressed here. RESULTS: A comprehensive microarray analysis was performed in order to compare gene-expression profiles in juvenile and mature cuttings of E. grandis, with or without auxin treatment on days, 0, 1, 3, 6, 9 and 12 post AR induction. Under these conditions AR primordia were formed only in auxin-treated juvenile cuttings. However, clustering the expression profiles revealed that the time after induction contributed more significantly to the differences in expression than the developmental phase of the cuttings or auxin treatment. Most detected differences which were related to the developmental phase and auxin treatment occurred on day 6, which correlated with the kinetics of AR-primordia formation. Among the functional groups of transcripts that differed between juvenile and mature cuttings was that of microtubules (MT). The expression of 42 transcripts annotated as coding for tubulin, MT-associated proteins and kinesin motor proteins was validated in the same RNA samples. The results suggest a coordinated developmental and auxin dependent regulation of several MT-related transcripts in these cuttings. To determine the relevance of MT remodeling to AR formation, MTs were subjected to subtle perturbations by trifluralin, a MT disrupting drug, applied during auxin induction. Juvenile cuttings were not affected by the treatment, but rooting of mature cuttings increased from 10 to more than 40 percent. CONCLUSIONS: The data suggest that juvenile-specific MT remodeling is involved in AR formation in E. grandis.