Summary• Phytosulfokine-α (PSK-α) is a disulfated pentapeptide described to act as a growth factor in suspension cells. In this study, the involvement of PSK signaling through the PSK receptor gene AtPSKR1 in Arabidopsis root growth was assessed.• Expression studies of PSK precursor genes and of AtPSKR1 were performed in roots with RT-PCR and P:GUS analyses. Root elongation, lateral root formation, cell production and root cell elongation were analyzed in wild-type (wt) and in the receptor knockout mutant Atpskr1-T treated with or without synthetic PSK-α.• Phytosulfokine and AtPSKR1 genes are differentially expressed in roots. PSK-α induced root growth in a dose-dependent manner without affecting lateral root density. Kinematic analysis established that enhancement of root growth by PSK-α was mainly caused by an increase in cell size. In Atpskr1-T, the primary roots were shorter as a result of reduced mature cell size and a smaller root apical meristem composed of fewer cells than in wt.• The results indicate that PSK-α signaling through AtPSKR1 affects root elongation primarily via control of mature cell size. Root organogenesis, on the other hand, is not controlled by PSK-α.

Phytosulfokine (PSK) is a peptide growth factor that requires tyrosine sulfation carried out by tyrosylprotein sulfotransferase (TPST) for its activity. PSK is processed from precursor proteins encoded by five genes in Arabidopsis thaliana and perceived by receptor kinases encoded by two genes in Arabidopsis. pskr1-3 pskr2-1 and tpst-1 knockout mutants displayed reduced seed production, indicative of a requirement for PSK peptide signaling in sexual plant reproduction. Expression analysis revealed PSK precursor and PSK receptor gene activity in reproductive organs with strong expression of PSK2 in pollen. In support of a role for PSK signaling in pollen, in vitro pollen tube (PT) growth was enhanced by exogenously added PSK while PTs of pskr1-3 pskr2-1 and of tpst-1 were shorter. In planta, growth of wild-type pollen in pskr1-3 pskr2-1 and tpst-1 flowers appeared slower than growth in wild-type flowers. But PTs did eventually reach the base of the style, suggesting that PT elongation rate may not be responsible for the reduced fertility. Detailed analysis of anthers, style and ovules did not reveal obvious developmental defects. By contrast, a high percentage of unfertilized ovules in pskr1-3 pskr2-1 and in tpst-1 siliques displayed loss of funicular PT guidance, suggesting that PSK signaling is required to guide the PT from the transmitting tract to the embryo sac. Cross-pollination experiments with wild-type, pskr1-3 pskr2-1 and tpst-1 male and female parents revealed that both the PT and the female sporophytic tissue and/or female gametophyte contribute to successful PT guidance via PSK signaling and to fertilization success.

By prospectively studying immunoglobulin heavy chain gene (IgH) and T cell receptor gamma (TCRgamma) gene rearrangements in 398 lymphoma cases, a dual genotype was observed in 13% of B cell and 11% of T cell lymphomas. According to histological subtype, the highest incidence was observed for mantle cell lymphomas (32%) and lymphoplasmacytic lymphoma (21%) among B cell lymphomas, and for angioimmunoblastic lymphoma (AILT) (46%) and Sézary syndrome (SS) (50%) among T cell lymphomas. To determine whether the dual genotype corresponds to the presence of two distinct monoclonal populations or to the presence of both rearrangements within the same lymphoma cells, single-cell microdissection was used after immunohistochemistry and a single-cell combined IgH and TCRgamma gene analysis was designed after a whole-genome amplification step. This protocol was applied to the study of two nodal B cell lymphomas (one diffuse large B cell lymphoma and one mantle cell lymphoma) and two cutaneous T cell lymphomas (one AILT and one SS). Two cases (SS and mantle cell lymphoma) were true bigenotypic lymphomas, as both IgH and TCRgamma monoclonal rearrangements were detected in the same cells. Conversely, in the diffuse large B cell lymphoma and AILT cases, large CD22+ single cells exhibited only the monoclonal IgH rearrangement but not the TCRgamma gene that was detected in CD3+ single cells. Such an approach allows the identification of true bigenotypic lymphoma among dual genotypic lymphoma. Specific genetic alterations may be further amplified from microdissected cryopreserved material, such as the t(11;14) breakpoint detected in bigenotypic B cells of the mantle cell lymphoma case.